From: Theo Kramer <theo@flame.co.za>
To: pgsql-performance@postgresql.org
Subject: Re: Performance problems with prepared statements
Date: Wed, 10 Oct 2007 21:34:00 +0200
Message-ID: <1192044840.2549.21.camel@fedmac> (raw)
In-Reply-To: <470CE918.4060608@dalibo.com>
References: <1192027540.2495.3.camel@localhost.localdomain>
<470CE918.4060608@dalibo.com>
On Wed, 2007-10-10 at 17:00 +0200, Cédric Villemain wrote:
> <snip>
> Reading the manual, you can learn that prepared statement can (not)
> follow the same plan as direct query:
> the plan is make before pg know the value of the variable.
>
> See 'Notes' http://www.postgresql.org/docs/8.2/interactive/sql-prepare.html
Thanks, had missed that, however, I am afraid that I fail to see how
preparing a query using PQprepare() and then executing it using
PQexecPrepared(), is 8 thousand times slower than directly executing
it.,, ( 403386.583ms/50.0ms = 8067 ).
When doing a 'manual' prepare and explain analyze I get the following
rascal=# prepare cq (char(12), smallint, integer) as SELECT oid,
calllog_mainteng, calllog_phase, calllog_self FROM calllog
WHERE calllog_mainteng = $1
AND calllog_phase = $2
AND calllog_self < $3
OR calllog_mainteng = $1
AND calllog_phase < $2
ORDER BY calllog_mainteng DESC,
calllog_phase DESC,
calllog_self DESC limit 25;
PREPARE
rascal=# explain analyze execute cq ('124 ', 8, 366942);
QUERY
PLAN
---------------------------------------------------------------------------
Limit (cost=0.00..232.73 rows=25 width=26) (actual time=2.992..3.178
rows=25 loops=1)
-> Index Scan Backward using calllog_rmc_idx on calllog
(cost=0.00..38651.38 rows=4152 width=26) (actual time=2.986..3.116
rows=25 loops=1)
Index Cond: (calllog_mainteng = $1)
Filter: (((calllog_phase = $2) AND (calllog_self < $3)) OR
(calllog_phase < $2))
Total runtime: 3.272 ms
So I suspect that there is something more fundamental here...
--
Regards
Theo
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To: pgsql-performance@postgresql.org
Cc: theo@flame.co.za
Subject: Re: Performance problems with prepared statements
In-Reply-To: <1192044840.2549.21.camel@fedmac>
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