From owner-pgsql-hackers@hub.org Tue Oct 13 15:05:53 1998
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From: Terry Mackintosh <terry@terrym.com>
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Subject: Re: [HACKERS] What about LIMIT in SELECT ?
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Hi, my 2 cents...

I agree completely, LIMIT would be VERY usefull in web based apps, which
is all I run.  It does not matter to me if it is not part of a formal
standard.  The idea is so common that it is a defacto standard.

I would not expect it for this release, but could it get put on the TODO
list for next time?  I am even willing to work at an apprentise level on
this with a more expeireanced person that knows this stuff.

A note on implimentation:
I *used to* :) work with VFP on NT's :(
And the way VFP did LIMIT, it would only return the number of rows asked
for, BUT it still did the WHOLE search!
So on a larger table, which we had (property tax database for the county),
if some one put in too vague a query, it would try to collect ALL of the
rows as the initial result set, then give you the first x rows of that.

This did save on pushing mass amounts of data out to the browser, but it
would have been even better if it could have simply aborted the select
after having found x rows.

Also, it did not have the concept of an offset, so one could not select
100 rows, starting 200 rows in, which would be REALLY usefull for "paging"
through data.  I do not know if mySQL or any other has such a concept
either, but it would be nice.

So a properly implemented "LIMIT" could:
1.  Save pushing mass amounts of data across the web, that no one wants
any way.
2.  Stop vague queries from bogging down the server.
(On very larg tables this could be critical!)
3.  Enable "Paging" of data. (easyer then now (app. level))
4.  Would be a very nice feather in PostgreSQL's cap that could make it
even more attractive to those looking at all sorts of databases out there.

Have a great day.

On Tue, 13 Oct 1998, Oleg Bartunov wrote:

> Hi,
> 
> I took a look at mysql and was very impressed with   possibility
> to limit number of rows returned from select. This is very useful
> feature for Web applications when user need to browse results of
> selection page by page. In my application I have to do full
> select every time user press button [Next] and show requested page
> using perl. This works more or less ok for several thousands rows but
> totally unusable for large selections. But now I'm about to work
> with big database and I don't know how I'll stay with postgres :-)
> It'll just doesn't work if customer will wait several minutes just browse
> next page. Mysql lacks some useful features postgres has 
> (subselects, transaction ..) but for most Web applications I need
> just select :-) I dont' know how LIMIT is implemented in Mysql and
> I know it's not in SQL92 standart, but this makes Mysql very popular.
> 
> Is it difficult to implement this feature in postgres ?
> 
> 	Regards,
> 
> 		Oleg
> 
> 
> _____________________________________________________________
> Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
> Sternberg Astronomical Institute, Moscow University (Russia)
> Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
> phone: +007(095)939-16-83, +007(095)939-23-83
> 
> 

Terry Mackintosh <terry@terrym.com>          http://www.terrym.com
sysadmin/owner  Please! No MIME encoded or HTML mail, unless needed.

Proudly powered by R H Linux 4.2, Apache 1.3, PHP 3, PostgreSQL 6.3
-------------------------------------------------------------------
Success Is A Choice ... book by Rick Patino, get it, read it!




From owner-pgsql-hackers@hub.org Tue Oct 13 18:12:41 1998
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From: "Jeff Hoffmann" <jeff@remapcorp.com>
To: "Marc G. Fournier" <scrappy@hub.org>, "Eric Lee Green" <eric@linux-hw.com>
Cc: "PostgreSQL-development" <hackers@postgreSQL.org>
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
Date: Tue, 13 Oct 1998 16:56:48 -0500
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>On Tue, 13 Oct 1998, Eric Lee Green wrote:
>
>> On Tue, 13 Oct 1998, Jeff Hoffmann wrote:
>> > >I agree completely, LIMIT would be VERY usefull in web based apps,
which
>> > >is all I run.  It does not matter to me if it is not part of a formal
>> > >standard.  The idea is so common that it is a defacto standard.
>> >
>> > i'm not familiar with mysql and using "LIMIT" but wouldn't this same
effect
>> > be achieved by declaring a cursor and fetching however many records in
the
>> > cursor?  it's a very noticeable improvement when you only want the
first 20
>> > out of 500 in a 200k record database, at least.
>>
>> The problem with declaring a cursor vs. the "LIMIT" clause is that the
>> "LIMIT" clause, if used properly by the database engine (along with the
>> database engine using indexes in "ORDER BY" clauses) allows the database
>> engine to short-circuit the tail end of the query. That is, if you have
25
>> names and the last one ends with BEAVIS, the database engine doesn't have
>> to go through the BUTTHEADS and KENNYs and etc.
>>
>> Theoretically a cursor is superior to the "LIMIT" clause because you're
>> eventually going to want the B's and K's and etc. anyhow -- but only in a
>> stateful enviornment. In the stateless web environment, a cursor is
>> useless because the connection can close at any time even when you're
>> using "persistent" connections (and of course when the connection closes
>> the cursor closes).
>
>Ookay, I'm sorry, butyou lost me here.  I haven't gotten into using
>CURSORs/FETCHs yet, since I haven't need it...but can you give an example
>of what you would want to do using a LIMIT?  I may be missing something,
>but wha is the different between using LIMIT to get X records, and
>definiing a cursor to FETCH X records?
>
>Practical example of *at least* the LIMIT side would be good, so that we
>can at least see a physical example of what LIMIT can do that
>CURSORs/FETCH can't...
>


fetch with cursors should work properly (i.e., you can short circuit it by
just ending your transaction)  my understanding on how this works is exactly
how you explained LIMIT to work.  here's some empirical proof from one of my
sample databases:

the sample table i'm using has 156k records (names of people)
i'm using a PP180 with 128MB RAM and some old slow SCSI drives.

public_mn=> select count(*) from public_ramsey;
 count
------
156566
(1 row)

i did the following query:
public_mn=> select * from public_ramsey where ownerlname ~ 'SMITH';

which returned 711 matches and took about 12 seconds.

i did the same thing with a cursor:

public_mn=> begin;
BEGIN
public_mn=> declare test cursor for select * from public_ramsey where
ownerlname  ~ 'SMITH';
SELECT

the select was instantaneous.

public_mn=> fetch 20 in test;

returns 20 records almost instantaneously.  each additional 20 took less
than a second, as well.

if this isn't what you're talking about, i don't understand what you're
saying.

jeff



From eric@ireland.linux-hw.com Tue Oct 13 18:52:42 1998
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From: Eric Lee Green <eric@linux-hw.com>
To: Bruce Momjian <maillist@candle.pha.pa.us>
cc: jeff@remapcorp.com, hackers@postgreSQL.org
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <199810132116.RAA11249@candle.pha.pa.us>
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On Tue, 13 Oct 1998, Bruce Momjian wrote:
> > Theoretically a cursor is superior to the "LIMIT" clause because you're
> > eventually going to want the B's and K's and etc. anyhow -- but only in a
> > stateful enviornment. In the stateless web environment, a cursor is
> > useless because the connection can close at any time even when you're
> > using "persistent" connections (and of course when the connection closes
> What we could do is _if_ there is only one table(no joins), and an index
> exists that matches the ORDER BY, we could use the index to
> short-circuit the query.

This is exactly what MySQL does in this situation, except that it can use
the ORDER BY to do the short circuiting even if there is a join involved
if all of the elements of the ORDER BY belong to one table. Obviously if
I'm doing an "ORDER BY table1.foo table2.bar" that isn't going to work!
But "select table1.fsname,table1.lname,table2.receivables where 
table2.receivables > 0 and table1.custnum=table2.custnum order by
(table1.lname,table1.fsname) limit 50" can be short-circuited by fiddling
with the join order -- table1.fsname table1.lname have to be the first two
things in the join order. 

Whether this is feasible in PostgreSQL I have no earthly idea. This would
seem to conflict with the join optimizer.

> happier?  If there is an ORDER BY and no index, or a join, I can't
> figure out how we would short-circuit the query.

If there is an ORDER BY and no index you can't short-circuit the query.
MySQL doesn't either. Under certain circumstances (such as above) you can
short-circuit a join, but it's unclear whether it'd be easy to add such
a capability to PostgreSQL given the current structure of the query
optimizer. (And I certainly am not in a position to tackle it, at the
moment MySQL is sufficing for my project despite the fact that it is 
quite limited compared to PostgreSQL, I need to get my project finished
first).  

--
Eric Lee Green         eric@linux-hw.com     http://www.linux-hw.com/~eric
"To call Microsoft an innovator is like calling the Pope Jewish ..." 
            -- James Love (Consumer Project on Technology)


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: eric@linux-hw.com (Eric Lee Green)
Date: Wed, 14 Oct 1998 13:09:21 +0200 (MET DST)
Cc: jeff@remapcorp.com, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <Pine.LNX.3.96.981013161955.30555A-100000@ireland.linux-hw.com> from "Eric Lee Green" at Oct 13, 98 04:24:20 pm
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Eric Lee Green wrote:
>
> On Tue, 13 Oct 1998, Jeff Hoffmann wrote:
> > >I agree completely, LIMIT would be VERY usefull in web based apps, which
> > >is all I run.  It does not matter to me if it is not part of a formal
> > >standard.  The idea is so common that it is a defacto standard.
> >
> > i'm not familiar with mysql and using "LIMIT" but wouldn't this same effect
> > be achieved by declaring a cursor and fetching however many records in the
> > cursor?  it's a very noticeable improvement when you only want the first 20
> > out of 500 in a 200k record database, at least.
>
> The problem with declaring a cursor vs. the "LIMIT" clause is that the
> "LIMIT" clause, if used properly by the database engine (along with the
> database engine using indexes in "ORDER BY" clauses) allows the database
> engine to short-circuit the tail end of the query. That is, if you have 25
> names and the last one ends with BEAVIS, the database engine doesn't have
> to go through the BUTTHEADS and KENNYs and etc.
>
> Theoretically a cursor is superior to the "LIMIT" clause because you're
> eventually going to want the B's and K's and etc. anyhow -- but only in a
> stateful enviornment. In the stateless web environment, a cursor is
> useless because the connection can close at any time even when you're
> using "persistent" connections (and of course when the connection closes
> the cursor closes).

    I'm  missing something. Well it's right that in the stateless
    web environment a cursor has to be declared  and  closed  for
    any  single  CGI  call.  But even if you have a LIMIT clause,
    your CGI must know with which key to start.

    So your query must look like

        SELECT ... WHERE key > 'last processed key' ORDER BY key;

    And your key must be unique (or at least contain no duplicate
    entries)  or you might miss some rows between the pages (have
    100 Brown's in the table and last processed key was  a  Brown
    while using LIMIT).

    In  postgres you could actually do the following (but read on
    below - it's not optimized correct)

        BEGIN;
        DECLARE c CURSOR FOR SELECT ... WHERE key > 'last' ORDER BY key;
        FETCH 20 IN c;
        (process the 20 rows in CGI)
        CLOSE c;
        COMMIT;

    Having LIMIT looks more elegant and has less overhead in CGI-
    backend   communication.   But  the  cursor  version  is  SQL
    standard and portable.

>
> I wanted very badly to use PostgreSQL for a web project I'm working on,
> but it just wouldn't do the job :-(.

    I've done some tests and what I found out might be a  bug  in
    PostgreSQL's  query  optimizer.  Having a table with 25k rows
    where key is a text field with a unique  index.  Now  I  used
    EXPLAIN for some queries

        SELECT * FROM tab;

        results in a seqscan - expected.

        SELECT * FROM tab ORDER BY key;

        results in a sort->seqscan - I would have
        expected an indexscan!

        SELECT * FROM tab WHERE key > 'G';

        results in an indexscan - expected.

        SELECT * FROM tab WHERE key > 'G' ORDER BY key;

        results in a sort->indexscan - hmmm.

    These  results  stay  the same even if I blow up the table by
    duplicating all rows (now with a non-unique  index)  to  100k
    rows and have them presorted in the table.

    Needless to say that everything is vacuum'd for statistics.

    The  last  one  is  the  query  we  would  need  in  the  web
    environment used over a cursor as in the example  above.  But
    due  to  the  sort,  the backend selects until the end of the
    table, sorts them and then returns only  the  first  20  rows
    (out of sorts result).

    This  is very painful if the qualification (key > ...) points
    to the beginning of the key list.

    Looking at planner.c I can see, that if there is a sortClause
    in  the  parsetree,  the planner creates a sort node and does
    absolutely not check if there is an index that could be  used
    to  do  it.  In  the  examples  above, the sort is absolutely
    needless because the  index  scan  will  already  return  the
    tuples in the right order :-).

    Somewhere  deep  in my brain I found a statement that sorting
    sorted  data  isn't  only  unnecessary  (except   the   order
    changes),  it  is  slow too compared against sorting randomly
    ordered data.

    Can we fix this before 6.4 release, will it be a past 6.4  or
    am I doing something wrong here? I think it isn't a fix (it's
    a planner enhancement) so it should  really  be  a  past  6.4
    item.

    For  now, the only possibility is to omit the ORDER BY in the
    query and hope the planner will always generate an index scan
    (because  of  the  qualification  'key  >  ...').  Doing so I
    selected multiple times 20 rows (with the last key qual  like
    a  CGI  would do) in separate transactions.  Using cursor and
    fetch speeds up the access by a factor of 1000!   But  it  is
    unsafe  and thus NOT RECOMMENDED! It's only a test if cursors
    can do the LIMIT job - and they could if the planner would do
    a better job.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Wed Oct 14 11:02:04 1998
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From: Oleg Bartunov <oleg@sai.msu.su>
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Reply-To: Oleg Bartunov <oleg@sai.msu.su>
To: hackers@postgreSQL.org
cc: t-ishii@sra.co.jp
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <m0zTOnx-000EBRC@orion.SAPserv.Hamburg.dsh.de>
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On Wed, 14 Oct 1998, Jan Wieck wrote:

> Date: Wed, 14 Oct 1998 13:09:21 +0200 (MET DST)
> From: Jan Wieck <jwieck@debis.com>
> To: Eric Lee Green <eric@linux-hw.com>
> Cc: jeff@remapcorp.com, hackers@postgreSQL.org
> Subject: Re: [HACKERS] What about LIMIT in SELECT ?
> 
> Eric Lee Green wrote:
> >
> > On Tue, 13 Oct 1998, Jeff Hoffmann wrote:
> > > >I agree completely, LIMIT would be VERY usefull in web based apps, which
> > > >is all I run.  It does not matter to me if it is not part of a formal
> > > >standard.  The idea is so common that it is a defacto standard.
> > >
> > > i'm not familiar with mysql and using "LIMIT" but wouldn't this same effect
> > > be achieved by declaring a cursor and fetching however many records in the
> > > cursor?  it's a very noticeable improvement when you only want the first 20
> > > out of 500 in a 200k record database, at least.
> >
> > The problem with declaring a cursor vs. the "LIMIT" clause is that the
> > "LIMIT" clause, if used properly by the database engine (along with the
> > database engine using indexes in "ORDER BY" clauses) allows the database
> > engine to short-circuit the tail end of the query. That is, if you have 25
> > names and the last one ends with BEAVIS, the database engine doesn't have
> > to go through the BUTTHEADS and KENNYs and etc.
> >
> > Theoretically a cursor is superior to the "LIMIT" clause because you're
> > eventually going to want the B's and K's and etc. anyhow -- but only in a
> > stateful enviornment. In the stateless web environment, a cursor is
> > useless because the connection can close at any time even when you're
> > using "persistent" connections (and of course when the connection closes
> > the cursor closes).
> 
>     I'm  missing something. Well it's right that in the stateless
>     web environment a cursor has to be declared  and  closed  for
>     any  single  CGI  call.  But even if you have a LIMIT clause,
>     your CGI must know with which key to start.
> 
      This is not a problem for CGI-script to know which key to start.
      Without LIMIT every CGI call backend will do *FULL* selection
      and cursor helps just in fetching a definite number of rows,
      in principle I can do this with CGI-script. Also, cursor
      returns data back in  ASCII  format (man l declare) and this requires
      additional job for backend to convert data from intrinsic (binary)
      format. Right implementation of LIMIT offset,number_of_rows could be
      a great win and make postgres superior free database engine for
      Web applications. Many colleagues of mine used mysql instead of
      postgres just because of lacking LIMIT. Tatsuo posted a patch
      for set query_limit to 'num', I just tested it and seems it
      works fine. Now, we need only possibility to specify offset,
      say 
         set query_limit to 'offset,num'
      ( Tatsuo, How difficult to do this ?)
      and LIMIT problem will ne gone.

      I'm wonder how many useful patches could be hidden from people  :-),
      
	Regards,

		Oleg

PS.

	Tatsuo, do you have patch for 6.3.2 ?
        I can't wait for 6.4 :-)
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83





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From: "Thomas G. Lockhart" <lockhart@alumni.caltech.edu>
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To: Jan Wieck <jwieck@debis.com>
CC: Eric Lee Green <eric@linux-hw.com>, jeff@remapcorp.com,
        hackers@postgreSQL.org
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
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>     I've done some tests and what I found out might be a  bug  in
>     PostgreSQL's  query  optimizer.
>         SELECT * FROM tab ORDER BY key;
>         results in a sort->seqscan - I would have
>         expected an indexscan!

Given that a table _could_ be completely unsorted on disk, it is
probably reasonable to suck the data in for a possible in-memory sort
rather than skipping around the disk to pick up individual tuples via
the index. Don't know if vacuum has a statistic on "orderness"...

>         SELECT * FROM tab WHERE key > 'G' ORDER BY key;
>         results in a sort->indexscan - hmmm.
>     The  last  one  is  the  query  we  would  need  in  the  web
>     environment used over a cursor as in the example  above.  But
>     due  to  the  sort,  the backend selects until the end of the
>     table, sorts them and then returns only  the  first  20  rows
>     (out of sorts result).

So you are saying that for this last case the sort was unnecessary? Does
the backend traverse the index in the correct order to guarantee that
the tuples are coming out already sorted? Does a hash index give the
same plan (I would expect a sort->seqscan for a hash index)?

                      - Tom


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: oleg@sai.msu.su
Date: Wed, 14 Oct 1998 16:24:56 +0200 (MET DST)
Cc: hackers@postgreSQL.org, t-ishii@sra.co.jp
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <Pine.GSO.3.96.SK.981014163020.10948B-100000@ra> from "Oleg Bartunov" at Oct 14, 98 04:53:53 pm
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Oleg Bartunov wrote:
>       This is not a problem for CGI-script to know which key to start.

    Never  meant that would be a problem. A FORM variable will of
    course do this.

>       Without LIMIT every CGI call backend will do *FULL* selection
>       and cursor helps just in fetching a definite number of rows,
>       in principle I can do this with CGI-script. Also, cursor
>       returns data back in  ASCII  format (man l declare) and this requires
>       additional job for backend to convert data from intrinsic (binary)
>       format. Right implementation of LIMIT offset,number_of_rows could be
>       a great win and make postgres superior free database engine for
>       Web applications. Many colleagues of mine used mysql instead of

    That's the point I was missing. The offset!

>       postgres just because of lacking LIMIT. Tatsuo posted a patch
>       for set query_limit to 'num', I just tested it and seems it
>       works fine. Now, we need only possibility to specify offset,
>       say
>          set query_limit to 'offset,num'
>       ( Tatsuo, How difficult to do this ?)
>       and LIMIT problem will ne gone.

    Think you haven't read my posting completely. Even  with  the
    executor  limit,  the  complete scan into the sort is done by
    the backend.  You need to specify ORDER BY to  get  the  same
    list  again  (without  the  offset  doesn't  make sense). But
    currently, ORDER BY forces a sort node into the query plan.

    What the executor limit  tells  is  how  many  rows  will  be
    returned  from  the sorted data. Not what goes into the sort.
    Filling the sort and sorting the data consumes the most  time
    of the queries execution.

    I  haven't  looked  at  Tatsuo's  patch  very well. But if it
    limits the amount of data going into the sort (on ORDER  BY),
    it  will  break it! The requested ordering could be different
    from what the choosen index might return. The used  index  is
    choosen by the planner upon the qualifications given, not the
    ordering wanted.

    So if you select WHERE b = 1 ORDER BY a, then it will use  an
    index on attribute b to match the qualification. The complete
    result of that index scan goes into the sort to  get  ordered
    by  a. If now the executor limit stops sort filling after the
    limit is exceeded, only the same tuples will go into the sort
    every  time.  But  they have nothing to do with the requested
    order by a.

    What LIMIT first needs is  a  planner  enhancement.  In  file
    backend/optimizer/plan/planner.c  line 284 it must be checked
    if the actual plan is an indexscan, if the indexed attributes
    are  all  the same as those in the given sort clause and that
    the requested sort order (operator) is that  what  the  index
    will  return.   If  that  all matches, it can ignore the sort
    clause and return the index scan itself.

    Second enhancement must be the handling of  the  offset.   In
    the  executor,  the  index scan must skip offset index tuples
    before returning the first. But  NOT  if  the  plan  isn't  a
    1-table-index-scan.  In that case the result tuples (from the
    topmost unique/join/whatever node) have to be skipped.

    With these enhancements,  the  index  tuples  to  be  skipped
    (offset)  will still be scanned, but not the data tuples they
    point to. Index scanning might be somewhat faster.

    This all will only speedup simple 1-table-queries,  no  joins
    or  if  the requested order isn't that what the index exactly
    returns.

    Anyway, I'll take a look if I can change the planner to  omit
    the  sort  if  the tests described above are true. I think it
    would be good anyway.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: lockhart@alumni.caltech.edu (Thomas G. Lockhart)
Date: Wed, 14 Oct 1998 16:34:47 +0200 (MET DST)
Cc: jwieck@debis.com, eric@linux-hw.com, jeff@remapcorp.com,
        hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <3624AE5C.752E4E7F@alumni.caltech.edu> from "Thomas G. Lockhart" at Oct 14, 98 01:59:56 pm
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>
> >         SELECT * FROM tab WHERE key > 'G' ORDER BY key;
> >         results in a sort->indexscan - hmmm.
> >     The  last  one  is  the  query  we  would  need  in  the  web
> >     environment used over a cursor as in the example  above.  But
> >     due  to  the  sort,  the backend selects until the end of the
> >     table, sorts them and then returns only  the  first  20  rows
> >     (out of sorts result).
>
> So you are saying that for this last case the sort was unnecessary? Does
> the backend traverse the index in the correct order to guarantee that
> the tuples are coming out already sorted? Does a hash index give the
> same plan (I would expect a sort->seqscan for a hash index)?

    Good  point!  As  far as I can see, the planner chooses index
    usage only depending on the WHERE clause.  A  hash  index  is
    only  usable  when  the  given  qualification  uses  = on the
    indexed attribute(s).

    If the sortClause exactly matches the indexed  attributes  of
    the  ONE used btree index and all operators request ascending
    order I think the index  scan  already  returns  the  correct
    order. Who know's definitely?

    Addition  to  my  last  posting: ... and if the index scan is
    using a btree index ...


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Wed Oct 14 13:55:58 1998
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From: Bruce Momjian <maillist@candle.pha.pa.us>
Message-Id: <199810141721.NAA28746@candle.pha.pa.us>
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <3624AE5C.752E4E7F@alumni.caltech.edu> from "Thomas G. Lockhart" at "Oct 14, 1998  1:59:56 pm"
To: lockhart@alumni.caltech.edu (Thomas G. Lockhart)
Date: Wed, 14 Oct 1998 13:21:15 -0400 (EDT)
Cc: jwieck@debis.com, eric@linux-hw.com, jeff@remapcorp.com,
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> >     I've done some tests and what I found out might be a  bug  in
> >     PostgreSQL's  query  optimizer.
> >         SELECT * FROM tab ORDER BY key;
> >         results in a sort->seqscan - I would have
> >         expected an indexscan!
> 
> Given that a table _could_ be completely unsorted on disk, it is
> probably reasonable to suck the data in for a possible in-memory sort
> rather than skipping around the disk to pick up individual tuples via
> the index. Don't know if vacuum has a statistic on "orderness"...

Thomas is correct on this.  Vadim has run some tests, and with our
optimized psort() code, the in-memory sort is often faster than using
the index to get the tuple, because you are jumping all over the drive. 
I don't remember, but obviously there is a break-even point where
getting X rows using the index on a table of Y rows is faster , but
getting X+1 rows on a table of Y rows is faster getting all the rows
sequentailly, and doing the sort.

You would have to pick only certain queries(no joins, index matches
ORDER BY), take the number of rows requested, and the number of rows
selected, and figure out if it is faster to use the index, or a
sequential scan and do the ORDER BY yourself.


Add to this the OFFSET capability.  I am not sure how you are going to
get into the index and start at the n-th entry, unless perhaps you just
sequential scan the index.

In fact, many queries just get column already indexed, and we could just
pull the data right out of the index.

I have added this to the TODO list:

	* Pull requested data directly from indexes, bypassing heap data        

I think this has to be post-6.4 work, but I think we need to work in
this direction.  I am holding off any cnfify fixes for post-6.4, but a
6.4.1 performance release certainly is possible.


But, you are correct that certain cases where in index is already being
used on a query, you could just skip the sort IF you used the index to
get the rows from the base table.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist@candle.pha.pa.us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026


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From: Terry Mackintosh <terry@terrym.com>
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To: Jeff Hoffmann <jeff@remapcorp.com>
cc: PostgreSQL-development <hackers@postgreSQL.org>
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
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On Tue, 13 Oct 1998, Jeff Hoffmann wrote:

> >Hi, my 2 cents...
> >
> >I agree completely, LIMIT would be VERY usefull in web based apps, which
> >is all I run.  It does not matter to me if it is not part of a formal
> >standard.  The idea is so common that it is a defacto standard.
> 
> i'm not familiar with mysql and using "LIMIT" but wouldn't this same effect
> be achieved by declaring a cursor and fetching however many records in the
> cursor?  it's a very noticeable improvement when you only want the first 20
> out of 500 in a 200k record database, at least.

Yes, while this is an improvement, it still has to do the entire query,
would be nice if the query could be terminated after a designated number
of rows where found, thus freeing system resources that are other wise
consumed.  
I have seen web users run ridculous querys, like search for the
letter 'a', and it happens to be a substring search, now the box go'es ape
shit for 5 or 10 min.s while it basically gets the whole db as the search
result.  All this befor you can do a 'FETCH', as I understand FETCH, I
will need to read up on it.

Note that I do not have any databases that larg on my box, I was thinking
back to my VFP/NT experiances.

Have a great day
Terry Mackintosh <terry@terrym.com>          http://www.terrym.com
sysadmin/owner  Please! No MIME encoded or HTML mail, unless needed.

Proudly powered by R H Linux 4.2, Apache 1.3, PHP 3, PostgreSQL 6.3
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From owner-pgsql-hackers@hub.org Wed Oct 14 13:59:05 1998
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From: Terry Mackintosh <terry@terrym.com>
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Subject: Re: [HACKERS] What about LIMIT in SELECT ?
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On Tue, 13 Oct 1998, Bruce Momjian wrote:

> What we could do is _if_ there is only one table(no joins), and an index
> exists that matches the ORDER BY, we could use the index to
> short-circuit the query.
> 
> I have added this item to the TODO list:
> 
> * Allow LIMIT ability on single-table queries that have no ORDER BY or
>         a matching index
>        
> This looks do-able, and a real win.  Would this make web applications
> happier?  If there is an ORDER BY and no index, or a join, I can't
> figure out how we would short-circuit the query.
> 
Yes, this would do for most of my apps.
It may just be my lack of sophistication, but I find that most web apps
are very simple in nature/table layout, and thus queries are often on only
a single table.

Thanks
Terry Mackintosh <terry@terrym.com>          http://www.terrym.com
sysadmin/owner  Please! No MIME encoded or HTML mail, unless needed.

Proudly powered by R H Linux 4.2, Apache 1.3, PHP 3, PostgreSQL 6.3
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From wieck@sapserv.debis.de Wed Oct 14 13:55:53 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Wed, 14 Oct 1998 19:57:27 +0200 (MET DST)
Cc: lockhart@alumni.caltech.edu, jwieck@debis.com, eric@linux-hw.com,
        jeff@remapcorp.com, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810141721.NAA28746@candle.pha.pa.us> from "Bruce Momjian" at Oct 14, 98 01:21:15 pm
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> But, you are correct that certain cases where in index is already being
> used on a query, you could just skip the sort IF you used the index to
> get the rows from the base table.

    Especially in the case where

        SELECT ... WHERE key > 'val' ORDER BY key;

    creates  a Sort->IndexScan plan. The index scan already jumps
    around on the disc to collect the sorts input  and  the  sort
    finally returns exactly the same output (if the used index is
    only on key).

    And this is the case for  large  tables.  The  planner  first
    decides  to  use  an  index  scan due to the WHERE clause and
    later it notices the ORDER BY clause and creates a sort  over
    the scan.

    I'm  actually  hacking  around on it to see what happens if I
    suppress the sort node in some cases.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #



From owner-pgsql-hackers@hub.org Wed Oct 14 16:31:07 1998
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From: Bruce Momjian <maillist@candle.pha.pa.us>
Message-Id: <199810141827.OAA29639@candle.pha.pa.us>
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <199810141721.NAA28746@candle.pha.pa.us> from Bruce Momjian at "Oct 14, 1998  1:21:15 pm"
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Wed, 14 Oct 1998 14:27:05 -0400 (EDT)
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        jeff@remapcorp.com, hackers@postgreSQL.org
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> Thomas is correct on this.  Vadim has run some tests, and with our
> optimized psort() code, the in-memory sort is often faster than using
> the index to get the tuple, because you are jumping all over the drive. 
> I don't remember, but obviously there is a break-even point where
> getting X rows using the index on a table of Y rows is faster , but
> getting X+1 rows on a table of Y rows is faster getting all the rows
> sequentailly, and doing the sort.
> 
> You would have to pick only certain queries(no joins, index matches
> ORDER BY), take the number of rows requested, and the number of rows
> selected, and figure out if it is faster to use the index, or a
> sequential scan and do the ORDER BY yourself.
> 
> Add to this the OFFSET capability.  I am not sure how you are going to
> get into the index and start at the n-th entry, unless perhaps you just
> sequential scan the index.
> 
> In fact, many queries just get column already indexed, and we could just
> pull the data right out of the index.
> 
> I have added this to the TODO list:
> 
> 	* Pull requested data directly from indexes, bypassing heap data        
> 
> I think this has to be post-6.4 work, but I think we need to work in
> this direction.  I am holding off any cnfify fixes for post-6.4, but a
> 6.4.1 performance release certainly is possible.
> 
> 
> But, you are correct that certain cases where in index is already being
> used on a query, you could just skip the sort IF you used the index to
> get the rows from the base table.

I have had more time to think about this.  Basically, for pre-sorted
data, our psort code is very fast, because it does not need to sort
anything.  It just moves the rows in and out of the sort memory.  Yes,
it could be removed in some cases, and probably should be, but it is not
going to produce great speedups.

The more general case I will describe below.

First, let's look at a normal query:

	SELECT *
	FROM tab
	ORDER BY col1

This is not going to use an index, and probably should not because it is
faster for large tables to sort them in memory, rather than moving all
over the disk.  For small tables, if the entire table fits in the buffer
cache, it may be faster to use the index, but on a small table the sort
doesn't take very long either, and the buffer cache effectiveness is
affected by other backends using it, so it may be better not to count on
it for a speedup.

However, if you only want the first 10 rows, that is a different story. 
We pull all the rows into the backend, sort them, then return 10 rows. 
The query, if we could do it, should be written as:

	SELECT *
	FROM tab
	WHERE col1 < some_unknown_value
	ORDER BY col1

In this case, the optimizer looks at the column statistics, and properly
uses an index to pull only a small subset of the table.  This is the
type of behavior people want for queries returning only a few values.

But, unfortunately, we don't know that mystery value.

Now, everyone agrees we need an index matching the ORDER BY to make this
query quick, but we don't know that mystery value, so currently we
execute the whole query, and do a fetch.

What I am now thinking is that maybe we need a way to walk around that
index.  Someone months ago asked how to do that, and we told him he
couldn't, because this not a C-ISAM/dbm type database.  However, if we
could somehow pass into the query the index location we want to start
at, and how many rows we need, that would solve our problem, and perhaps
even allow joined queries to work, assuming the table in the ORDER BY is
in an outer join loop.

	SELECT *
	FROM tab
	WHERE col1 < some_unknown_value
	ORDER BY col1
	USING INDEX tab_idx(452) COUNT 100

where 452 is an 452th index entry, and COUNT is the number of index rows
you want to process.  The query may return more or less than 100 rows if
there is a join and it joins to zero or more than one row in the joined
table, but this seems like perhaps a good way to go at it.  We need to
do it this way because if a single index row returns 4 result rows, and
only two of the four rows fit in the number of rows returnd as set by the
user, it is hard to re-start the query at the proper point, because you
would have to process the index rows a second time, and return just part
of the result, and that is hard.

If the index changes, or rows are added, the results are going to be
unreliable, but that is probably going to be true of any state-less
implementation we can devise.

I think this may be fairly easy to implement.  We could sequential scan
the index to get to the 452th row.  That is going to be quick.  We can
pass the 452 into the btree index code, so only a certain range of index
tuples are returned, and the system believes it has processed the entire
query, while we know it hasn't.  Doesn't really work with hash, so we
will not allow it for those indexes. 

To make it really easy, we could implement it as a 'SET' command, so we
don't actually have it as part of the query, and have to pass it around
through all the modules.  You would do the proper 'SET' before running
the query.  Optimizer would look at 'SET' value to force index use.

	SET INDEX TO tab_idx START 452 COUNT 100

or

	SET INDEX TO tab_idx FROM 452 COUNT 451

There would have to be some way to signal that the end of the index had
been reached, because returning zero rows is not enough of a guarantee
in a joined SELECT.

Comments?

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist@candle.pha.pa.us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Wed, 14 Oct 1998 23:05:07 +0200 (MET DST)
Cc: maillist@candle.pha.pa.us, lockhart@alumni.caltech.edu, jwieck@debis.com,
        eric@linux-hw.com, jeff@remapcorp.com, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810141827.OAA29639@candle.pha.pa.us> from "Bruce Momjian" at Oct 14, 98 02:27:05 pm
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> I have had more time to think about this.  Basically, for pre-sorted
> data, our psort code is very fast, because it does not need to sort
> anything.  It just moves the rows in and out of the sort memory.  Yes,
> it could be removed in some cases, and probably should be, but it is not
> going to produce great speedups.

    And I got the time to hack around about this.

    I  hacked  in  a little check into the planner, that compares
    the sortClause against the key field list of  an  index  scan
    and  just  suppresses  the sort node if it exactly matchs and
    all sort operators are "<".

    I tested with a 10k row table where key is a text field.  The
    base query is a

        SELECT ... WHERE key > 'val' ORDER BY key;

    The  used 'val' is always a key that is close to the first of
    all keys in the table ('' on the first  query  and  the  last
    selected value on subsequent ones).

    Scenario  1  (S1)  uses exactly the above query but processes
    only the first 20 rows  from  the  result  buffer.  Thus  the
    frontend receives nearly the whole table.

    Scenario  2  (S2)  uses a cursor and FETCH 20. But closes the
    cursor and creates a new one for  the  next  selection  (only
    with another 'val') as it would occur in a web application.

    If  there  is  no index on key, the backend will allways do a
    Sort->SeqScan and due  to  the  'val'  close  to  the  lowest
    existing  key  nearly all tuples get scanned and put into the
    sort. S1 here runs about 10 seconds and S2 about  6  seconds.
    The  speedup  in  S2  results  from  the  reduced overhead of
    sending not wanted tuples into the frontend.

    Now with a btree index  on  key  and  an  unpatched  backend.
    Produced  plan  is  always  a  Sort->IndexScan.   S1 needs 16
    seconds and S2 needs 12 seconds. Again nearly all data is put
    into  the  sort but this time over the index scan and that is
    slower.

    Last with the btree index on key  and  the  patched  backend.
    This  time the plan is a plain IndexScan because the ORDER BY
    clause exactly matches the sort order of the  choosen  index.
    S1  needs  13  seconds  and  S2 less than 0.2!  This dramatic
    speedup comes from the fact, that this time the index scan is
    the  toplevel  executor  node and the executor run is stopped
    after 20 tuples have been selected.

    Analysis of the above timings:

    If there is an ORDER BY clause, using an index  scan  is  the
    clever  way  if  the  indexqual  dramatically reduces the the
    amount of data selected and sorted.   I  think  this  is  the
    normal case (who really selects nearly all rows from a 5M row
    table?). So choosing the index path  is  correct.  This  will
    hurt if someone really selects most of the rows and the index
    scan jumps over the disc.  But here the programmer should use
    an  unqualified  query  to  perform  a  seqscan  and  do  the
    qualification in the frontend application.

    The speedup for the cursor/fetch scenario  is  so  impressive
    that  I'll  create  a  post 6.4 patch. I don't want it in 6.4
    because there is absolutely no query in the whole  regression
    test,  where  it  suppresses  the  sort  node.   So  we  have
    absolutely no check that it doesn't break anything.

    For a web application, that can use a unique  key  to  select
    the next amount of rows, it will be a big win.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Thu Oct 15 00:01:10 1998
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Message-Id: <199810150234.LAA08260@srapc451.sra.co.jp>
To: jwieck@debis.com (Jan Wieck)
cc: oleg@sai.msu.su, hackers@postgreSQL.org, t-ishii@sra.co.jp
Subject: Re: [HACKERS] What about LIMIT in SELECT ? 
From: Tatsuo Ishii <t-ishii@sra.co.jp>
Reply-To: t-ishii@sra.co.jp
In-reply-to: Your message of Wed, 14 Oct 1998 16:24:56 +0200.
             <m0zTRrE-000EBRC@orion.SAPserv.Hamburg.dsh.de> 
Date: Thu, 15 Oct 1998 11:34:54 +0900
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>>       postgres just because of lacking LIMIT. Tatsuo posted a patch
>>       for set query_limit to 'num', I just tested it and seems it
>>       works fine. Now, we need only possibility to specify offset,
>>       say
>>          set query_limit to 'offset,num'
>>       ( Tatsuo, How difficult to do this ?)
>>       and LIMIT problem will ne gone.
>
>    Think you haven't read my posting completely. Even  with  the
>    executor  limit,  the  complete scan into the sort is done by
>    the backend.  You need to specify ORDER BY to  get  the  same
>    list  again  (without  the  offset  doesn't  make sense). But
>    currently, ORDER BY forces a sort node into the query plan.

I think we have understanded your point. set query_limit is just a
easy alternative of using cursor and fetch.

>    I  haven't  looked  at  Tatsuo's  patch  very well. But if it
>    limits the amount of data going into the sort (on ORDER  BY),
>    it  will  break it! The requested ordering could be different
>    from what the choosen index might return. The used  index  is
>    choosen by the planner upon the qualifications given, not the
>    ordering wanted.

I think it limits the final result. When query_limit is set,
the arg "numberTuples" of ExecutePlan() is set to it instead of 0
(this means no limit).

Talking about "offset," it shouldn't be very difficult. I guess all we
have to do is adding a new arg "offset" to ExecutePlan() then making
obvious modifications. (and of course we have to modify set
query_limit syntax but it's trivial)

However, before going ahead, I would like to ask other hackers about
this direction. This might be convenient for some users, but still the 
essential performance issue would remain. In another word, this is a
short-term solution not a intrinsic one, IMHO.
--
Tatsuo Ishii
t-ishii@sra.co.jp


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: t-ishii@sra.co.jp
Date: Thu, 15 Oct 1998 14:23:43 +0200 (MET DST)
Cc: jwieck@debis.com, oleg@sai.msu.su, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810150234.LAA08260@srapc451.sra.co.jp> from "Tatsuo Ishii" at Oct 15, 98 11:34:54 am
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Tatsuo Ishii wrote:

> I think we have understanded your point. set query_limit is just a
> easy alternative of using cursor and fetch.
>
> >    I  haven't  looked  at  Tatsuo's  patch  very well. But if it
> >    limits the amount of data going into the sort (on ORDER  BY),
> >    it  will  break it! The requested ordering could be different
> >    from what the choosen index might return. The used  index  is
> >    choosen by the planner upon the qualifications given, not the
> >    ordering wanted.
>
> I think it limits the final result. When query_limit is set,
> the arg "numberTuples" of ExecutePlan() is set to it instead of 0
> (this means no limit).
>
> Talking about "offset," it shouldn't be very difficult. I guess all we
> have to do is adding a new arg "offset" to ExecutePlan() then making
> obvious modifications. (and of course we have to modify set
> query_limit syntax but it's trivial)

    The offset could become

        FETCH n IN cursor [OFFSET n];

    and

        SELECT ... [LIMIT offset,count];

    The  FETCH command already calls ExecutorRun() with the given
    count (the tuple limit). Telling it the offset too is  really
    simple.   And  ExecutorRun()  could  check  if  the  toplevel
    executor node is an index scan. Skipping  tuples  during  the
    index  scan  requires,  that  all  qualifications  are in the
    indexqual, thus any tuple returned by it will become a  final
    result  row  (as it would be in the simple 1-table-queries we
    discussed).  If  that  isn't  the  case,  the  executor  must
    fallback to skip the final result tuples and that is after an
    eventually processed sort/merge of the complete  result  set.
    That would only reduce communication to the client and memory
    required there to buffer the result  set  (not  a  bad  thing
    either).

    ProcessQueryDesc()  in tcop/pquery.c also calls ExecutorRun()
    but with a constant 0 tuple count. Having offset and count in
    the  parsetree  would  make it without any state variables or
    SET command. And it's the only clean way to restrict LIMIT to
    SELECT  queries.  Any  thrown  in LIMIT to ExecutorRun() from
    another place could badly hurt the rewrite  system.  Remember
    that   non-instead   actions   on   insert/update/delete  are
    processed before the  original  query!  And  what  about  SQL
    functions that get processed during the evaluation of another
    query (view using an SQL function for count(*))?

    A little better would it be to make the LIMIT values able  to
    be  parameter  nodes. C or PL functions use the prepared plan
    feature  of  the  SPI  manager   for   performance   reasons.
    Especially  the  offset  value  might  there  need  to  be  a
    parameter that the executor has to pick  out  first.   If  we
    change  the  count  argument of ExecutorRun to a List *limit,
    this one could be NIL (to mean  the  old  0  count  0  offset
    behaviour)  or a list of two elements that both can be either
    a Const or a Param of type int4.  Easy for  the  executor  to
    evaluate.

    The   only   places   where   ExecutorRun()   is  called  are
    tcop/pquery.c  (queries  from  frontend),  commands/command.c
    (FETCH  command),  executor/functions.c  (SQL  functions) and
    executor/spi.c (SPI manager). So it is  easy  to  change  the
    call interface too.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Thu Oct 15 14:32:34 1998
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Date: Thu, 15 Oct 1998 20:01:23 +0400 (MSD)
From: Oleg Bartunov <oleg@sai.msu.su>
X-Sender: megera@ra
To: Jan Wieck <jwieck@debis.com>
cc: t-ishii@sra.co.jp, hackers@postgreSQL.org
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <m0zTmRT-000EBRC@orion.SAPserv.Hamburg.dsh.de>
Message-ID: <Pine.GSO.3.96.SK.981015193853.19322D-100000@ra>
Organization: Sternberg Astronomical Institute (Moscow University)
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This is a little bit off-topic,
I did some timings with latest cvs on my real database 
( all output redirected to /dev/null ), table contains 8798 records,
31 columns, order key have indices.

1.select count(*) from work_flats;
0.02user 0.00system 0:00.18elapsed 10%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (131major+21minor)pagefaults 0swaps

2.select * from work_flats order by rooms, metro_id;
2.35user 0.25system 0:10.11elapsed 25%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (131major+2799minor)pagefaults 0swaps

3.set query_limit to '150';
SET VARIABLE
select * from work_flats order by rooms, metro_id;
0.06user 0.00system 0:02.75elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (131major+67minor)pagefaults 0swaps

4.begin;
declare tt cursor  for
select * from work_flats order by rooms, metro_id;
fetch 150 in tt;
end;
0.05user 0.01system 0:02.76elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (131major+67minor)pagefaults 0swaps

As you can see timings for query_limit and cursor are very similar,
I didn't expected this. So, in principle, enhanced version of fetch
(with offset) would cover all we need from LIMIT, but query_limit would be
still useful, for example to restrict loadness of server.
Will all enhancements you discussed go to the 6.4 ?
I'm really interested in testing this stuff because I begin new project
and everything we discussed here are badly needed.


	Regards,

	 Oleg



On Thu, 15 Oct 1998, Jan Wieck wrote:

> Date: Thu, 15 Oct 1998 14:23:43 +0200 (MET DST)
> From: Jan Wieck <jwieck@debis.com>
> To: t-ishii@sra.co.jp
> Cc: jwieck@debis.com, oleg@sai.msu.su, hackers@postgreSQL.org
> Subject: Re: [HACKERS] What about LIMIT in SELECT ?
> 
> Tatsuo Ishii wrote:
> 
> > I think we have understanded your point. set query_limit is just a
> > easy alternative of using cursor and fetch.
> >
> > >    I  haven't  looked  at  Tatsuo's  patch  very well. But if it
> > >    limits the amount of data going into the sort (on ORDER  BY),
> > >    it  will  break it! The requested ordering could be different
> > >    from what the choosen index might return. The used  index  is
> > >    choosen by the planner upon the qualifications given, not the
> > >    ordering wanted.
> >
> > I think it limits the final result. When query_limit is set,
> > the arg "numberTuples" of ExecutePlan() is set to it instead of 0
> > (this means no limit).
> >
> > Talking about "offset," it shouldn't be very difficult. I guess all we
> > have to do is adding a new arg "offset" to ExecutePlan() then making
> > obvious modifications. (and of course we have to modify set
> > query_limit syntax but it's trivial)
> 
>     The offset could become
> 
>         FETCH n IN cursor [OFFSET n];
> 
>     and
> 
>         SELECT ... [LIMIT offset,count];
> 
>     The  FETCH command already calls ExecutorRun() with the given
>     count (the tuple limit). Telling it the offset too is  really
>     simple.   And  ExecutorRun()  could  check  if  the  toplevel
>     executor node is an index scan. Skipping  tuples  during  the
>     index  scan  requires,  that  all  qualifications  are in the
>     indexqual, thus any tuple returned by it will become a  final
>     result  row  (as it would be in the simple 1-table-queries we
>     discussed).  If  that  isn't  the  case,  the  executor  must
>     fallback to skip the final result tuples and that is after an
>     eventually processed sort/merge of the complete  result  set.
>     That would only reduce communication to the client and memory
>     required there to buffer the result  set  (not  a  bad  thing
>     either).
> 
>     ProcessQueryDesc()  in tcop/pquery.c also calls ExecutorRun()
>     but with a constant 0 tuple count. Having offset and count in
>     the  parsetree  would  make it without any state variables or
>     SET command. And it's the only clean way to restrict LIMIT to
>     SELECT  queries.  Any  thrown  in LIMIT to ExecutorRun() from
>     another place could badly hurt the rewrite  system.  Remember
>     that   non-instead   actions   on   insert/update/delete  are
>     processed before the  original  query!  And  what  about  SQL
>     functions that get processed during the evaluation of another
>     query (view using an SQL function for count(*))?
> 
>     A little better would it be to make the LIMIT values able  to
>     be  parameter  nodes. C or PL functions use the prepared plan
>     feature  of  the  SPI  manager   for   performance   reasons.
>     Especially  the  offset  value  might  there  need  to  be  a
>     parameter that the executor has to pick  out  first.   If  we
>     change  the  count  argument of ExecutorRun to a List *limit,
>     this one could be NIL (to mean  the  old  0  count  0  offset
>     behaviour)  or a list of two elements that both can be either
>     a Const or a Param of type int4.  Easy for  the  executor  to
>     evaluate.
> 
>     The   only   places   where   ExecutorRun()   is  called  are
>     tcop/pquery.c  (queries  from  frontend),  commands/command.c
>     (FETCH  command),  executor/functions.c  (SQL  functions) and
>     executor/spi.c (SPI manager). So it is  easy  to  change  the
>     call interface too.
> 
> 
> Jan
> 
> --
> 
> #======================================================================#
> # It's easier to get forgiveness for being wrong than for being right. #
> # Let's break this rule - forgive me.                                  #
> #======================================== jwieck@debis.com (Jan Wieck) #
> 
> 

_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83



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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: hannu@trust.ee (Hannu Krosing)
Date: Thu, 15 Oct 1998 19:01:33 +0200 (MET DST)
Cc: jwieck@debis.com, pgsql-hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <36261DF7.D20368A0@trust.ee> from "Hannu Krosing" at Oct 15, 98 07:08:23 pm
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Hannu Krosing wrote:

> Jan Wieck wrote:
> >    The speedup for the cursor/fetch scenario  is  so  impressive
> >    that  I'll  create  a  post 6.4 patch. I don't want it in 6.4
> >    because there is absolutely no query in the whole  regression
> >    test,  where  it  suppresses  the  sort  node.
>
> Good, then it works as expected ;)
>
> More seriously, it is not within powers of current regression test
> framework to test speed improvements (only the case where
> performance-wise bad implementation will actually crash the backend,
> as in the cnfify problem, but AFAIK we dont test for those now)
>
> >   So  we  have absolutely no check that it doesn't break anything.
>
> If it did pass the regression, then IMHO it did not break anything.

    Thats  the  point.  The  check  if  the sort node is required
    returns TRUE for  ALL  queries  of  the  regression.  So  the
    behaviour when it returns FALSE is absolutely not tested.

>
> I would vote for putting it in (maybe with a
> 'set fix_optimiser_stupidity on' safeguard to enable it). I see no
> reason to postpone it to 6.4.1 and force almost everybody to first
> patch their copy and then upgrade very soon.
>
> I would even go far enough to call it a bugfix, as it does not really
> introduce any new functionality only fixes some existing functionality
> so that much bigger databases can be actually used.

    I can't call it a bugfix because it is only a performance win
    in some situations. And I feel the risk is too  high  to  put
    untested  code  into  the  backend  at BETA2 time. The max we
    should do is to take this one  and  the  LIMIT  thing  (maybe
    implemented  as  I  suggested  lately),  and  put  out a Web-
    Performance-Release at the same time we release 6.4.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From Inoue@tpf.co.jp Thu Oct 15 20:31:01 1998
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Jan Wieck" <jwieck@debis.com>,
        "Bruce Momjian" <maillist@candle.pha.pa.us>
Subject: RE: [HACKERS] What about LIMIT in SELECT ?
Date: Fri, 16 Oct 1998 09:12:55 +0900
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> -----Original Message-----
> From: owner-pgsql-hackers@postgreSQL.org
> [mailto:owner-pgsql-hackers@postgreSQL.org]On Behalf Of Bruce Momjian
> Sent: Thursday, October 15, 1998 2:52 PM
> To: jwieck@debis.com
> Cc: lockhart@alumni.caltech.edu; jwieck@debis.com; eric@linux-hw.com;
> jeff@remapcorp.com; hackers@postgreSQL.org
> Subject: Re: [HACKERS] What about LIMIT in SELECT ?
>
>
> > > I have had more time to think about this.  Basically, for pre-sorted
> > > data, our psort code is very fast, because it does not need to sort
> > > anything.  It just moves the rows in and out of the sort memory.  Yes,
> > > it could be removed in some cases, and probably should be,
> but it is not
> > > going to produce great speedups.
> >
> >     And I got the time to hack around about this.
> >
> >     I  hacked  in  a little check into the planner, that compares
> >     the sortClause against the key field list of  an  index  scan
> >     and  just  suppresses  the sort node if it exactly matchs and
> >     all sort operators are "<".
> >
> >     I tested with a 10k row table where key is a text field.  The
> >     base query is a
> >
> >         SELECT ... WHERE key > 'val' ORDER BY key;
> >
> >     The  used 'val' is always a key that is close to the first of
> >     all keys in the table ('' on the first  query  and  the  last
> >     selected value on subsequent ones).
>
> This is good stuff.  I want to think about it for a day.  Sounds very
> promising.
>

Did you see my contribution about this subject ?
I have already implemented above cases and used on trial for three
months or more.
It is good to be formally supported by PostgreSQL community.

And please remember that there are descending order cases.
(Moreover there are compound cases such as
 SELECT * from ... order by key1 desc,key2 asc;
 I didn't implement such cases.)

Thanks.

Hiroshi Inoue
Inoue@tpf.co.jp


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From: Bruce Momjian <maillist@candle.pha.pa.us>
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Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <m0zTY6V-000EBRC@orion.SAPserv.Hamburg.dsh.de> from Jan Wieck at "Oct 14, 1998 11: 5: 7 pm"
To: jwieck@debis.com
Date: Fri, 16 Oct 1998 01:34:33 -0400 (EDT)
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OK, I have had my day of thinking, and will address this specific
posting first, because it is the most fundamental concerning the future
direction of the optimization.

> 
>     And I got the time to hack around about this.
> 
>     I  hacked  in  a little check into the planner, that compares
>     the sortClause against the key field list of  an  index  scan
>     and  just  suppresses  the sort node if it exactly matchs and
>     all sort operators are "<".
> 
>     I tested with a 10k row table where key is a text field.  The
>     base query is a
> 
>         SELECT ... WHERE key > 'val' ORDER BY key;
> 
>     The  used 'val' is always a key that is close to the first of
>     all keys in the table ('' on the first  query  and  the  last
>     selected value on subsequent ones).
> 
>     Scenario  1  (S1)  uses exactly the above query but processes
>     only the first 20 rows  from  the  result  buffer.  Thus  the
>     frontend receives nearly the whole table.

OK.

> 
>     Scenario  2  (S2)  uses a cursor and FETCH 20. But closes the
>     cursor and creates a new one for  the  next  selection  (only
>     with another 'val') as it would occur in a web application.
> 
>     If  there  is  no index on key, the backend will allways do a
>     Sort->SeqScan and due  to  the  'val'  close  to  the  lowest
>     existing  key  nearly all tuples get scanned and put into the
>     sort. S1 here runs about 10 seconds and S2 about  6  seconds.
>     The  speedup  in  S2  results  from  the  reduced overhead of
>     sending not wanted tuples into the frontend.

Makes sense.  All rows are processed, but not sent to client.

> 
>     Now with a btree index  on  key  and  an  unpatched  backend.
>     Produced  plan  is  always  a  Sort->IndexScan.   S1 needs 16
>     seconds and S2 needs 12 seconds. Again nearly all data is put
>     into  the  sort but this time over the index scan and that is
>     slower.

VACUUM ANALYZE could affect this.  Because it had no stats, it thought
index use would be faster, but in fact because 'val' was near the lowest
value, it as selecting 90% of the table, and would have been better with
a sequential scan.  pg_statistics's low/hi values for a column could
have told that to the optimizer.

I know the good part of the posting is coming.

>     Last with the btree index on key  and  the  patched  backend.
>     This  time the plan is a plain IndexScan because the ORDER BY
>     clause exactly matches the sort order of the  chosen  index.
>     S1  needs  13  seconds  and  S2 less than 0.2!  This dramatic
>     speedup comes from the fact, that this time the index scan is
>     the  toplevel  executor  node and the executor run is stopped
>     after 20 tuples have been selected.

OK, seems like in the S1 case, the use of the psort/ORDER BY code on top
of the index was taking and extra 3 seconds, which is 23%.  That is a
lot more than I thought for the psort code, and shows we could gain a
lot by removing unneeded sorts from queries that are already using
matching indexes.

Just for clarity, added to TODO.  I think everyone is clear on this one,
and its magnitude is a surprise to me:

  * Prevent psort() usage when query already using index matching ORDER BY


>     Analysis of the above timings:
> 
>     If there is an ORDER BY clause, using an index  scan  is  the
>     clever  way  if  the  indexqual  dramatically reduces the the
>     amount of data selected and sorted.   I  think  this  is  the
>     normal case (who really selects nearly all rows from a 5M row
>     table?). So choosing the index path  is  correct.  This  will
>     hurt if someone really selects most of the rows and the index
>     scan jumps over the disc.  But here the programmer should use
>     an  unqualified  query  to  perform  a  seqscan  and  do  the
>     qualification in the frontend application.

Fortunately, the optimizer already does the index selection for us, and
guesses pretty well if the index or sequential scan is better.  Once we
implement the above removal of psort(), we will have to change the
timings because now you have to compare index scan against sequential
scan AND psort(), because in the index scan situation, you don't need
the psort(), assuming the ORDER BY matches the index exactly.

>     The speedup for the cursor/fetch scenario  is  so  impressive
>     that  I'll  create  a  post 6.4 patch. I don't want it in 6.4
>     because there is absolutely no query in the whole  regression
>     test,  where  it  suppresses  the  sort  node.   So  we  have
>     absolutely no check that it doesn't break anything.
> 
>     For a web application, that can use a unique  key  to  select
>     the next amount of rows, it will be a big win.

OK, I think the reason the regression test did not show your code being
used is important.

First, most of the tables are small in the regression test, so sequential
scans are faster.  Second, most queries using indexes are either joins,
which do the entire table, or equality tests, like col = 3, where there
is no matching ORDER BY because all the col values are 3.  Again, your
code can't help with these.

The only regression-type code that would use it would be a 'col > 3'
qualification with a col ORDER BY, and there aren't many of those.

However, if we think of the actual application you are addressing, it is
a major win.  If we are going after only one row of the index, it is
fast.  If we are going after the entire table, it is faster to
sequential scan and psort().  You big win is with the partial queries,
where you end up doing a full sequential scan or index scan, then and
ORDER BY, while you really only need a few rows from the query, and if
you deal directly with the index, you can prevent many rows from being
processed.  It is the ability to skip processing those extra rows that
makes it a big win, not so much the removal of the ORDER BY, though that
helps too.

Your solution really is tailored for this 'partial' query application,
and I think it is a big need for certain applications that can't use
cursors, like web apps.  Most other apps have long-time connections to
the database, and are better off with cursors.

I did profiling to improve startup time, because the database
requirements of web apps are different from normal db apps, and we have
to adjust to that.

So, to reiterate, full queries are not benefited as much from the new
code, because sequential scan/psort is faster, or because the index only
retrieves a small number of rows because the qualification of values is
very specific.

Those open-ended, give me the rows from 100 to 199 really need your
modifications.

OK, we have QUERY_LIMIT, and that allows us to throw any query at the
system, and it will return that many of the first rows for the ORDER BY.
No fancy stuff required.  If we can get a matching index, we may be able
to remove the requirement of scanning all the row (with Jan's patch),
and that is a big win.  If not, we at least prevent the rows from being
returned to the client.

However, there is the OFFSET issue.  This is really a case where the
user wants to _restart_ the query where they left off.  That is a
different problem.  All of a sudden, we need to evaluate more of the
query, and return a segment from the middle of the result set.

I think we need to decide how to handle such a restart.  Do we
re-evaluate the entire query, skipping all the rows up to OFFSET, and
return the number of rows they requested after OFFSET.  I would think we
don't want to do that, do we.  It would be much easier to code.  If it
is a single table, skipping forward has to be done anyway, because we
can't just _jump_ to the 100th entry in the index, unless we pass some
_tid_ to the user, and expect them to pass that back to start the query.
I don't think we went to do that.  It is ugly, and the row may have
moved since we started.  So, for a single table, adding a QUERY_OFFSET
would do exactly what we need, with Jan's patches.

For a joined query, I think you will have to do the entire _join_ before
returning anything. 

You can't just process all the joins up to the OFFSET location, and you
can't just jump to the 100th index location, because you don't know that
the 100th index location produced the 100th result just returned to the
user.  You have to process the whole query, and because of the join and
not knowing which data row from each table is going to make which entry
in the final result.  If you are really craft, and the ORDER BY table is
in the outer part of the join loop, you could start processing the table
that is part of the outer loop in _index_ order, because you know that
the rows processed in index order are going to produce the output in
result order.  You then could process and throw away the results up to
offset, and generate the needed rows and stop.

The other way of doing it is to specify a query limit based on specific
index entries, so you say I want the query returned by the first 20
index entries matching the ORDER BY, or entries 100-199, and the query
is limited to using only those entries in the index.  In that case,
though, in joins, you could return more or less rows in the result
depending on the other tables, and that may be unacceptable.  However,
for this case, the advantage is that you don't need to process the rows
from 1 to 99 because you have been told the user only wants rows from
certain index slots.  If the user requests rows 50000-50100, this would
be much faster because you don't have to process the 50000 rows before
returning any data.  However, I question how often people grab stuff
from the center of large data sets.  Seems the QUERY_OFFSET idea may be
easier for users.

I will be commenting on the rest of the optimization postings tomorrow.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist@candle.pha.pa.us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026


From Inoue@tpf.co.jp Fri Oct 16 03:31:02 1998
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <maillist@candle.pha.pa.us>
Cc: <jwieck@debis.com>
Subject: RE: [HACKERS] What about LIMIT in SELECT ?
Date: Fri, 16 Oct 1998 16:16:29 +0900
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Where's my contibution to hackers@potsgreSQL.org ?
I will resend it.

> -----Original Message-----
> From: Bruce Momjian [mailto:maillist@candle.pha.pa.us]
> Sent: Friday, October 16, 1998 3:22 PM
> To: Hiroshi Inoue
> Cc: jwieck@debis.com
> Subject: Re: [HACKERS] What about LIMIT in SELECT ?
>
>
> [Charset iso-8859-1 unsupported, filtering to ASCII...]
> > > >     The  used 'val' is always a key that is close to the first of
> > > >     all keys in the table ('' on the first  query  and  the  last
> > > >     selected value on subsequent ones).
> > >
> > > This is good stuff.  I want to think about it for a day.  Sounds very
> > > promising.
> > >
> >
> > Did you see my contribution about this subject ?
>
> I am sorry.  I have not seen it, and I am confused how I could have
> missed it.
>
> > I have already implemented above cases and used on trial for three
> > months or more.
> > It is good to be formally supported by PostgreSQL community.
> >
> > And please remember that there are descending order cases.
> > (Moreover there are compound cases such as
> >  SELECT * from ... order by key1 desc,key2 asc;
> >  I didn't implement such cases.)
>
> Where is the discussion of this?  I am confused.  You have been using
> code for three months that does this?
>

Hi all.
I didn't follow all the posts about this thread.
So this post may be out of center.

I think current PostgreSQL lacks the concern to the response to get first
rows quickly.
For example,queries with ORDER BY clause necessarily include sort steps
and process all target rows to get first rows only.
So I modified my code for ORDER BY cases and use on trial.
I don't understand PostgreSQL sources,so my code is not complete.

I modified my code for the following 2 cases.

1.In many cases the following query uses index scan.
      SELECT * from ... where key > ...; (where (key) is an index)
   If so,we can omit sort steps from the access plan for the following
  query.
      SELECT * from ... where key > ... order by key;

  Currently cursors without sort steps may be sensitive diffrent from
  cursors with sort steps.  But no one mind it.

2.In many cases the following query uses index scan same as case 1.
        SELECT * from ... where key < ...;(where (key) is an index)
   If so and if we scan the index backward,we can omit sort steps from
   the access plan for the following query.
        SELECT * from ... where key < ... order by key desc;

  To achive this(backward scan),I used hidden(provided for the future ?)code
  that is never executed and is not necessarily correct.

In the following cases I didn't modify my code to use index scan,
because I couldn't formulate how to tell PostgreSQL optimizer whether
the response to get first rows is needed or the throughput to process
sufficiently many target rows is needed.

3.The access plan made by current PostgreSQL optimizer for a query with
   ORDER BY clause doesn't include index scan.

I thought the use of Tatsuo's QUERY_LIMIT to decide that the responce
is needed. It is sufficient but not necessary ?
In Oracle the hints FIRST_ROWS,ALL_ROWS are used.

Thanks.

Hiroshi Inoue
Inoue@tpf.co.jp


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: Inoue@tpf.co.jp (Hiroshi Inoue)
Date: Fri, 16 Oct 1998 10:20:47 +0200 (MET DST)
Cc: maillist@candle.pha.pa.us, jwieck@debis.com
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <000001bdf8d4$e4cdf520$2801007e@cadzone.tpf.co.jp> from "Hiroshi Inoue" at Oct 16, 98 04:16:29 pm
X-Mailer: ELM [version 2.4 PL25]
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Status: RO

Hiroshi Inoue wrote:

> In the following cases I didn't modify my code to use index scan,
> because I couldn't formulate how to tell PostgreSQL optimizer whether
> the response to get first rows is needed or the throughput to process
> sufficiently many target rows is needed.
>
> 3.The access plan made by current PostgreSQL optimizer for a query with
>    ORDER BY clause doesn't include index scan.
>
> I thought the use of Tatsuo's QUERY_LIMIT to decide that the responce
> is needed. It is sufficient but not necessary ?
> In Oracle the hints FIRST_ROWS,ALL_ROWS are used.

    I  still  think  that  the  QUERY LIMIT should be part of the
    parse tree and not thrown in  by  a  magic  SET  command.  If
    rewriting  or  function  calls turn the one query sent to the
    backend into multiple queries processed internal, how  should
    this QUERY LIMIT variable know to which of all the queries it
    has to be applied?  It can really break functions and rewrite
    rules  if  this  variable  is used on all queries while it is
    set.

    For your case 3 I think, if there is a  QUERY  LIMIT  in  the
    parse  tree, the (future) optimizer definitely knows that not
    all rows will get processed even if there is no qualification
    given.   So  if  there is an index, that matches the ORDER BY
    clause and it is no a join and the (future) executor  handles
    OFFSET  in  single table index scans fast, it could choose an
    index scan for this query too.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #



From owner-pgsql-hackers@hub.org Fri Oct 16 12:02:27 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: [HACKERS] SELECT ... LIMIT (trial implementation)
To: pgsql-hackers@postgreSQL.org (PostgreSQL HACKERS)
Date: Fri, 16 Oct 1998 17:35:39 +0200 (MET DST)
Reply-To: jwieck@debis.com (Jan Wieck)
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Sender: owner-pgsql-hackers@postgreSQL.org
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Status: ROr

Here we go,

    this is up to now only for discussion, do not apply to CVS!

    Those involved into the LIMIT discussion please comment.

    Here is what I had in mind for the SELECT ... LIMIT.  It adds

        SELECT ... [LIMIT count [, offset]]

    to the parser and arranges that these values are passed  down
    to the executor.

    It is a clean implementation of LIMIT (regression tested) and
    the open items on it are to enable parameters and  handle  it
    in  SQL  functions and SPI stuff (currently ignored in both).
    Optimizing the executor would require  the  other  sort  node
    stuff  discussion  first to come to a conclusion.  For now it
    skips final result rows - but that's already one step forward
    since  it  reduces  the  rows sent to the frontend to exactly
    that what LIMIT requested.

    I've seen the queryLimit  by  SET  variable  stuff  and  that
    really  can  break rewrite rules, triggers or functions. This
    is because the query limit will be  inherited  by  any  query
    (inserts, updates, deletes too) done by them. Have a rule for
    constraint deletes of referencing tuples

        CREATE RULE del_table1 AS ON DELETE TO table1 DO
            DELETE FROM table2 WHERE ref = OLD.key;

    If the user now sets the query limit to 1 via SET and deletes
    a row from table1, only the first found record in table2 will
    be constraint deleted, not all of them.

    This is a feature where  users  can  get  around  rules  that
    ensure data integrity.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


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`
end


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Date: Sun, 18 Oct 1998 21:45:24 +0400 (MSD)
From: Oleg Bartunov <oleg@sai.msu.su>
X-Sender: megera@ra
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Bruce Momjian <maillist@candle.pha.pa.us>, pgsql-hackers@postgreSQL.org,
        jwieck@debis.com
Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation) 
In-Reply-To: <2292.908726689@sss.pgh.pa.us>
Message-ID: <Pine.GSO.3.96.SK.981018213213.17519C-100000@ra>
Organization: Sternberg Astronomical Institute (Moscow University)
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On Sun, 18 Oct 1998, Tom Lane wrote:

> Date: Sun, 18 Oct 1998 12:04:49 -0400
> From: Tom Lane <tgl@sss.pgh.pa.us>
> To: Bruce Momjian <maillist@candle.pha.pa.us>
> Cc: pgsql-hackers@postgreSQL.org
> Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation) 
> 
> Bruce Momjian <maillist@candle.pha.pa.us> writes:
> > What if someone wants the rows from 500 to the end.  Should we allow
> > the syntax to be:
> >          SELECT ... [LIMIT count] [OFFSET offset]
> > LIMIT and OFFSET are independent.
> 
> I like that syntax the best, but remember we are not inventing in
> a green field here.  Isn't this a feature that already exists in
> other DBMs?  We should probably copy their syntax, unless it's
> truly spectacularly awful...
> 
> 			regards, tom lane
> 

Mysql uses LIMIT [offset,] rows 
>From documentation:

      LIMIT takes one or two numeric arguments. A single argument
      represents the maximum number of rows to return in a result. If two
      arguments are given the first argument is the offset to the first row to
      return, while the second is the maximum number of rows to return in the
      result. 

What would be nice if somehow total number of rows could be returned.
This is often needed for altavista-like application.
Of course, I can do
select count(*) from sometable ... LIMIT offset, rows
and then
select ... from sometable ... LIMIT offset, rows
but this seems not elegant solution.

	Regards,

		Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83


From owner-pgsql-hackers@hub.org Sun Oct 18 14:31:12 1998
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Date: Sun, 18 Oct 1998 21:58:41 +0400 (MSD)
From: Oleg Bartunov <oleg@sai.msu.su>
X-Sender: megera@ra
To: Jan Wieck <jwieck@debis.com>
cc: PostgreSQL HACKERS <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation)
In-Reply-To: <m0zUBum-000EBQC@orion.SAPserv.Hamburg.dsh.de>
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Jan,

I tested your patch on my Linux box and it works ok, except
aggregates functions doesn't work properly, for example
count(*) always produces 0

kdo=> select count(*)  from work_flats limit 10,1000;
count
-----
(0 rows)

while

kdo=> select rooms from work_flats limit 10,1000;
rooms
-----
    3
    3
    3
    3
    3
    3
    3
    3
    3
    3
(10 rows)


	Regards,

		Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83



From wieck@sapserv.debis.de Sun Oct 18 15:17:53 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation)
To: oleg@sai.msu.su (Oleg Bartunov)
Date: Sun, 18 Oct 1998 21:29:43 +0200 (MET DST)
Cc: tgl@sss.pgh.pa.us, maillist@candle.pha.pa.us, pgsql-hackers@postgreSQL.org,
        jwieck@debis.com
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <Pine.GSO.3.96.SK.981018213213.17519C-100000@ra> from "Oleg Bartunov" at Oct 18, 98 09:45:24 pm
X-Mailer: ELM [version 2.4 PL25]
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Oleg Bartunov wrote:

> On Sun, 18 Oct 1998, Tom Lane wrote:
>
> > Bruce Momjian <maillist@candle.pha.pa.us> writes:
> > > What if someone wants the rows from 500 to the end.  Should we allow
> > > the syntax to be:
> > >          SELECT ... [LIMIT count] [OFFSET offset]
> > > LIMIT and OFFSET are independent.
> >
> > I like that syntax the best, but remember we are not inventing in
> > a green field here.  Isn't this a feature that already exists in
> > other DBMs?  We should probably copy their syntax, unless it's
> > truly spectacularly awful...
> >
> >            regards, tom lane
> >
>
> Mysql uses LIMIT [offset,] rows
> >From documentation:
>
>       LIMIT takes one or two numeric arguments. A single argument
>       represents the maximum number of rows to return in a result. If two
>       arguments are given the first argument is the offset to the first row to
>       return, while the second is the maximum number of rows to return in the
>       result.

    Simple change, just flip them in gram.y.

    And for the 500 to end:

        SELECT ... LIMIT 500, 0 (after flipped)

    The  0  has  the  same meaning as ALL. And that could also be
    added to the parser easily so one can say

        SELECT ... LIMIT 500, ALL

    too.

>
> What would be nice if somehow total number of rows could be returned.
> This is often needed for altavista-like application.
> Of course, I can do
> select count(*) from sometable ... LIMIT offset, rows
> and then
> select ... from sometable ... LIMIT offset, rows
> but this seems not elegant solution.

    Absolutely makes no sense  for  me.  As  said  in  the  other
    posting,  aggregates  do  the counting scan in a deeper level
    and thus cannot get limited. So if you invoke  an  aggregate,
    the whole scan is always done.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #



From owner-pgsql-hackers@hub.org Sun Oct 18 19:08:47 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation)
To: terry@terrym.com (Terry Mackintosh)
Date: Sun, 18 Oct 1998 22:05:31 +0200 (MET DST)
Cc: hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <Pine.LNX.3.95.981018155322.29282B-100000@terry1.acun.com> from "Terry Mackintosh" at Oct 18, 98 03:58:57 pm
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>
> On Sun, 18 Oct 1998, Tom Lane wrote:
>
> > Bruce Momjian <maillist@candle.pha.pa.us> writes:
> > > What if someone wants the rows from 500 to the end.  Should we allow
> > > the syntax to be:
> > >          SELECT ... [LIMIT count] [OFFSET offset]
> > > LIMIT and OFFSET are independent.
> >
> > I like that syntax the best, but remember we are not inventing in
> > a green field here.  Isn't this a feature that already exists in
> > other DBMs?  We should probably copy their syntax, unless it's
> > truly spectacularly awful...
> >
> >            regards, tom lane
>
> None that I have used (VFP, M$ SQL Server) that had 'LIMIT', had 'OFFSET'.
> So it would seem that the very idea of OFFSET is to break with what others
> are doing.
>
> I too like the above syntax.
> Why mimic, when you can do better?  Go for it!
>

    We have a powerful parser. So we can provide

        ... [ LIMIT { rows | ALL } ] [ OFFSET skip ]

        or

        ... [ LIMIT [ skip , ] { rows | ALL } ]

    at the same time.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Sun Oct 18 19:08:39 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation)
To: jwieck@debis.com
Date: Sun, 18 Oct 1998 22:44:15 +0200 (MET DST)
Cc: terry@terrym.com, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <m0zUz52-000EBPC@orion.SAPserv.Hamburg.dsh.de> from "Jan Wieck" at Oct 18, 98 10:05:31 pm
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>     We have a powerful parser. So we can provide
> [...]

    This version now accepts all of the following

        ... [ LIMIT rows ] [ OFFSET skip ]
        ... [ OFFSET skip ] [ LIMIT rows ]
        ... [ LIMIT [ skip , ] rows ]

    rows  can be a positive integer constant greater that 0, a $n
    parameter (in SPI_prepare()) or  the  keyword  ALL.  0  isn't
    accepted  as  constant to force ALL in that case making clear
    that this is wanted. In the  parameter  version  the  integer
    value 0 still is used to mean ALL.

    skip can be a positive integer constant greater or equal to 0
    or a $n parameter for SPI_prepare.

    If any of these syntaxes is used  in  SPI_prepare()'d  plans,
    the  given tcount argument for SPI_execp() is ignored and the
    plan or parameter values are used.

    Anyone happy now?


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #

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`
end


From owner-pgsql-hackers@hub.org Mon Oct 19 07:31:10 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: Inoue@tpf.co.jp (Hiroshi Inoue)
Date: Mon, 19 Oct 1998 12:30:52 +0200 (MET DST)
Cc: jwieck@debis.com, pgsql-hackers@postgreSQL.org, maillist@candle.pha.pa.us
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <002801bdfb46$39ad8ec0$2801007e@cadzone.tpf.co.jp> from "Hiroshi Inoue" at Oct 19, 98 06:52:46 pm
X-Mailer: ELM [version 2.4 PL25]
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Status: RO

Hiroshi Inoue wrote:

> When using cursors,in most cases the response to get first(next) rows
> is necessary for me,not the throughput.
> How can we tell PostgreSQL optimzer that the response is necessary ?

    With my LIMIT patch, the offset and the row count are part of
    the querytree. And if a LIMIT is given, the limitCount elemet
    of the querytree (a Node *) isn't NULL what it is by default.

    When a LIMIT is given, the optimizer could assume that  first
    rows  is  wanted (even if the limit is ALL maybe - but I have
    to think about this some more). And this assumption might let
    it  decide  to use an index to resolve an ORDER BY even if no
    qualification was given.

    Telling the optimizer that first  rows  wanted  in  a  cursor
    operation would read

        DECLARE CURSOR c FOR SELECT * FROM mytab ORDER BY a LIMIT ALL;


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From owner-pgsql-hackers@hub.org Tue Oct 20 06:01:49 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: Inoue@tpf.co.jp (Hiroshi Inoue)
Date: Tue, 20 Oct 1998 11:25:22 +0200 (MET DST)
Cc: maillist@candle.pha.pa.us, jwieck@debis.com, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <000601bdfc03$02e67100$2801007e@cadzone.tpf.co.jp> from "Hiroshi Inoue" at Oct 20, 98 05:24:09 pm
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Hiroshi Inoue wrote:

> >   * Prevent psort() usage when query already using index matching ORDER BY
> >
> >
>
> I can't find the reference to descending order cases except my posting.
> If  we use an index scan to remove sorts in those cases,backward positioning
> and scanning are necessary.

    I  think  it's  only thought as a reminder that the optimizer
    needs some optimization.

    That topic, and the LIMIT stuff too I think, is past 6.4 work
    and  may  go into a 6.4.1 performance release. So when we are
    after 6.4, we have enough time to work out a  real  solution,
    instead of just throwing in a patch as a quick shot.

    What  we  two did where steps in the same direction. Your one
    covers more situations, but after all if multiple people have
    the  same  idea  there  is a good chance that it is the right
    thing to do.

>
> Let t be a table with 2 indices, index1(key1,key2), index2(key1,key3).
> i.e. key1 is common to index1 and index2.
>
> And for the query
>   select * from t where key1>....;
>
> If   PosgreSQL optimizer choose [ index scan on index1 ] we can't remove
> sorts from the following query.
>    select * from t where key1>... order by key1,key3;
>
> Similarly if  [ index scan on index2 ] are chosen  we can't remove sorts
> from the following query.
>    select * from t where key1>... order by key1,key2;
>
> But in both cases (clever) optimizer can choose another index for scan.

    Right. As I remember, your solution does basically  the  same
    as  my  one. It does not change the optimizers decision about
    the index or if an index at all is used.  So  I  assume  they
    hook  into  the same position where depending on the order by
    clause the sort node is added. And that is at the very end of
    the optimizer.

    What  you  describe above requires changes in upper levels of
    optimization.  Doing that is far away from my knowledge about
    the  optimizer.   And  some of your earlier statements let me
    think you aren't familiar enough with  it  too.  We  need  at
    least help from others to do it well.

    I  don't want to dive that deep into the optimizer. There was
    a far too long time where the rule system was broken and  got
    out  of  sync with the parser/optimizer capabilities. I fixed
    many things in it for 6.4. My first priority now is,  not  to
    let such a situation come up again.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Tue, 20 Oct 1998 19:12:19 +0200 (MET DST)
Cc: jwieck@debis.com, Inoue@tpf.co.jp, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810201645.MAA07946@candle.pha.pa.us> from "Bruce Momjian" at Oct 20, 98 12:45:49 pm
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>
> I agree.  Another good thing is that the LIMIT thing will not require a
> dump/reload, so it is a good candidate for a minor release.

    That's wrong, sorry.

    The  limit  thing as I implemented it adds 2 new variables to
    the Query structure. Rewrite rules are stored  as  querytrees
    and in the existing pg_rewrite entries that would be missing.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #



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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Tue, 20 Oct 1998 19:12:19 +0200 (MET DST)
Cc: jwieck@debis.com, Inoue@tpf.co.jp, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810201645.MAA07946@candle.pha.pa.us> from "Bruce Momjian" at Oct 20, 98 12:45:49 pm
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>
> I agree.  Another good thing is that the LIMIT thing will not require a
> dump/reload, so it is a good candidate for a minor release.

    That's wrong, sorry.

    The  limit  thing as I implemented it adds 2 new variables to
    the Query structure. Rewrite rules are stored  as  querytrees
    and in the existing pg_rewrite entries that would be missing.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #




From wieck@sapserv.debis.de Tue Oct 20 13:10:22 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Tue, 20 Oct 1998 19:22:40 +0200 (MET DST)
Cc: jwieck@debis.com, Inoue@tpf.co.jp, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810201702.NAA08286@candle.pha.pa.us> from "Bruce Momjian" at Oct 20, 98 01:02:58 pm
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> 
> > >
> > > I agree.  Another good thing is that the LIMIT thing will not require a
> > > dump/reload, so it is a good candidate for a minor release.
> > 
> >     That's wrong, sorry.
> > 
> >     The  limit  thing as I implemented it adds 2 new variables to
> >     the Query structure. Rewrite rules are stored  as  querytrees
> >     and in the existing pg_rewrite entries that would be missing.
> 
> Oh, sorry.  I forgot.  That could be tough.

    But it wouldn't hurt to add them now to have them in
    place. The required out-, read- and copyfuncs are in
    my patch too. This  would prevent  dump/load when we
    later add the real LIMIT functionality. And  it does
    not change anything now.


Jan

-- 

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Tue, 20 Oct 1998 19:22:40 +0200 (MET DST)
Cc: jwieck@debis.com, Inoue@tpf.co.jp, hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810201702.NAA08286@candle.pha.pa.us> from "Bruce Momjian" at Oct 20, 98 01:02:58 pm
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> 
> > >
> > > I agree.  Another good thing is that the LIMIT thing will not require a
> > > dump/reload, so it is a good candidate for a minor release.
> > 
> >     That's wrong, sorry.
> > 
> >     The  limit  thing as I implemented it adds 2 new variables to
> >     the Query structure. Rewrite rules are stored  as  querytrees
> >     and in the existing pg_rewrite entries that would be missing.
> 
> Oh, sorry.  I forgot.  That could be tough.

    But it wouldn't hurt to add them now to have them in
    place. The required out-, read- and copyfuncs are in
    my patch too. This  would prevent  dump/load when we
    later add the real LIMIT functionality. And  it does
    not change anything now.


Jan

-- 

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #



From owner-pgsql-hackers@hub.org Wed Oct 21 02:35:54 1998
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From: Bruce Momjian <maillist@candle.pha.pa.us>
Message-Id: <199810210609.CAA27774@candle.pha.pa.us>
Subject: Re: [HACKERS] What about LIMIT in SELECT ?
In-Reply-To: <m0zVfUW-000EBPC@orion.SAPserv.Hamburg.dsh.de> from Jan Wieck at "Oct 20, 1998  7:22:40 pm"
To: jwieck@debis.com
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> > 
> > > >
> > > > I agree.  Another good thing is that the LIMIT thing will not require a
> > > > dump/reload, so it is a good candidate for a minor release.
> > > 
> > >     That's wrong, sorry.
> > > 
> > >     The  limit  thing as I implemented it adds 2 new variables to
> > >     the Query structure. Rewrite rules are stored  as  querytrees
> > >     and in the existing pg_rewrite entries that would be missing.
> > 
> > Oh, sorry.  I forgot.  That could be tough.
> 
>     But it wouldn't hurt to add them now to have them in
>     place. The required out-, read- and copyfuncs are in
>     my patch too. This  would prevent  dump/load when we
>     later add the real LIMIT functionality. And  it does
>     not change anything now.
> 

Jan, we found that I am having to require an initdb for the INET/CIDR
type, so if you want stuff to change the views/rules for the limit
addition post 6.4, please send them in and I will apply them.

You clearly have the syntax down, so I think you should go ahead.


-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist@candle.pha.pa.us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026


From wieck@sapserv.debis.de Thu Oct 22 10:20:58 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] psql's help (the LIMIT stuff)
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Thu, 22 Oct 1998 16:33:50 +0200 (MET DST)
Cc: jwieck@debis.com, jose@sferacarta.com, pgsql-hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810221351.JAA19663@candle.pha.pa.us> from "Bruce Momjian" at Oct 22, 98 09:51:19 am
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> >
> >     I hope the QUERY_LIMIT too.
>
> I still have that cnfify() possible fix to review for KQSO.  Are you
> still thinking limit for 6.4 final, or a minor release after that?

    I  posted the part that is the minimum applied to 6.4 to make
    adding LIMIT later  non-initdb  earlier.  Anyway,  here  it's
    again.

    My  LIMIT  implementation  that  does  it like the SET in the
    toplevel executor (but via parsetree  values)  is  ready  for
    production.  I  only  held  it back because it's feature, not
    bugfix.

    Do you want it in 6.4 final?


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


diff -cr src.orig/backend/nodes/copyfuncs.c src/backend/nodes/copyfuncs.c
*** src.orig/backend/nodes/copyfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/copyfuncs.c	Fri Oct 16 13:32:35 1998
***************
*** 1578,1583 ****
--- 1578,1586 ----
  		newnode->unionClause = temp_list;
  	}
  
+ 	Node_Copy(from, newnode, limitOffset);
+ 	Node_Copy(from, newnode, limitCount);
+ 
  	return newnode;
  }
  
diff -cr src.orig/backend/nodes/outfuncs.c src/backend/nodes/outfuncs.c
*** src.orig/backend/nodes/outfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/outfuncs.c	Fri Oct 16 13:30:50 1998
***************
*** 259,264 ****
--- 259,268 ----
  	appendStringInfo(str, (node->hasSubLinks ? "true" : "false"));
  	appendStringInfo(str, " :unionClause ");
  	_outNode(str, node->unionClause);
+ 	appendStringInfo(str, " :limitOffset ");
+ 	_outNode(str, node->limitOffset);
+ 	appendStringInfo(str, " :limitCount ");
+ 	_outNode(str, node->limitCount);
  }
  
  static void
diff -cr src.orig/backend/nodes/readfuncs.c src/backend/nodes/readfuncs.c
*** src.orig/backend/nodes/readfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/readfuncs.c	Fri Oct 16 13:31:43 1998
***************
*** 163,168 ****
--- 163,174 ----
  	token = lsptok(NULL, &length);		/* skip :unionClause */
  	local_node->unionClause = nodeRead(true);
  
+ 	token = lsptok(NULL, &length);		/* skip :limitOffset */
+ 	local_node->limitOffset = nodeRead(true);
+ 
+ 	token = lsptok(NULL, &length);		/* skip :limitCount */
+ 	local_node->limitCount = nodeRead(true);
+ 
  	return local_node;
  }
  
diff -cr src.orig/include/nodes/parsenodes.h src/include/nodes/parsenodes.h
*** src.orig/include/nodes/parsenodes.h	Fri Oct 16 11:53:58 1998
--- src/include/nodes/parsenodes.h	Fri Oct 16 13:35:32 1998
***************
*** 60,65 ****
--- 60,67 ----
  
  	List	   *unionClause;	/* unions are linked under the previous
  								 * query */
+ 	Node	   *limitOffset;	/* # of result tuples to skip */
+ 	Node	   *limitCount;		/* # of result tuples to return */
  
  	/* internal to planner */
  	List	   *base_rel_list;	/* base relation list */
***************
*** 639,644 ****
--- 641,648 ----
  	char	   *portalname;		/* the portal (cursor) to create */
  	bool		binary;			/* a binary (internal) portal? */
  	bool		unionall;		/* union without unique sort */
+ 	Node	   *limitOffset;	/* # of result tuples to skip */
+ 	Node	   *limitCount;		/* # of result tuples to return */
  } SelectStmt;
  
  

From owner-pgsql-hackers@hub.org Thu Oct 22 11:33:41 1998
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Message-Id: <m0zWLoE-000EBPC@orion.SAPserv.Hamburg.dsh.de>
From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] psql's help (the LIMIT stuff)
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Thu, 22 Oct 1998 16:33:50 +0200 (MET DST)
Cc: jwieck@debis.com, jose@sferacarta.com, pgsql-hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810221351.JAA19663@candle.pha.pa.us> from "Bruce Momjian" at Oct 22, 98 09:51:19 am
X-Mailer: ELM [version 2.4 PL25]
Content-Type: text
Sender: owner-pgsql-hackers@postgreSQL.org
Precedence: bulk
Status: RO

> >
> >     I hope the QUERY_LIMIT too.
>
> I still have that cnfify() possible fix to review for KQSO.  Are you
> still thinking limit for 6.4 final, or a minor release after that?

    I  posted the part that is the minimum applied to 6.4 to make
    adding LIMIT later  non-initdb  earlier.  Anyway,  here  it's
    again.

    My  LIMIT  implementation  that  does  it like the SET in the
    toplevel executor (but via parsetree  values)  is  ready  for
    production.  I  only  held  it back because it's feature, not
    bugfix.

    Do you want it in 6.4 final?


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


diff -cr src.orig/backend/nodes/copyfuncs.c src/backend/nodes/copyfuncs.c
*** src.orig/backend/nodes/copyfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/copyfuncs.c	Fri Oct 16 13:32:35 1998
***************
*** 1578,1583 ****
--- 1578,1586 ----
  		newnode->unionClause = temp_list;
  	}
  
+ 	Node_Copy(from, newnode, limitOffset);
+ 	Node_Copy(from, newnode, limitCount);
+ 
  	return newnode;
  }
  
diff -cr src.orig/backend/nodes/outfuncs.c src/backend/nodes/outfuncs.c
*** src.orig/backend/nodes/outfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/outfuncs.c	Fri Oct 16 13:30:50 1998
***************
*** 259,264 ****
--- 259,268 ----
  	appendStringInfo(str, (node->hasSubLinks ? "true" : "false"));
  	appendStringInfo(str, " :unionClause ");
  	_outNode(str, node->unionClause);
+ 	appendStringInfo(str, " :limitOffset ");
+ 	_outNode(str, node->limitOffset);
+ 	appendStringInfo(str, " :limitCount ");
+ 	_outNode(str, node->limitCount);
  }
  
  static void
diff -cr src.orig/backend/nodes/readfuncs.c src/backend/nodes/readfuncs.c
*** src.orig/backend/nodes/readfuncs.c	Fri Oct 16 11:53:40 1998
--- src/backend/nodes/readfuncs.c	Fri Oct 16 13:31:43 1998
***************
*** 163,168 ****
--- 163,174 ----
  	token = lsptok(NULL, &length);		/* skip :unionClause */
  	local_node->unionClause = nodeRead(true);
  
+ 	token = lsptok(NULL, &length);		/* skip :limitOffset */
+ 	local_node->limitOffset = nodeRead(true);
+ 
+ 	token = lsptok(NULL, &length);		/* skip :limitCount */
+ 	local_node->limitCount = nodeRead(true);
+ 
  	return local_node;
  }
  
diff -cr src.orig/include/nodes/parsenodes.h src/include/nodes/parsenodes.h
*** src.orig/include/nodes/parsenodes.h	Fri Oct 16 11:53:58 1998
--- src/include/nodes/parsenodes.h	Fri Oct 16 13:35:32 1998
***************
*** 60,65 ****
--- 60,67 ----
  
  	List	   *unionClause;	/* unions are linked under the previous
  								 * query */
+ 	Node	   *limitOffset;	/* # of result tuples to skip */
+ 	Node	   *limitCount;		/* # of result tuples to return */
  
  	/* internal to planner */
  	List	   *base_rel_list;	/* base relation list */
***************
*** 639,644 ****
--- 641,648 ----
  	char	   *portalname;		/* the portal (cursor) to create */
  	bool		binary;			/* a binary (internal) portal? */
  	bool		unionall;		/* union without unique sort */
+ 	Node	   *limitOffset;	/* # of result tuples to skip */
+ 	Node	   *limitCount;		/* # of result tuples to return */
  } SelectStmt;
  
  


From wieck@sapserv.debis.de Thu Oct 22 11:01:05 1998
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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] psql's help (the LIMIT stuff)
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Thu, 22 Oct 1998 16:50:40 +0200 (MET DST)
Cc: jwieck@debis.com, jose@sferacarta.com, pgsql-hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810221424.KAA20601@candle.pha.pa.us> from "Bruce Momjian" at Oct 22, 98 10:24:08 am
X-Mailer: ELM [version 2.4 PL25]
Content-Type: text
Status: RO

>
> > > >
> > > >     I hope the QUERY_LIMIT too.
> > >
> > > I still have that cnfify() possible fix to review for KQSO.  Are you
> > > still thinking limit for 6.4 final, or a minor release after that?
> >
> >     I  posted the part that is the minimum applied to 6.4 to make
> >     adding LIMIT later  non-initdb  earlier.  Anyway,  here  it's
> >     again.
>
> Already applied.  I assume it is the same as the one I applied.

    Seen,  thanks.  Your 'Applied' just arrived after I packed it
    again. It's the same.

> We are close to final, and can easily put it in 6.4.1, which I am sure
> we will need, and if we split CVS trees, you'll have lots of minor
> versions to pick from.  :-)
>
> Seems like it would be a nice minor release item, but the problem is
> that minor releases aren't tested as much as major ones.  How confident
> are you in the code?  What do others thing?

    I regression tested it,  and  did  additional  tests  in  the
    SPI/PL  area.  It  works.  It only touches the parser and the
    executor. Rules, planner/optimizer just bypass the values  in
    the  parsetree.  The  parser  and  the  executor are parts of
    Postgres I feel very familiar with (not so in the optimizer).
    I  trust  in  the  code  and  would  use  it  in a production
    environment.

    It's below.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


diff -cr src.orig/backend/commands/command.c src/backend/commands/command.c
*** src.orig/backend/commands/command.c	Fri Oct 16 11:53:38 1998
--- src/backend/commands/command.c	Fri Oct 16 12:56:44 1998
***************
*** 39,44 ****
--- 39,45 ----
  #include "utils/mcxt.h"
  #include "utils/portal.h"
  #include "utils/syscache.h"
+ #include "string.h"
  
  /* ----------------
   *		PortalExecutorHeapMemory stuff
***************
*** 101,106 ****
--- 102,108 ----
  	int			feature;
  	QueryDesc  *queryDesc;
  	MemoryContext context;
+ 	Const		limcount;
  
  	/* ----------------
  	 *	sanity checks
***************
*** 113,118 ****
--- 115,134 ----
  	}
  
  	/* ----------------
+ 	 *	Create a const node from the given count value
+ 	 * ----------------
+ 	 */
+ 	memset(&limcount, 0, sizeof(limcount));
+ 	limcount.type		= T_Const;
+ 	limcount.consttype	= INT4OID;
+ 	limcount.constlen	= sizeof(int4);
+ 	limcount.constvalue	= (Datum)count;
+ 	limcount.constisnull	= FALSE;
+ 	limcount.constbyval	= TRUE;
+ 	limcount.constisset	= FALSE;
+ 	limcount.constiscast	= FALSE;
+ 
+ 	/* ----------------
  	 *	get the portal from the portal name
  	 * ----------------
  	 */
***************
*** 176,182 ****
  	PortalExecutorHeapMemory = (MemoryContext)
  		PortalGetHeapMemory(portal);
  
! 	ExecutorRun(queryDesc, PortalGetState(portal), feature, count);
  
  	if (dest == None)			/* MOVE */
  		pfree(queryDesc);
--- 192,198 ----
  	PortalExecutorHeapMemory = (MemoryContext)
  		PortalGetHeapMemory(portal);
  
! 	ExecutorRun(queryDesc, PortalGetState(portal), feature, (Node *)NULL, (Node *)&limcount);
  
  	if (dest == None)			/* MOVE */
  		pfree(queryDesc);
diff -cr src.orig/backend/executor/execMain.c src/backend/executor/execMain.c
*** src.orig/backend/executor/execMain.c	Fri Oct 16 11:53:38 1998
--- src/backend/executor/execMain.c	Fri Oct 16 20:05:19 1998
***************
*** 64,69 ****
--- 64,70 ----
  static void EndPlan(Plan *plan, EState *estate);
  static TupleTableSlot *ExecutePlan(EState *estate, Plan *plan,
  			Query *parseTree, CmdType operation,
+ 			int offsetTuples,
  			int numberTuples, ScanDirection direction,
  			void (*printfunc) ());
  static void ExecRetrieve(TupleTableSlot *slot, void (*printfunc) (),
***************
*** 163,169 ****
   * ----------------------------------------------------------------
   */
  TupleTableSlot *
! ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, int count)
  {
  	CmdType		operation;
  	Query	   *parseTree;
--- 164,170 ----
   * ----------------------------------------------------------------
   */
  TupleTableSlot *
! ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, Node *limoffset, Node *limcount)
  {
  	CmdType		operation;
  	Query	   *parseTree;
***************
*** 171,176 ****
--- 172,179 ----
  	TupleTableSlot *result;
  	CommandDest dest;
  	void		(*destination) ();
+ 	int		offset = 0;
+ 	int		count = 0;
  
  	/******************
  	 *	sanity checks
***************
*** 191,196 ****
--- 194,289 ----
  	estate->es_processed = 0;
  	estate->es_lastoid = InvalidOid;
  
+ 	/******************
+ 	 *	if given get the offset of the LIMIT clause
+ 	 ******************
+ 	 */
+ 	if (limoffset != NULL)
+ 	{
+ 		Const		*coffset;
+ 		Param		*poffset;
+ 		ParamListInfo	paramLI;
+ 		int		i;
+ 
+ 		switch (nodeTag(limoffset))
+ 		{
+ 			case T_Const:
+ 				coffset = (Const *)limoffset;
+ 				offset = (int)(coffset->constvalue);
+ 				break;
+ 
+ 			case T_Param:
+ 				poffset = (Param *)limoffset;
+ 				paramLI = estate->es_param_list_info;
+ 
+ 				if (paramLI == NULL)
+ 					elog(ERROR, "parameter for limit offset not in executor state");
+ 				for (i = 0; paramLI[i].kind != PARAM_INVALID; i++)
+ 				{
+ 					if (paramLI[i].kind == PARAM_NUM && paramLI[i].id == poffset->paramid)
+ 						break;
+ 				}
+ 				if (paramLI[i].kind == PARAM_INVALID)
+ 					elog(ERROR, "parameter for limit offset not in executor state");
+ 				if (paramLI[i].isnull)
+ 					elog(ERROR, "limit offset cannot be NULL value");
+ 				offset = (int)(paramLI[i].value);
+ 
+ 				break;
+ 
+ 			default:
+ 				elog(ERROR, "unexpected node type %d as limit offset", nodeTag(limoffset));
+ 		}
+ 
+ 		if (offset < 0)
+ 			elog(ERROR, "limit offset cannot be negative");
+ 	}
+ 
+ 	/******************
+ 	 *	if given get the count of the LIMIT clause
+ 	 ******************
+ 	 */
+ 	if (limcount != NULL)
+ 	{
+ 		Const		*ccount;
+ 		Param		*pcount;
+ 		ParamListInfo	paramLI;
+ 		int		i;
+ 
+ 		switch (nodeTag(limcount))
+ 		{
+ 			case T_Const:
+ 				ccount = (Const *)limcount;
+ 				count = (int)(ccount->constvalue);
+ 				break;
+ 
+ 			case T_Param:
+ 				pcount = (Param *)limcount;
+ 				paramLI = estate->es_param_list_info;
+ 
+ 				if (paramLI == NULL)
+ 					elog(ERROR, "parameter for limit count not in executor state");
+ 				for (i = 0; paramLI[i].kind != PARAM_INVALID; i++)
+ 				{
+ 					if (paramLI[i].kind == PARAM_NUM && paramLI[i].id == pcount->paramid)
+ 						break;
+ 				}
+ 				if (paramLI[i].kind == PARAM_INVALID)
+ 					elog(ERROR, "parameter for limit count not in executor state");
+ 				if (paramLI[i].isnull)
+ 					elog(ERROR, "limit count cannot be NULL value");
+ 				count = (int)(paramLI[i].value);
+ 
+ 				break;
+ 
+ 			default:
+ 				elog(ERROR, "unexpected node type %d as limit count", nodeTag(limcount));
+ 		}
+ 
+ 		if (count < 0)
+ 			elog(ERROR, "limit count cannot be negative");
+ 	}
+ 
  	switch (feature)
  	{
  
***************
*** 199,205 ****
  								 plan,
  								 parseTree,
  								 operation,
! 								 ALL_TUPLES,
  								 ForwardScanDirection,
  								 destination);
  			break;
--- 292,299 ----
  								 plan,
  								 parseTree,
  								 operation,
! 								 offset,
! 								 count,
  								 ForwardScanDirection,
  								 destination);
  			break;
***************
*** 208,213 ****
--- 302,308 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 offset,
  								 count,
  								 ForwardScanDirection,
  								 destination);
***************
*** 222,227 ****
--- 317,323 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 offset,
  								 count,
  								 BackwardScanDirection,
  								 destination);
***************
*** 237,242 ****
--- 333,339 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 0,
  								 ONE_TUPLE,
  								 ForwardScanDirection,
  								 destination);
***************
*** 691,696 ****
--- 788,794 ----
  			Plan *plan,
  			Query *parseTree,
  			CmdType operation,
+ 			int offsetTuples,
  			int numberTuples,
  			ScanDirection direction,
  			void (*printfunc) ())
***************
*** 742,747 ****
--- 840,859 ----
  		{
  			result = NULL;
  			break;
+ 		}
+ 
+ 		/******************
+ 		 *	For now we completely execute the plan and skip
+ 		 *	result tuples if requested by LIMIT offset.
+ 		 *	Finally we should try to do it in deeper levels
+ 		 *	if possible (during index scan)
+ 		 *	- Jan
+ 		 ******************
+ 		 */
+ 		if (offsetTuples > 0)
+ 		{
+ 			--offsetTuples;
+ 			continue;
  		}
  
  		/******************
diff -cr src.orig/backend/executor/functions.c src/backend/executor/functions.c
*** src.orig/backend/executor/functions.c	Fri Oct 16 11:53:38 1998
--- src/backend/executor/functions.c	Fri Oct 16 19:01:02 1998
***************
*** 130,135 ****
--- 130,138 ----
  									 None);
  		estate = CreateExecutorState();
  
+ 		if (queryTree->limitOffset != NULL || queryTree->limitCount != NULL)
+ 			elog(ERROR, "LIMIT clause from SQL functions not yet implemented");
+ 
  		if (nargs > 0)
  		{
  			int			i;
***************
*** 200,206 ****
  
  	feature = (LAST_POSTQUEL_COMMAND(es)) ? EXEC_RETONE : EXEC_RUN;
  
! 	return ExecutorRun(es->qd, es->estate, feature, 0);
  }
  
  static void
--- 203,209 ----
  
  	feature = (LAST_POSTQUEL_COMMAND(es)) ? EXEC_RETONE : EXEC_RUN;
  
! 	return ExecutorRun(es->qd, es->estate, feature, (Node *)NULL, (Node *)NULL);
  }
  
  static void
diff -cr src.orig/backend/executor/spi.c src/backend/executor/spi.c
*** src.orig/backend/executor/spi.c	Fri Oct 16 11:53:39 1998
--- src/backend/executor/spi.c	Fri Oct 16 19:25:33 1998
***************
*** 791,796 ****
--- 791,798 ----
  	bool		isRetrieveIntoRelation = false;
  	char	   *intoName = NULL;
  	int			res;
+ 	Const		tcount_const;
+ 	Node		*count = NULL;
  
  	switch (operation)
  	{
***************
*** 825,830 ****
--- 827,865 ----
  			return SPI_ERROR_OPUNKNOWN;
  	}
  
+ 	/* ----------------
+ 	 *	Get the query LIMIT tuple count
+ 	 * ----------------
+ 	 */
+ 	if (parseTree->limitCount != NULL)
+ 	{
+ 		/* ----------------
+ 		 *      A limit clause in the parsetree overrides the
+ 		 *	tcount parameter
+ 		 * ----------------
+ 		 */
+ 		count = parseTree->limitCount;
+ 	}
+ 	else
+ 	{
+ 		/* ----------------
+ 		 *      No LIMIT clause in parsetree. Use a local Const node
+ 		 *	to put tcount into it
+ 		 * ----------------
+ 		 */
+ 		memset(&tcount_const, 0, sizeof(tcount_const));
+ 		tcount_const.type           = T_Const;
+ 		tcount_const.consttype      = INT4OID;
+ 		tcount_const.constlen       = sizeof(int4);
+ 		tcount_const.constvalue     = (Datum)tcount;
+ 		tcount_const.constisnull    = FALSE;
+ 		tcount_const.constbyval     = TRUE;
+ 		tcount_const.constisset     = FALSE;
+ 		tcount_const.constiscast    = FALSE;
+ 
+ 		count = (Node *)&tcount_const;
+ 	}
+ 
  	if (state == NULL)			/* plan preparation */
  		return res;
  #ifdef SPI_EXECUTOR_STATS
***************
*** 845,851 ****
  		return SPI_OK_CURSOR;
  	}
  
! 	ExecutorRun(queryDesc, state, EXEC_FOR, tcount);
  
  	_SPI_current->processed = state->es_processed;
  	if (operation == CMD_SELECT && queryDesc->dest == SPI)
--- 880,886 ----
  		return SPI_OK_CURSOR;
  	}
  
! 	ExecutorRun(queryDesc, state, EXEC_FOR, parseTree->limitOffset, count);
  
  	_SPI_current->processed = state->es_processed;
  	if (operation == CMD_SELECT && queryDesc->dest == SPI)
diff -cr src.orig/backend/parser/analyze.c src/backend/parser/analyze.c
*** src.orig/backend/parser/analyze.c	Fri Oct 16 11:53:41 1998
--- src/backend/parser/analyze.c	Fri Oct 16 13:29:27 1998
***************
*** 180,186 ****
--- 180,190 ----
  
  		case T_SelectStmt:
  			if (!((SelectStmt *) parseTree)->portalname)
+ 			{
  				result = transformSelectStmt(pstate, (SelectStmt *) parseTree);
+ 				result->limitOffset = ((SelectStmt *)parseTree)->limitOffset;
+ 				result->limitCount = ((SelectStmt *)parseTree)->limitCount;
+ 			}
  			else
  				result = transformCursorStmt(pstate, (SelectStmt *) parseTree);
  			break;
diff -cr src.orig/backend/parser/gram.y src/backend/parser/gram.y
*** src.orig/backend/parser/gram.y	Fri Oct 16 11:53:42 1998
--- src/backend/parser/gram.y	Sun Oct 18 22:20:36 1998
***************
*** 45,50 ****
--- 45,51 ----
  #include "catalog/catname.h"
  #include "utils/elog.h"
  #include "access/xact.h"
+ #include "catalog/pg_type.h"
  
  #ifdef MULTIBYTE
  #include "mb/pg_wchar.h"
***************
*** 163,169 ****
  		sort_clause, sortby_list, index_params, index_list, name_list,
  		from_clause, from_list, opt_array_bounds, nest_array_bounds,
  		expr_list, attrs, res_target_list, res_target_list2,
! 		def_list, opt_indirection, group_clause, TriggerFuncArgs
  
  %type <node>	func_return
  %type <boolean>	set_opt
--- 164,171 ----
  		sort_clause, sortby_list, index_params, index_list, name_list,
  		from_clause, from_list, opt_array_bounds, nest_array_bounds,
  		expr_list, attrs, res_target_list, res_target_list2,
! 		def_list, opt_indirection, group_clause, TriggerFuncArgs,
! 		opt_select_limit
  
  %type <node>	func_return
  %type <boolean>	set_opt
***************
*** 192,197 ****
--- 194,201 ----
  
  %type <ival>	fetch_how_many
  
+ %type <node>	select_limit_value select_offset_value
+ 
  %type <list>	OptSeqList
  %type <defelt>	OptSeqElem
  
***************
*** 267,273 ****
  		FALSE_P, FETCH, FLOAT, FOR, FOREIGN, FROM, FULL,
  		GRANT, GROUP, HAVING, HOUR_P,
  		IN, INNER_P, INSENSITIVE, INSERT, INTERVAL, INTO, IS,
! 		JOIN, KEY, LANGUAGE, LEADING, LEFT, LIKE, LOCAL,
  		MATCH, MINUTE_P, MONTH_P, NAMES,
  		NATIONAL, NATURAL, NCHAR, NEXT, NO, NOT, NULL_P, NUMERIC,
  		OF, ON, ONLY, OPTION, OR, ORDER, OUTER_P,
--- 271,277 ----
  		FALSE_P, FETCH, FLOAT, FOR, FOREIGN, FROM, FULL,
  		GRANT, GROUP, HAVING, HOUR_P,
  		IN, INNER_P, INSENSITIVE, INSERT, INTERVAL, INTO, IS,
! 		JOIN, KEY, LANGUAGE, LEADING, LEFT, LIKE, LIMIT, LOCAL,
  		MATCH, MINUTE_P, MONTH_P, NAMES,
  		NATIONAL, NATURAL, NCHAR, NEXT, NO, NOT, NULL_P, NUMERIC,
  		OF, ON, ONLY, OPTION, OR, ORDER, OUTER_P,
***************
*** 299,305 ****
  		INCREMENT, INDEX, INHERITS, INSTEAD, ISNULL,
  		LANCOMPILER, LISTEN, LOAD, LOCATION, LOCK_P, MAXVALUE, MINVALUE, MOVE,
  		NEW, NOCREATEDB, NOCREATEUSER, NONE, NOTHING, NOTIFY, NOTNULL,
! 		OIDS, OPERATOR, PASSWORD, PROCEDURAL,
  		RECIPE, RENAME, RESET, RETURNS, ROW, RULE,
  		SEQUENCE, SERIAL, SETOF, SHOW, START, STATEMENT, STDIN, STDOUT, TRUSTED, 
  		UNLISTEN, UNTIL, VACUUM, VALID, VERBOSE, VERSION
--- 303,309 ----
  		INCREMENT, INDEX, INHERITS, INSTEAD, ISNULL,
  		LANCOMPILER, LISTEN, LOAD, LOCATION, LOCK_P, MAXVALUE, MINVALUE, MOVE,
  		NEW, NOCREATEDB, NOCREATEUSER, NONE, NOTHING, NOTIFY, NOTNULL,
! 		OFFSET, OIDS, OPERATOR, PASSWORD, PROCEDURAL,
  		RECIPE, RENAME, RESET, RETURNS, ROW, RULE,
  		SEQUENCE, SERIAL, SETOF, SHOW, START, STATEMENT, STDIN, STDOUT, TRUSTED, 
  		UNLISTEN, UNTIL, VACUUM, VALID, VERBOSE, VERSION
***************
*** 2591,2596 ****
--- 2595,2601 ----
  			 result from_clause where_clause
  			 group_clause having_clause
  			 union_clause sort_clause
+ 			 opt_select_limit
  				{
  					SelectStmt *n = makeNode(SelectStmt);
  					n->unique = $2;
***************
*** 2602,2607 ****
--- 2607,2622 ----
  					n->havingClause = $8;
  					n->unionClause = $9;
  					n->sortClause = $10;
+ 					if ($11 != NIL)
+ 					{
+ 						n->limitOffset = nth(0, $11);
+ 						n->limitCount = nth(1, $11);
+ 					}
+ 					else
+ 					{
+ 						n->limitOffset = NULL;
+ 						n->limitCount = NULL;
+ 					}
  					$$ = (Node *)n;
  				}
  		;
***************
*** 2699,2704 ****
--- 2714,2794 ----
  		| ASC									{ $$ = "<"; }
  		| DESC									{ $$ = ">"; }
  		| /*EMPTY*/								{ $$ = "<"; /*default*/ }
+ 		;
+ 
+ opt_select_limit:	LIMIT select_offset_value ',' select_limit_value
+ 	{ $$ = lappend(lappend(NIL, $2), $4); }
+ 		| LIMIT select_limit_value OFFSET select_offset_value
+ 	{ $$ = lappend(lappend(NIL, $4), $2); }
+ 		| LIMIT select_limit_value
+ 	{ $$ = lappend(lappend(NIL, NULL), $2); }
+ 		| OFFSET select_offset_value LIMIT select_limit_value
+ 	{ $$ = lappend(lappend(NIL, $2), $4); }
+ 		| OFFSET select_offset_value
+ 	{ $$ = lappend(lappend(NIL, $2), NULL); }
+ 		| /* EMPTY */
+ 	{ $$ = NIL; }
+ 		;
+ 
+ select_limit_value:	Iconst
+ 			{
+ 				Const	*n = makeNode(Const);
+ 
+ 				if ($1 < 1)
+ 					elog(ERROR, "selection limit must be ALL or a positive integer value > 0");
+ 
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)$1;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| ALL
+ 			{
+ 				Const	*n = makeNode(Const);
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)0;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| PARAM
+ 			{
+ 				Param	*n = makeNode(Param);
+ 				n->paramkind = PARAM_NUM;
+ 				n->paramid = $1;
+ 				n->paramtype = INT4OID;
+ 				$$ = (Node *)n;
+ 			}
+ 		;
+ 
+ select_offset_value:	Iconst
+ 			{
+ 				Const	*n = makeNode(Const);
+ 
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)$1;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| PARAM
+ 			{
+ 				Param	*n = makeNode(Param);
+ 				n->paramkind = PARAM_NUM;
+ 				n->paramid = $1;
+ 				n->paramtype = INT4OID;
+ 				$$ = (Node *)n;
+ 			}
  		;
  
  /*
diff -cr src.orig/backend/parser/keywords.c src/backend/parser/keywords.c
*** src.orig/backend/parser/keywords.c	Fri Oct 16 11:53:42 1998
--- src/backend/parser/keywords.c	Sun Oct 18 22:13:29 1998
***************
*** 128,133 ****
--- 128,134 ----
  	{"leading", LEADING},
  	{"left", LEFT},
  	{"like", LIKE},
+ 	{"limit", LIMIT},
  	{"listen", LISTEN},
  	{"load", LOAD},
  	{"local", LOCAL},
***************
*** 156,161 ****
--- 157,163 ----
  	{"null", NULL_P},
  	{"numeric", NUMERIC},
  	{"of", OF},
+ 	{"offset", OFFSET},
  	{"oids", OIDS},
  	{"old", CURRENT},
  	{"on", ON},
diff -cr src.orig/backend/rewrite/rewriteDefine.c src/backend/rewrite/rewriteDefine.c
*** src.orig/backend/rewrite/rewriteDefine.c	Fri Oct 16 11:53:46 1998
--- src/backend/rewrite/rewriteDefine.c	Fri Oct 16 13:48:55 1998
***************
*** 312,317 ****
--- 312,323 ----
  		heap_close(event_relation);
  
  		/*
+ 		 * LIMIT in view is not supported
+ 		 */
+ 		if (query->limitOffset != NULL || query->limitCount != NULL)
+ 			elog(ERROR, "LIMIT clause not supported in views");
+ 
+ 		/*
  		 * ... and finally the rule must be named _RETviewname.
  		 */
  		sprintf(expected_name, "_RET%s", event_obj->relname);
diff -cr src.orig/backend/tcop/pquery.c src/backend/tcop/pquery.c
*** src.orig/backend/tcop/pquery.c	Fri Oct 16 11:53:47 1998
--- src/backend/tcop/pquery.c	Fri Oct 16 14:02:36 1998
***************
*** 40,46 ****
  #include "commands/command.h"
  
  static char *CreateOperationTag(int operationType);
! static void ProcessQueryDesc(QueryDesc *queryDesc);
  
  
  /* ----------------------------------------------------------------
--- 40,46 ----
  #include "commands/command.h"
  
  static char *CreateOperationTag(int operationType);
! static void ProcessQueryDesc(QueryDesc *queryDesc, Node *limoffset, Node *limcount);
  
  
  /* ----------------------------------------------------------------
***************
*** 205,211 ****
   * ----------------------------------------------------------------
   */
  static void
! ProcessQueryDesc(QueryDesc *queryDesc)
  {
  	Query	   *parseTree;
  	Plan	   *plan;
--- 205,211 ----
   * ----------------------------------------------------------------
   */
  static void
! ProcessQueryDesc(QueryDesc *queryDesc, Node *limoffset, Node *limcount)
  {
  	Query	   *parseTree;
  	Plan	   *plan;
***************
*** 330,336 ****
  	 *	 actually run the plan..
  	 * ----------------
  	 */
! 	ExecutorRun(queryDesc, state, EXEC_RUN, 0);
  
  	/* save infos for EndCommand */
  	UpdateCommandInfo(operation, state->es_lastoid, state->es_processed);
--- 330,336 ----
  	 *	 actually run the plan..
  	 * ----------------
  	 */
! 	ExecutorRun(queryDesc, state, EXEC_RUN, limoffset, limcount);
  
  	/* save infos for EndCommand */
  	UpdateCommandInfo(operation, state->es_lastoid, state->es_processed);
***************
*** 373,377 ****
  		print_plan(plan, parsetree);
  	}
  	else
! 		ProcessQueryDesc(queryDesc);
  }
--- 373,377 ----
  		print_plan(plan, parsetree);
  	}
  	else
! 		ProcessQueryDesc(queryDesc, parsetree->limitOffset, parsetree->limitCount);
  }
diff -cr src.orig/include/executor/executor.h src/include/executor/executor.h
*** src.orig/include/executor/executor.h	Fri Oct 16 11:53:56 1998
--- src/include/executor/executor.h	Fri Oct 16 12:04:17 1998
***************
*** 83,89 ****
   * prototypes from functions in execMain.c
   */
  extern TupleDesc ExecutorStart(QueryDesc *queryDesc, EState *estate);
! extern TupleTableSlot *ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, int count);
  extern void ExecutorEnd(QueryDesc *queryDesc, EState *estate);
  extern HeapTuple ExecConstraints(char *caller, Relation rel, HeapTuple tuple);
  #ifdef QUERY_LIMIT
--- 83,89 ----
   * prototypes from functions in execMain.c
   */
  extern TupleDesc ExecutorStart(QueryDesc *queryDesc, EState *estate);
! extern TupleTableSlot *ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, Node *limoffset, Node *limcount);
  extern void ExecutorEnd(QueryDesc *queryDesc, EState *estate);
  extern HeapTuple ExecConstraints(char *caller, Relation rel, HeapTuple tuple);
  #ifdef QUERY_LIMIT

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From: jwieck@debis.com (Jan Wieck)
Subject: Re: [HACKERS] psql's help (the LIMIT stuff)
To: maillist@candle.pha.pa.us (Bruce Momjian)
Date: Thu, 22 Oct 1998 16:50:40 +0200 (MET DST)
Cc: jwieck@debis.com, jose@sferacarta.com, pgsql-hackers@postgreSQL.org
Reply-To: jwieck@debis.com (Jan Wieck)
In-Reply-To: <199810221424.KAA20601@candle.pha.pa.us> from "Bruce Momjian" at Oct 22, 98 10:24:08 am
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Status: ROr

>
> > > >
> > > >     I hope the QUERY_LIMIT too.
> > >
> > > I still have that cnfify() possible fix to review for KQSO.  Are you
> > > still thinking limit for 6.4 final, or a minor release after that?
> >
> >     I  posted the part that is the minimum applied to 6.4 to make
> >     adding LIMIT later  non-initdb  earlier.  Anyway,  here  it's
> >     again.
>
> Already applied.  I assume it is the same as the one I applied.

    Seen,  thanks.  Your 'Applied' just arrived after I packed it
    again. It's the same.

> We are close to final, and can easily put it in 6.4.1, which I am sure
> we will need, and if we split CVS trees, you'll have lots of minor
> versions to pick from.  :-)
>
> Seems like it would be a nice minor release item, but the problem is
> that minor releases aren't tested as much as major ones.  How confident
> are you in the code?  What do others thing?

    I regression tested it,  and  did  additional  tests  in  the
    SPI/PL  area.  It  works.  It only touches the parser and the
    executor. Rules, planner/optimizer just bypass the values  in
    the  parsetree.  The  parser  and  the  executor are parts of
    Postgres I feel very familiar with (not so in the optimizer).
    I  trust  in  the  code  and  would  use  it  in a production
    environment.

    It's below.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #


diff -cr src.orig/backend/commands/command.c src/backend/commands/command.c
*** src.orig/backend/commands/command.c	Fri Oct 16 11:53:38 1998
--- src/backend/commands/command.c	Fri Oct 16 12:56:44 1998
***************
*** 39,44 ****
--- 39,45 ----
  #include "utils/mcxt.h"
  #include "utils/portal.h"
  #include "utils/syscache.h"
+ #include "string.h"
  
  /* ----------------
   *		PortalExecutorHeapMemory stuff
***************
*** 101,106 ****
--- 102,108 ----
  	int			feature;
  	QueryDesc  *queryDesc;
  	MemoryContext context;
+ 	Const		limcount;
  
  	/* ----------------
  	 *	sanity checks
***************
*** 113,118 ****
--- 115,134 ----
  	}
  
  	/* ----------------
+ 	 *	Create a const node from the given count value
+ 	 * ----------------
+ 	 */
+ 	memset(&limcount, 0, sizeof(limcount));
+ 	limcount.type		= T_Const;
+ 	limcount.consttype	= INT4OID;
+ 	limcount.constlen	= sizeof(int4);
+ 	limcount.constvalue	= (Datum)count;
+ 	limcount.constisnull	= FALSE;
+ 	limcount.constbyval	= TRUE;
+ 	limcount.constisset	= FALSE;
+ 	limcount.constiscast	= FALSE;
+ 
+ 	/* ----------------
  	 *	get the portal from the portal name
  	 * ----------------
  	 */
***************
*** 176,182 ****
  	PortalExecutorHeapMemory = (MemoryContext)
  		PortalGetHeapMemory(portal);
  
! 	ExecutorRun(queryDesc, PortalGetState(portal), feature, count);
  
  	if (dest == None)			/* MOVE */
  		pfree(queryDesc);
--- 192,198 ----
  	PortalExecutorHeapMemory = (MemoryContext)
  		PortalGetHeapMemory(portal);
  
! 	ExecutorRun(queryDesc, PortalGetState(portal), feature, (Node *)NULL, (Node *)&limcount);
  
  	if (dest == None)			/* MOVE */
  		pfree(queryDesc);
diff -cr src.orig/backend/executor/execMain.c src/backend/executor/execMain.c
*** src.orig/backend/executor/execMain.c	Fri Oct 16 11:53:38 1998
--- src/backend/executor/execMain.c	Fri Oct 16 20:05:19 1998
***************
*** 64,69 ****
--- 64,70 ----
  static void EndPlan(Plan *plan, EState *estate);
  static TupleTableSlot *ExecutePlan(EState *estate, Plan *plan,
  			Query *parseTree, CmdType operation,
+ 			int offsetTuples,
  			int numberTuples, ScanDirection direction,
  			void (*printfunc) ());
  static void ExecRetrieve(TupleTableSlot *slot, void (*printfunc) (),
***************
*** 163,169 ****
   * ----------------------------------------------------------------
   */
  TupleTableSlot *
! ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, int count)
  {
  	CmdType		operation;
  	Query	   *parseTree;
--- 164,170 ----
   * ----------------------------------------------------------------
   */
  TupleTableSlot *
! ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, Node *limoffset, Node *limcount)
  {
  	CmdType		operation;
  	Query	   *parseTree;
***************
*** 171,176 ****
--- 172,179 ----
  	TupleTableSlot *result;
  	CommandDest dest;
  	void		(*destination) ();
+ 	int		offset = 0;
+ 	int		count = 0;
  
  	/******************
  	 *	sanity checks
***************
*** 191,196 ****
--- 194,289 ----
  	estate->es_processed = 0;
  	estate->es_lastoid = InvalidOid;
  
+ 	/******************
+ 	 *	if given get the offset of the LIMIT clause
+ 	 ******************
+ 	 */
+ 	if (limoffset != NULL)
+ 	{
+ 		Const		*coffset;
+ 		Param		*poffset;
+ 		ParamListInfo	paramLI;
+ 		int		i;
+ 
+ 		switch (nodeTag(limoffset))
+ 		{
+ 			case T_Const:
+ 				coffset = (Const *)limoffset;
+ 				offset = (int)(coffset->constvalue);
+ 				break;
+ 
+ 			case T_Param:
+ 				poffset = (Param *)limoffset;
+ 				paramLI = estate->es_param_list_info;
+ 
+ 				if (paramLI == NULL)
+ 					elog(ERROR, "parameter for limit offset not in executor state");
+ 				for (i = 0; paramLI[i].kind != PARAM_INVALID; i++)
+ 				{
+ 					if (paramLI[i].kind == PARAM_NUM && paramLI[i].id == poffset->paramid)
+ 						break;
+ 				}
+ 				if (paramLI[i].kind == PARAM_INVALID)
+ 					elog(ERROR, "parameter for limit offset not in executor state");
+ 				if (paramLI[i].isnull)
+ 					elog(ERROR, "limit offset cannot be NULL value");
+ 				offset = (int)(paramLI[i].value);
+ 
+ 				break;
+ 
+ 			default:
+ 				elog(ERROR, "unexpected node type %d as limit offset", nodeTag(limoffset));
+ 		}
+ 
+ 		if (offset < 0)
+ 			elog(ERROR, "limit offset cannot be negative");
+ 	}
+ 
+ 	/******************
+ 	 *	if given get the count of the LIMIT clause
+ 	 ******************
+ 	 */
+ 	if (limcount != NULL)
+ 	{
+ 		Const		*ccount;
+ 		Param		*pcount;
+ 		ParamListInfo	paramLI;
+ 		int		i;
+ 
+ 		switch (nodeTag(limcount))
+ 		{
+ 			case T_Const:
+ 				ccount = (Const *)limcount;
+ 				count = (int)(ccount->constvalue);
+ 				break;
+ 
+ 			case T_Param:
+ 				pcount = (Param *)limcount;
+ 				paramLI = estate->es_param_list_info;
+ 
+ 				if (paramLI == NULL)
+ 					elog(ERROR, "parameter for limit count not in executor state");
+ 				for (i = 0; paramLI[i].kind != PARAM_INVALID; i++)
+ 				{
+ 					if (paramLI[i].kind == PARAM_NUM && paramLI[i].id == pcount->paramid)
+ 						break;
+ 				}
+ 				if (paramLI[i].kind == PARAM_INVALID)
+ 					elog(ERROR, "parameter for limit count not in executor state");
+ 				if (paramLI[i].isnull)
+ 					elog(ERROR, "limit count cannot be NULL value");
+ 				count = (int)(paramLI[i].value);
+ 
+ 				break;
+ 
+ 			default:
+ 				elog(ERROR, "unexpected node type %d as limit count", nodeTag(limcount));
+ 		}
+ 
+ 		if (count < 0)
+ 			elog(ERROR, "limit count cannot be negative");
+ 	}
+ 
  	switch (feature)
  	{
  
***************
*** 199,205 ****
  								 plan,
  								 parseTree,
  								 operation,
! 								 ALL_TUPLES,
  								 ForwardScanDirection,
  								 destination);
  			break;
--- 292,299 ----
  								 plan,
  								 parseTree,
  								 operation,
! 								 offset,
! 								 count,
  								 ForwardScanDirection,
  								 destination);
  			break;
***************
*** 208,213 ****
--- 302,308 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 offset,
  								 count,
  								 ForwardScanDirection,
  								 destination);
***************
*** 222,227 ****
--- 317,323 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 offset,
  								 count,
  								 BackwardScanDirection,
  								 destination);
***************
*** 237,242 ****
--- 333,339 ----
  								 plan,
  								 parseTree,
  								 operation,
+ 								 0,
  								 ONE_TUPLE,
  								 ForwardScanDirection,
  								 destination);
***************
*** 691,696 ****
--- 788,794 ----
  			Plan *plan,
  			Query *parseTree,
  			CmdType operation,
+ 			int offsetTuples,
  			int numberTuples,
  			ScanDirection direction,
  			void (*printfunc) ())
***************
*** 742,747 ****
--- 840,859 ----
  		{
  			result = NULL;
  			break;
+ 		}
+ 
+ 		/******************
+ 		 *	For now we completely execute the plan and skip
+ 		 *	result tuples if requested by LIMIT offset.
+ 		 *	Finally we should try to do it in deeper levels
+ 		 *	if possible (during index scan)
+ 		 *	- Jan
+ 		 ******************
+ 		 */
+ 		if (offsetTuples > 0)
+ 		{
+ 			--offsetTuples;
+ 			continue;
  		}
  
  		/******************
diff -cr src.orig/backend/executor/functions.c src/backend/executor/functions.c
*** src.orig/backend/executor/functions.c	Fri Oct 16 11:53:38 1998
--- src/backend/executor/functions.c	Fri Oct 16 19:01:02 1998
***************
*** 130,135 ****
--- 130,138 ----
  									 None);
  		estate = CreateExecutorState();
  
+ 		if (queryTree->limitOffset != NULL || queryTree->limitCount != NULL)
+ 			elog(ERROR, "LIMIT clause from SQL functions not yet implemented");
+ 
  		if (nargs > 0)
  		{
  			int			i;
***************
*** 200,206 ****
  
  	feature = (LAST_POSTQUEL_COMMAND(es)) ? EXEC_RETONE : EXEC_RUN;
  
! 	return ExecutorRun(es->qd, es->estate, feature, 0);
  }
  
  static void
--- 203,209 ----
  
  	feature = (LAST_POSTQUEL_COMMAND(es)) ? EXEC_RETONE : EXEC_RUN;
  
! 	return ExecutorRun(es->qd, es->estate, feature, (Node *)NULL, (Node *)NULL);
  }
  
  static void
diff -cr src.orig/backend/executor/spi.c src/backend/executor/spi.c
*** src.orig/backend/executor/spi.c	Fri Oct 16 11:53:39 1998
--- src/backend/executor/spi.c	Fri Oct 16 19:25:33 1998
***************
*** 791,796 ****
--- 791,798 ----
  	bool		isRetrieveIntoRelation = false;
  	char	   *intoName = NULL;
  	int			res;
+ 	Const		tcount_const;
+ 	Node		*count = NULL;
  
  	switch (operation)
  	{
***************
*** 825,830 ****
--- 827,865 ----
  			return SPI_ERROR_OPUNKNOWN;
  	}
  
+ 	/* ----------------
+ 	 *	Get the query LIMIT tuple count
+ 	 * ----------------
+ 	 */
+ 	if (parseTree->limitCount != NULL)
+ 	{
+ 		/* ----------------
+ 		 *      A limit clause in the parsetree overrides the
+ 		 *	tcount parameter
+ 		 * ----------------
+ 		 */
+ 		count = parseTree->limitCount;
+ 	}
+ 	else
+ 	{
+ 		/* ----------------
+ 		 *      No LIMIT clause in parsetree. Use a local Const node
+ 		 *	to put tcount into it
+ 		 * ----------------
+ 		 */
+ 		memset(&tcount_const, 0, sizeof(tcount_const));
+ 		tcount_const.type           = T_Const;
+ 		tcount_const.consttype      = INT4OID;
+ 		tcount_const.constlen       = sizeof(int4);
+ 		tcount_const.constvalue     = (Datum)tcount;
+ 		tcount_const.constisnull    = FALSE;
+ 		tcount_const.constbyval     = TRUE;
+ 		tcount_const.constisset     = FALSE;
+ 		tcount_const.constiscast    = FALSE;
+ 
+ 		count = (Node *)&tcount_const;
+ 	}
+ 
  	if (state == NULL)			/* plan preparation */
  		return res;
  #ifdef SPI_EXECUTOR_STATS
***************
*** 845,851 ****
  		return SPI_OK_CURSOR;
  	}
  
! 	ExecutorRun(queryDesc, state, EXEC_FOR, tcount);
  
  	_SPI_current->processed = state->es_processed;
  	if (operation == CMD_SELECT && queryDesc->dest == SPI)
--- 880,886 ----
  		return SPI_OK_CURSOR;
  	}
  
! 	ExecutorRun(queryDesc, state, EXEC_FOR, parseTree->limitOffset, count);
  
  	_SPI_current->processed = state->es_processed;
  	if (operation == CMD_SELECT && queryDesc->dest == SPI)
diff -cr src.orig/backend/parser/analyze.c src/backend/parser/analyze.c
*** src.orig/backend/parser/analyze.c	Fri Oct 16 11:53:41 1998
--- src/backend/parser/analyze.c	Fri Oct 16 13:29:27 1998
***************
*** 180,186 ****
--- 180,190 ----
  
  		case T_SelectStmt:
  			if (!((SelectStmt *) parseTree)->portalname)
+ 			{
  				result = transformSelectStmt(pstate, (SelectStmt *) parseTree);
+ 				result->limitOffset = ((SelectStmt *)parseTree)->limitOffset;
+ 				result->limitCount = ((SelectStmt *)parseTree)->limitCount;
+ 			}
  			else
  				result = transformCursorStmt(pstate, (SelectStmt *) parseTree);
  			break;
diff -cr src.orig/backend/parser/gram.y src/backend/parser/gram.y
*** src.orig/backend/parser/gram.y	Fri Oct 16 11:53:42 1998
--- src/backend/parser/gram.y	Sun Oct 18 22:20:36 1998
***************
*** 45,50 ****
--- 45,51 ----
  #include "catalog/catname.h"
  #include "utils/elog.h"
  #include "access/xact.h"
+ #include "catalog/pg_type.h"
  
  #ifdef MULTIBYTE
  #include "mb/pg_wchar.h"
***************
*** 163,169 ****
  		sort_clause, sortby_list, index_params, index_list, name_list,
  		from_clause, from_list, opt_array_bounds, nest_array_bounds,
  		expr_list, attrs, res_target_list, res_target_list2,
! 		def_list, opt_indirection, group_clause, TriggerFuncArgs
  
  %type <node>	func_return
  %type <boolean>	set_opt
--- 164,171 ----
  		sort_clause, sortby_list, index_params, index_list, name_list,
  		from_clause, from_list, opt_array_bounds, nest_array_bounds,
  		expr_list, attrs, res_target_list, res_target_list2,
! 		def_list, opt_indirection, group_clause, TriggerFuncArgs,
! 		opt_select_limit
  
  %type <node>	func_return
  %type <boolean>	set_opt
***************
*** 192,197 ****
--- 194,201 ----
  
  %type <ival>	fetch_how_many
  
+ %type <node>	select_limit_value select_offset_value
+ 
  %type <list>	OptSeqList
  %type <defelt>	OptSeqElem
  
***************
*** 267,273 ****
  		FALSE_P, FETCH, FLOAT, FOR, FOREIGN, FROM, FULL,
  		GRANT, GROUP, HAVING, HOUR_P,
  		IN, INNER_P, INSENSITIVE, INSERT, INTERVAL, INTO, IS,
! 		JOIN, KEY, LANGUAGE, LEADING, LEFT, LIKE, LOCAL,
  		MATCH, MINUTE_P, MONTH_P, NAMES,
  		NATIONAL, NATURAL, NCHAR, NEXT, NO, NOT, NULL_P, NUMERIC,
  		OF, ON, ONLY, OPTION, OR, ORDER, OUTER_P,
--- 271,277 ----
  		FALSE_P, FETCH, FLOAT, FOR, FOREIGN, FROM, FULL,
  		GRANT, GROUP, HAVING, HOUR_P,
  		IN, INNER_P, INSENSITIVE, INSERT, INTERVAL, INTO, IS,
! 		JOIN, KEY, LANGUAGE, LEADING, LEFT, LIKE, LIMIT, LOCAL,
  		MATCH, MINUTE_P, MONTH_P, NAMES,
  		NATIONAL, NATURAL, NCHAR, NEXT, NO, NOT, NULL_P, NUMERIC,
  		OF, ON, ONLY, OPTION, OR, ORDER, OUTER_P,
***************
*** 299,305 ****
  		INCREMENT, INDEX, INHERITS, INSTEAD, ISNULL,
  		LANCOMPILER, LISTEN, LOAD, LOCATION, LOCK_P, MAXVALUE, MINVALUE, MOVE,
  		NEW, NOCREATEDB, NOCREATEUSER, NONE, NOTHING, NOTIFY, NOTNULL,
! 		OIDS, OPERATOR, PASSWORD, PROCEDURAL,
  		RECIPE, RENAME, RESET, RETURNS, ROW, RULE,
  		SEQUENCE, SERIAL, SETOF, SHOW, START, STATEMENT, STDIN, STDOUT, TRUSTED, 
  		UNLISTEN, UNTIL, VACUUM, VALID, VERBOSE, VERSION
--- 303,309 ----
  		INCREMENT, INDEX, INHERITS, INSTEAD, ISNULL,
  		LANCOMPILER, LISTEN, LOAD, LOCATION, LOCK_P, MAXVALUE, MINVALUE, MOVE,
  		NEW, NOCREATEDB, NOCREATEUSER, NONE, NOTHING, NOTIFY, NOTNULL,
! 		OFFSET, OIDS, OPERATOR, PASSWORD, PROCEDURAL,
  		RECIPE, RENAME, RESET, RETURNS, ROW, RULE,
  		SEQUENCE, SERIAL, SETOF, SHOW, START, STATEMENT, STDIN, STDOUT, TRUSTED, 
  		UNLISTEN, UNTIL, VACUUM, VALID, VERBOSE, VERSION
***************
*** 2591,2596 ****
--- 2595,2601 ----
  			 result from_clause where_clause
  			 group_clause having_clause
  			 union_clause sort_clause
+ 			 opt_select_limit
  				{
  					SelectStmt *n = makeNode(SelectStmt);
  					n->unique = $2;
***************
*** 2602,2607 ****
--- 2607,2622 ----
  					n->havingClause = $8;
  					n->unionClause = $9;
  					n->sortClause = $10;
+ 					if ($11 != NIL)
+ 					{
+ 						n->limitOffset = nth(0, $11);
+ 						n->limitCount = nth(1, $11);
+ 					}
+ 					else
+ 					{
+ 						n->limitOffset = NULL;
+ 						n->limitCount = NULL;
+ 					}
  					$$ = (Node *)n;
  				}
  		;
***************
*** 2699,2704 ****
--- 2714,2794 ----
  		| ASC									{ $$ = "<"; }
  		| DESC									{ $$ = ">"; }
  		| /*EMPTY*/								{ $$ = "<"; /*default*/ }
+ 		;
+ 
+ opt_select_limit:	LIMIT select_offset_value ',' select_limit_value
+ 	{ $$ = lappend(lappend(NIL, $2), $4); }
+ 		| LIMIT select_limit_value OFFSET select_offset_value
+ 	{ $$ = lappend(lappend(NIL, $4), $2); }
+ 		| LIMIT select_limit_value
+ 	{ $$ = lappend(lappend(NIL, NULL), $2); }
+ 		| OFFSET select_offset_value LIMIT select_limit_value
+ 	{ $$ = lappend(lappend(NIL, $2), $4); }
+ 		| OFFSET select_offset_value
+ 	{ $$ = lappend(lappend(NIL, $2), NULL); }
+ 		| /* EMPTY */
+ 	{ $$ = NIL; }
+ 		;
+ 
+ select_limit_value:	Iconst
+ 			{
+ 				Const	*n = makeNode(Const);
+ 
+ 				if ($1 < 1)
+ 					elog(ERROR, "selection limit must be ALL or a positive integer value > 0");
+ 
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)$1;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| ALL
+ 			{
+ 				Const	*n = makeNode(Const);
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)0;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| PARAM
+ 			{
+ 				Param	*n = makeNode(Param);
+ 				n->paramkind = PARAM_NUM;
+ 				n->paramid = $1;
+ 				n->paramtype = INT4OID;
+ 				$$ = (Node *)n;
+ 			}
+ 		;
+ 
+ select_offset_value:	Iconst
+ 			{
+ 				Const	*n = makeNode(Const);
+ 
+ 				n->consttype	= INT4OID;
+ 				n->constlen	= sizeof(int4);
+ 				n->constvalue	= (Datum)$1;
+ 				n->constisnull	= FALSE;
+ 				n->constbyval	= TRUE;
+ 				n->constisset	= FALSE;
+ 				n->constiscast	= FALSE;
+ 				$$ = (Node *)n;
+ 			}
+ 		| PARAM
+ 			{
+ 				Param	*n = makeNode(Param);
+ 				n->paramkind = PARAM_NUM;
+ 				n->paramid = $1;
+ 				n->paramtype = INT4OID;
+ 				$$ = (Node *)n;
+ 			}
  		;
  
  /*
diff -cr src.orig/backend/parser/keywords.c src/backend/parser/keywords.c
*** src.orig/backend/parser/keywords.c	Fri Oct 16 11:53:42 1998
--- src/backend/parser/keywords.c	Sun Oct 18 22:13:29 1998
***************
*** 128,133 ****
--- 128,134 ----
  	{"leading", LEADING},
  	{"left", LEFT},
  	{"like", LIKE},
+ 	{"limit", LIMIT},
  	{"listen", LISTEN},
  	{"load", LOAD},
  	{"local", LOCAL},
***************
*** 156,161 ****
--- 157,163 ----
  	{"null", NULL_P},
  	{"numeric", NUMERIC},
  	{"of", OF},
+ 	{"offset", OFFSET},
  	{"oids", OIDS},
  	{"old", CURRENT},
  	{"on", ON},
diff -cr src.orig/backend/rewrite/rewriteDefine.c src/backend/rewrite/rewriteDefine.c
*** src.orig/backend/rewrite/rewriteDefine.c	Fri Oct 16 11:53:46 1998
--- src/backend/rewrite/rewriteDefine.c	Fri Oct 16 13:48:55 1998
***************
*** 312,317 ****
--- 312,323 ----
  		heap_close(event_relation);
  
  		/*
+ 		 * LIMIT in view is not supported
+ 		 */
+ 		if (query->limitOffset != NULL || query->limitCount != NULL)
+ 			elog(ERROR, "LIMIT clause not supported in views");
+ 
+ 		/*
  		 * ... and finally the rule must be named _RETviewname.
  		 */
  		sprintf(expected_name, "_RET%s", event_obj->relname);
diff -cr src.orig/backend/tcop/pquery.c src/backend/tcop/pquery.c
*** src.orig/backend/tcop/pquery.c	Fri Oct 16 11:53:47 1998
--- src/backend/tcop/pquery.c	Fri Oct 16 14:02:36 1998
***************
*** 40,46 ****
  #include "commands/command.h"
  
  static char *CreateOperationTag(int operationType);
! static void ProcessQueryDesc(QueryDesc *queryDesc);
  
  
  /* ----------------------------------------------------------------
--- 40,46 ----
  #include "commands/command.h"
  
  static char *CreateOperationTag(int operationType);
! static void ProcessQueryDesc(QueryDesc *queryDesc, Node *limoffset, Node *limcount);
  
  
  /* ----------------------------------------------------------------
***************
*** 205,211 ****
   * ----------------------------------------------------------------
   */
  static void
! ProcessQueryDesc(QueryDesc *queryDesc)
  {
  	Query	   *parseTree;
  	Plan	   *plan;
--- 205,211 ----
   * ----------------------------------------------------------------
   */
  static void
! ProcessQueryDesc(QueryDesc *queryDesc, Node *limoffset, Node *limcount)
  {
  	Query	   *parseTree;
  	Plan	   *plan;
***************
*** 330,336 ****
  	 *	 actually run the plan..
  	 * ----------------
  	 */
! 	ExecutorRun(queryDesc, state, EXEC_RUN, 0);
  
  	/* save infos for EndCommand */
  	UpdateCommandInfo(operation, state->es_lastoid, state->es_processed);
--- 330,336 ----
  	 *	 actually run the plan..
  	 * ----------------
  	 */
! 	ExecutorRun(queryDesc, state, EXEC_RUN, limoffset, limcount);
  
  	/* save infos for EndCommand */
  	UpdateCommandInfo(operation, state->es_lastoid, state->es_processed);
***************
*** 373,377 ****
  		print_plan(plan, parsetree);
  	}
  	else
! 		ProcessQueryDesc(queryDesc);
  }
--- 373,377 ----
  		print_plan(plan, parsetree);
  	}
  	else
! 		ProcessQueryDesc(queryDesc, parsetree->limitOffset, parsetree->limitCount);
  }
diff -cr src.orig/include/executor/executor.h src/include/executor/executor.h
*** src.orig/include/executor/executor.h	Fri Oct 16 11:53:56 1998
--- src/include/executor/executor.h	Fri Oct 16 12:04:17 1998
***************
*** 83,89 ****
   * prototypes from functions in execMain.c
   */
  extern TupleDesc ExecutorStart(QueryDesc *queryDesc, EState *estate);
! extern TupleTableSlot *ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, int count);
  extern void ExecutorEnd(QueryDesc *queryDesc, EState *estate);
  extern HeapTuple ExecConstraints(char *caller, Relation rel, HeapTuple tuple);
  #ifdef QUERY_LIMIT
--- 83,89 ----
   * prototypes from functions in execMain.c
   */
  extern TupleDesc ExecutorStart(QueryDesc *queryDesc, EState *estate);
! extern TupleTableSlot *ExecutorRun(QueryDesc *queryDesc, EState *estate, int feature, Node *limoffset, Node *limcount);
  extern void ExecutorEnd(QueryDesc *queryDesc, EState *estate);
  extern HeapTuple ExecConstraints(char *caller, Relation rel, HeapTuple tuple);
  #ifdef QUERY_LIMIT


