Re: Bad prepare performance

Le Monday 31 March 2008, Martin Kjeldsen a écrit :
> Hello,
>
> I'm sorry if this has been discussed before, I couldn't find a discussion
> about this problem.
>
> I've done the same query on a 8.2.5 database. The first one is prepared
> first and the other is executed directly.
>
> I understand why the there is such a great difference between the two ways
> of executing the query (postgres has no way of knowing that $1 will be
> quite big and that the result is not too big).
>
> I could just avoid using prepare statements, but this is done automatically
> with Perl's DBD::Pg. I know how to avoid using prepare statements (avoid
> having placeholders in the statement), but that is not the prettiest of
> work arounds.

Did you saw this option :

$sth = $dbh->prepare("SELECT id FROM mytable WHERE val = ?",
{ pg_server_prepare => 0 });

Then, *this* query will not be prepared by the server.

> Is there any planner hints I can use or anything happened or
> happening with 8.3 or later that I can use?
>
> Thank you in advance and the following is the EXPLAIN ANALYZE for the
> queries.
>
> Best regards
>
>
> Martin Kjeldsen
>
> -----
>
> PREPARE test_x (INT) AS SELECT * FROM v_rt_trap_detailed WHERE guid > $1
> ORDER BY created LIMIT 3000;
>
> EXPLAIN ANALYZE EXECUTE test_x (116505531);
>
> QUERY PLAN
> ---------------------------------------------------------------------------
>----------------------------------------------------------------------------
>------------------------------- Limit (cost=1143870.95..1143878.45
> rows=3000 width=267) (actual time=83033.101..83033.111 rows=4 loops=1) ->
> Sort (cost=1143870.95..1148074.36 rows=1681367 width=267) (actual
> time=83033.099..83033.103 rows=4 loops=1) Sort Key: rt_trap.created
> -> Merge Left Join (cost=0.00..829618.73 rows=1681367 width=267)
> (actual time=83032.946..83033.051 rows=4 loops=1) Merge Cond: (rt_trap.guid
> = tp.trap_guid)
> -> Index Scan using idx_rt_trap_guid on rt_trap
> (cost=0.00..81738.88 rows=1681367 width=192) (actual time=0.012..0.020
> rows=4 loops=1) Index Cond: (guid > $1)
> Filter: (deleted IS NULL)
> -> Index Scan using idx_rt_trap_param_trap_guid on
> rt_trap_param tp (cost=0.00..706147.04 rows=4992440 width=79) (actual
> time=6.523..78594.750 rows=5044927 loops=1) Filter: (param_oid =
> 'snmpTrapOID.0'::text)
> Total runtime: 83033.411 ms
> (11 rows)
>
> dmon2=# EXPLAIN ANALYZE SELECT * FROM v_rt_trap_detailed WHERE guid >
> 116505531 ORDER BY created LIMIT 3000; QUERY PLAN
> ---------------------------------------------------------------------------
>----------------------------------------------------------------------------
>------------- Limit (cost=9866.45..9867.71 rows=504 width=267) (actual
> time=0.590..0.616 rows=12 loops=1) -> Sort (cost=9866.45..9867.71
> rows=504 width=267) (actual time=0.587..0.596 rows=12 loops=1) Sort Key:
> rt_trap.created
> -> Nested Loop Left Join (cost=0.00..9843.83 rows=504 width=267)
> (actual time=0.157..0.531 rows=12 loops=1) -> Index Scan using
> idx_rt_trap_guid on rt_trap (cost=0.00..26.78 rows=504 width=192) (actual
> time=0.022..0.034 rows=12 loops=1) Index Cond: (guid > 116505531)
> Filter: (deleted IS NULL)
> -> Index Scan using idx_rt_trap_param_trap_guid on
> rt_trap_param tp (cost=0.00..18.36 rows=89 width=79) (actual
> time=0.006..0.009 rows=1 loops=12) Index Cond: (rt_trap.guid =
> tp.trap_guid)
> Filter: (param_oid = 'snmpTrapOID.0'::text)
> Total runtime: 0.733 ms
> (11 rows)

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