Re: Improve performance of query

Richard Rowell <richard(at)bowmansystems(dot)com> writes:
> I'm trying to port our application from MS-SQL to Postgres. We have
> implemented all of our rather complicated application security in the
> database. The query that follows takes a half of a second or less on
> MS-SQL server and around 5 seconds on Postgres.

The EXPLAIN shows that most of the time is going into repeated
executions of svp_getparentproviderids() in the first UNION arm:

> -> Seq Scan on da_answer a (cost=0.00..63928.75 rows=10540 width=24) (actual time=279.080..4418.808 rows=161 loops=1)
> Filter: ((date_effective <= 9999999999::double precision) AND (inactive <> 1) AND (subplan))
> SubPlan
> -> Function Scan on svp_getparentproviderids (cost=0.00..15.00 rows=5 width=4) (actual time=0.203..0.203 rows=0 loops=21089)
> Filter: (svp_getparentproviderids = $1)

I'd suggest replacing the EXISTS coding by IN:
(EXISTS (SELECT * FROM svp_getparentproviderids(1) WHERE svp_getparentproviderids = a.provider_id))
to
(a.provider_id IN (SELECT * FROM svp_getparentproviderids(1)))
The latter form is likely to be significantly faster in PG 7.4.

It's also possible that the speed loss compared to MSSQL is really
inside the svp_getparentproviderids function; you should look into
that rather than assuming this query per se is at fault.

Also, do you actually need UNION as opposed to UNION ALL? The
duplicate-elimination behavior of UNION is a bit expensive if not
needed. It looks from the EXPLAIN output that some of the unions
aren't actually eliminating any rows.

regards, tom lane