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

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;

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