Re: Slow set-returning functions

Dean Rasheed wrote:
> I have been having difficulty with some functions which return sets of
> rows. The functions seem to run very slowly, even though the queries
> they run execute very quicky if I run them directly from psgl.
> Typically these queries are only returning a few hundred rows with my
> real data.
>
> I have had difficulty coming up with a simple test case, but the code
> below usually shows the same problem. Sometimes I have to run the
> setup code a few times before it happens - not sure why (I would
> expect this to be deterministic), but perhaps there is some randomness
> introduced by the sampling done by the analyse.
>
> The function foo() which has a hard-coded LIMIT always executes
> quickly (comparable to running the query directly).
>
> However, the function foo(int) which is passed the same LIMIT as a
> parameter executes around 30 times slower. The only difference is that
> the LIMIT is a parameter to the function, although the LIMIT isn't
> reached anyway in this case. Sometimes running this same script
> generates data for which this function executes as fast as the other
> one (which is always fast).

This is clearly because the planner doesn't know what the value for the
parameter will be at run time, so it chooses a plan that's not optimal
for LIMIT 100.

> Is there any way that I can see what execution plan is being used
> internally by the functions?

Not directly, but you can do this:

postgres=# PREPARE p (int4) AS SELECT id FROM foo WHERE lower(name) LIKE
'foo' ORDER BY id OFFSET 0 LIMIT $1;
PREPARE
postgres=# EXPLAIN EXECUTE p(100); QUERY
PLAN
-----------------------------------------------------------------------------
Limit (cost=0.00..49.18 rows=2 width=4)
-> Index Scan using foo_pkey on foo (cost=0.00..614.77 rows=25
width=4)
Filter: (lower(name) ~~ 'foo'::text)
(3 rows)

You could work around that by using EXECUTE in the plpgsql function, to
force the query to be planned on every execution with the actual value
of the LIMIT.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com