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Important speed difference between a query and a function with the same query

From: Frederic Jolliton <fred-pg(at)jolliton(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Important speed difference between a query and a function with the same query
Date: 2003-04-23 17:53:55
Message-ID: 8665p56qx8.fsf@mau.localdomain (view raw or flat)
Thread:
Lists: pgsql-performance
Hi,

(PostgreSQL 7.3.2 on i386-portbld-freebsd4.7, compiled by GCC 2.95.4)

I've a curious performance problem with a function returning set of
rows.  The query alone is very fast, but not when called from the
function.

To "emulate" a parametred view¹, I created a function as follow:

CREATE FUNCTION get_info (integer) RETURNS SETOF type_get_info
    AS '...' <- here the query show below, where 'LIMIT $1' is used instead of 'LIMIT 10'
    LANGUAGE sql;

The table table1 have 330K rows, and table2 have 3K rows.

When I run the following query (prefixed with SELECT * to try to get
the same behavior that the second query), I obtain very good time.

database=# SELECT * FROM (
	(SELECT a.field1,a.field2,a.field3,b.field3,b.field4,a.field5
		FROM table1 AS a, table1 AS b
		WHERE a.field6=b.field4
		ORDER BY a.field6 DESC
		LIMIT 10)
	UNION
	(SELECT a.field1,a.field2,b.field3,a.field3,a.field4,b.field5
		FROM table2 AS a, table1 AS b
		WHERE a.field4=b.field6
		ORDER BY a.field4 DESC
		LIMIT 10)
	ORDER BY field4 DESC
	LIMIT 10
) AS type_get_info;

[...]
(10 rows)

Time: 185.86 ms

But, when I run the function (with 10 as parameter, but even 1 is
slow) I get poor time:

database=# SELECT * FROM get_info(10);
[...]
(10 rows)

Time: 32782.26 ms
database=# 

(even after a VACUUM FULL ANALYZE, and REINDEX of indexes used in the
queries)

What is curious is that I remember that the function was fast at a
time..

What is the difference between the two case ?

[1] Is there another solution to this 'hack' ? I can't simply create a
view and use 'LIMIT 10' because intermediate SELECT have be limited
too (to avoid UNION with 300K rows where only the first 10 are of
interest to me.)

-- 
Frédéric Jolliton


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Subject: Re: Important speed difference between a query and a
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