Re: Calculating statistic via function rather than with query is slowing my query

Hello

2011/8/17 Anish Kejariwal <anishkej(at)gmail(dot)com>:
> Hi everyone,
> I'm using postgres 9.0.3, and here's the OS I'm running this on:
> Linux 2.6.18-238.12.1.el5xen #1 SMP Tue May 31 14:02:29 EDT 2011 x86_64
> x86_64 x86_64 GNU/Linux
> I have a fairly straight forward query.  I'm doing a group by on an ID, and
> then calculating some a statistic on the resulting data.  The problem I'm
> running into is that when I'm calculating the statistics via a function,
> it's twice as slow as when I'm calculating the statistics directly in my
> query.  I want to be able to use a function, since I'll be using this
> particular calculation in many places.
> Any idea of what's going on?  Below, I've included my function, and both
> queries (I removed the type_ids, and just wrote …ids…
> Here's my function (I also tried stable):
> CREATE OR REPLACE FUNCTION calc_test(a double precision, b integer, c
> integer)
> RETURNS double precision AS $$
> BEGIN
>         return a/b/c* 1000000000::double precision;
> END;
> $$ LANGUAGE plpgsql immutable;
>

this is overhead of plpgsql call. For this simple functions use a SQL
functions instead

CREATE OR REPLACE FUNCTION calc_test(a double precision, b integer, c
integer)
RETURNS double precision AS $$
> SELECT $1/$2/$3* 1000000000::double precision;
> $$ LANGUAGE sql;

Regards

Pavel Stehule

> The query that takes 7.6 seconds, when I calculate the statistic from within
> the query:
> explain analyze
> select
>    agg.primary_id,
>    avg(agg.a / agg.b / agg.c * 1000000000::double precision) foo,
>    stddev(agg.a / agg.b / agg.c * 1000000000::double precision) bar
> from mytable agg
> where agg.type_id in (....ids....)
> group by agg.primary_id;
> The execution plan:
>  HashAggregate  (cost=350380.58..350776.10 rows=9888 width=20) (actual
> time=7300.414..7331.659 rows=20993 loops=1)
>    ->  Bitmap Heap Scan on mytable agg  (cost=28667.90..337509.63
> rows=1716127 width=20) (actual time=200.064..2861.600 rows=2309230 loops=1)
>          Recheck Cond: (type_id = ANY ('{....ids....}'::integer[]))
>          ->  Bitmap Index Scan on mytable_type_id_idx  (cost=0.00..28238.87
> rows=1716127 width=0) (actual time=192.725..192.725 rows=2309230 loops=1)
>                Index Cond: (type_id = ANY ('{....ids....}'::integer[]))
>  Total runtime: 7358.337 ms
> (6 rows)
>
>
>
> The same query, but now I'm calling the function.  When I call the function
> it's taking 15.5 seconds.
> explain analyze select
>    agg.primary_id,
>    avg(calc_test(agg.a,agg.b,agg.c)) foo,
>    stddev(calc_test(agg.a,agg.b,agg.c)) bar
> from mytable agg
> where agg.type_id in (....ids....)
> group by agg.primary_id;
> and, here's the execution plan:
>  HashAggregate  (cost=350380.58..355472.90 rows=9888 width=20) (actual
> time=13660.838..13686.618 rows=20993 loops=1)
>    ->  Bitmap Heap Scan on mytable agg  (cost=28667.90..337509.63
> rows=1716127 width=20) (actual time=170.385..2881.122 rows=2309230 loops=1)
>          Recheck Cond: (type_id = ANY ('{....ids....}'::integer[]))
>          ->  Bitmap Index Scan on mytable_type_id_idx  (cost=0.00..28238.87
> rows=1716127 width=0) (actual time=162.834..162.834 rows=2309230 loops=1)
>                Index Cond: (type_id = ANY ('{....ids....}'::integer[]))
>  Total runtime: 13707.560 ms
>
> Thanks!
> Anish