<Chapter>
<DocInfo>
<AuthorGroup>
<Author>
<FirstName>Gene</FirstName>
<Surname>Selkov</Surname>
</Author>
</AuthorGroup>
<Date>Transcribed 1998-02-19</Date>
</DocInfo>
<Title>GiST Indices</Title>

<Para>
<Note>
<Title>Caveat</Title>
<Para>
This extraction from an e-mail sent by 
<ULink url="mailto:selkovjr@mcs.anl.gov">Eugene Selkov Jr.</ULink>
contains good information
on GiST. Hopefully we will learn more in the future and update this information.
- thomas
</Para>
</Note>

<Para>
Well, I can't say I quite understand what's going on, but at least
I (almost) succeeded in porting GiST examples to linux. The GiST access
method is already in the postgres tree (<FileName>src/backend/access/gist</FileName>).

<Para>
<ULink url="ftp://s2k-ftp.cs.berkeley.edu/pub/gist/pggist/pggist.tgz">Examples at Berkeley</ULink>
come with an overview of the methods and demonstrate spatial index 
mechanisms for 2D boxes, polygons, integer intervals and text
(see also <ULink url="http://gist.cs.berkeley.edu:8000/gist/">GiST at Berkeley</ULink>).
In the box example, we
are supposed to see a performance gain when using the GiST index; it did
work for me but I do not have a reasonably large collection of boxes
to check that. Other examples also worked, except polygons: I got an 
error doing

<ProgramListing>
test=> create index pix on polytmp using gist (p:box gist_poly_ops) with
(islossy);
ERROR:  cannot open pix

(PostgreSQL 6.3               Sun Feb  1 14:57:30 EST 1998)
</ProgramListing>

<Para>
I could not get sense of this error message; it appears to be something
we'd rather ask the developers about (see also Note 4 below). What I
would suggest here is that someone of you linux guys (linux==gcc?) fetch the 
original sources quoted above and apply my patch (see attachment) and 
tell us what you feel about it. Looks cool to me, but I would not like 
to hold it up while there are so many competent people around.

<Para>
A few notes on the sources:

<Para>
1. I failed to make use of the original (HPUX) Makefile and rearranged
   the Makefile from the ancient postgres95 tutorial to do the job. I
tried
   to keep it generic, but I am a very poor makefile writer -- just did
   some monkey work. Sorry about that, but I guess it is now a little
   more portable that the original makefile.

<Para>
2. I built the example sources right under pgsql/src (just extracted the
   tar file there). The aforementioned Makefile assumes it is one level
   below pgsql/src (in our case, in pgsql/src/pggist).

<Para>
3. The changes I made to the *.c files were all about #include's,
   function prototypes and typecasting. Other than that, I just threw 
   away a bunch of unused vars and added a couple parentheses to please
   gcc. I hope I did not screw up too much :)

<Para>
4. There is a comment in polyproc.sql:

<ProgramListing>
-- -- there's a memory leak in rtree poly_ops!!
-- -- create index pix2 on polytmp using rtree (p poly_ops);
</ProgramListing>

   Roger that!! I thought it could be related to a number of
   <ProductName>Postgres</ProductName> versions
   back and tried the query. My system went nuts and I had to shoot down
   the postmaster in about ten minutes.


<Para>
I will continue to look into GiST for a while, but I would also
appreciate
more examples of R-tree usage.

</Chapter>
