From ronz@ravensfield.com Tue May 22 17:35:37 2001
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From: Andrew Rawnsley <ronz@ravensfield.com>
Organization: Ravensfield Geographic
To: Bruce Momjian <pgman@candle.pha.pa.us>
Subject: Re: [GENERAL] Queries across multiple databases (was: SELECT from a table in another database).
Date: Tue, 22 May 2001 17:37:25 -0400
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cc: Tom Lane <tgl@sss.pgh.pa.us>
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Status: ORr

On Tuesday 22 May 2001 12:37am, Bruce Momjian wrote:
> Can you send me a little sample of SCHEMA use?   

Pardon if this is more long-winded or tangental than you are looking for...

What may beconfusing many people (not excluding myself from time to time) is 
that cross-schema queries may have nothing to do with cross-database queries, 
which is an entirely different kettle of trout.... SCHEMAs as used by at 
least by Oracle and Sybase are nothing more than users/object owners (I have 
no experience with DB2 or Informix, or anything more exotic than that).

Just off the top of my head, what would satisfy most people would be to be 
able to refer to objects as OWNER.OBJECT, with owner being 'within' the 
database (i.e. DATABASE.OWNER.OBJECT, which is how Sybase does it. Oracle has 
no 'database' parallel like that).  Whether you do it Oracle-fashion and use 
the term SCHEMA for owner pretty universally or Sybase fashion and just pay 
lip service to the word doesn't really matter (unless there is a standards 
compliance issue). 

As to creating schemas...In Oracle you have to execute the CREATE SCHEMA 
AUTHORIZATION <user> while logged in as that user before you can add objects 
under that user's ownership. While it seems trivial, if you have a situation 
where you do not want to grant a user session rights, you have to grant them 
session rights, log in as them, execute CREATE SCHEMA..., then revoke the 
session rights. Bah. A table created by user X in schema Y is also owned by 
user Y, and its user Y that has to have many of the object rights to create 
that table.

In Sybase, its essentially the same except the only real use for the CREATE 
SCHEMA command is for compliance and to group some DDL commands together. 
Other than that, Sybase always refers to schemas as owners. You don't have to 
execute CREATE SCHEMA... to create objects - you just need the rights. I've 
never used it at least - the only thing I see in it is eliminating the need 
to type 'go' after every DDL command.

As for examples from Oracle space -

Here is a foreign key reference with delete triggers from a table in 
schema/user PROJECT to tables in schemas/users SERVICES and WEBCAL:

CREATE TABLE PROJECT.tasks_users (
   event_id INTEGER REFERENCES WEBCAL.tasks(event_id) ON DELETE CASCADE,
   user_id VARCHAR2(25) REFERENCES SERVICES.users(user_id) ON DELETE CASCADE,
   confirmed CHAR(1),
   PRIMARY KEY (event_id,user_id)
);

A join between tables in  would be 
SELECT   A.SAMPLE_ID, 
                 A.CONCENTRATION, 
                 A.CASNO, 
                 B.PARAMETER,
                 C.DESCRIPTION AS STYPE 
         FROM HAI.RESULTS A, SAMPLETRACK.PARAMETERS B, 
SAMPLETRACK.SAMPLE_TYPE C
                 WHERE A.CASNO = B.CASNO AND A.SAMPLE_TYPE = B.SAMPLE_TYPE

In both Oracle and Sybase, all the objects are in the same 'database' 
(instance in Oracle), as I assume they would be in Postgres.  There is I 
assume a name space issue - one should be able to create a FOO.BAR and a 
BAR.BAR in the same database.

> I may be adding it to
> 7.2 inside the same code that maps temp table names to real tables.
>

Excellent!  I see light at the end of the tunnel (I will say the Postgres 
maintainers are among the most solidly competent around - one never has any 
real doubts about the system's progress).

-- 
Regards,

Andrew Rawnsley
Ravensfield Digital Resource Group, Ltd.
(740) 587-0114
www.ravensfield.com

From reedstrm@rice.edu Wed May 23 10:59:42 2001
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Date: Wed, 23 May 2001 09:56:41 -0500
From: "Ross J. Reedstrom" <reedstrm@rice.edu>
To: Bruce Momjian <pgman@candle.pha.pa.us>
Subject: Re: [HACKERS] Re: [GENERAL] Re: [GENERAL] Queries across multiple databases ?(was: SELECT from a table in another database).
Message-ID: <20010523095641.D23741@rice.edu>
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In-Reply-To: <200105221131.f4MBVIc28574@candle.pha.pa.us>; from pgman@candle.pha.pa.us on Tue, May 22, 2001 at 07:31:18AM -0400
Status: ORr

Bruce - 
Around the first of the year, I started playing around with a schema
implementation. As you may recall, my first crack at changing file storage
names about a year ago was motivated by the need to avoid collisions
once schema were available.

Anyway, now that Vadim has removed a lot of the internal dependence
on relname for keeping track of relations, using the new relfinenode
many places relname used to be used, it seems to me that adding a
parallel schemaname to all the data structures that use relname isn't
as cumbersome as Peter might think. I hadn't tackled the relcache yet:
perhaps concatenating the schemaname and relname to use as the hash key
is the way to go for that.

Unfortunately, all that code is now 4 months old, and on my machine
at home.  I has started with the rangetable entries, because hacked the
parser to allow 'SELECT * FROM schemaname.tablename' was easier than
'select schemaname.tablename.fieldname FROM', since the dot function
calling convention isn't allowed in the range list, while it is in the
target list. 

I seem to recall tripping up on the query plan printer, of all things,
before other, paying work pushed it aside. I'll see if I can update that
code to the current tree, and send you something, if you'd like.

Ross



On Tue, May 22, 2001 at 07:31:18AM -0400, Bruce Momjian wrote:
> > > I'm not sure whether it is quite the way to do it, but I'd have a better
> > > time with things if I could span databases in a single request. Are
> > > there theoretical problems with spanning databases in a single query? Is
> > > it a feature of bad database design & implementation?
> > 
> > I think the developers are planning full schema support for the relatively
> > near future (possibly even 7.2, but check the archives and see what's been
> > said). Although it looks easy to access a table from another database,
> > things can rapidly become more complicated as you start having to deal with
> > transactions, triggers, rules, constraints...
> 
> Schema is on my radar screen for 7.2.  I am waiting to do some research
> in what needs to be done, but my initial idea is to use the system cache
> to do namespace mapping, just like is done now for temp tables.
> 
> -- 
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@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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