Re: GiST index performance

On Thu, 16 Apr 2009, Tom Lane wrote:
> Matthew, can you put together a self-contained test case with a similar
> slowdown?

It isn't the smoking gun I thought it would be, but:

CREATE TABLE a AS SELECT a FROM generate_series(1,1000000) AS a(a);
CREATE TABLE b AS SELECT b FROM generate_series(1,1000000) AS b(b);

ANALYSE;

CREATE INDEX a_a ON a (a);

EXPLAIN ANALYSE SELECT * FROM a, b WHERE a.a BETWEEN b.b AND b.b + 2;

DROP INDEX a_a;
CREATE INDEX a_a ON a USING gist (a);

EXPLAIN ANALYSE SELECT * FROM a, b WHERE a.a BETWEEN b.b AND b.b + 2;

I see four seconds versus thirty seconds. The difference was much greater
on my non-test-case - I wonder if multi-column indexing has something to
do with it.

> Also, what are the physical sizes of the two indexes?

relname | pg_size_pretty
----------------------------+----------------
location_object_start_gist | 193 MB
location_object_start | 75 MB
(2 rows)

> I notice that the inner nestloop join gets slower too, when it's not
> changed at all --- that suggests that the overall I/O load is a lot
> worse, so maybe the reason the query is falling off a performance cliff
> is that the GIST index fails to fit in cache.

Memory in the machine is 16GB.

Matthew

--
[About NP-completeness] These are the problems that make efficient use of
the Fairy Godmother. -- Computer Science Lecturer