Re: Performance problems with prepared statements

Theo Kramer a écrit :
> 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...
>
my two cents:
perhaps ... please check that with your C code
And be sure you are not providing time from application. If you have a
lot of data and/or  a lag  on your lan, it can be the cause of your so
big difference between psql and C