From: Davor J. <DavorJ@live.com>
To: pgsql-performance@postgresql.org
Subject: Slow function in queries SELECT clause.
Date: Sat, 19 Jun 2010 21:38:14 +0200
Message-ID: <hvj6cg$2dna$1@news.hub.org> (raw)
I think I have read what is to be read about queries being prepared in
plpgsql functions, but I still can not explain the following, so I thought
to post it here:
Suppose 2 functions: factor(int,int) and offset(int, int).
Suppose a third function: convert(float,int,int) which simply returns
$1*factor($2,$3)+offset($2,$3)
All three functions are IMMUTABLE.
Very simple, right? Now I have very fast AND very slow executing queries on
some 150k records:
VERY FAST (half a second):
----------------
SELECT data*factor(1,2)+offset(1,2) FROM tbl_data;
VERY SLOW (a minute):
----------------
SELECT convert(data, 1, 2) FROM tbl_data;
The slowness cannot be due to calling a function 150k times. If I define
convert2(float,int,int) to return a constant value, then it executes in
about a second. (still half as slow as the VERY FAST query).
I assume that factor and offset are cached in the VERY FAST query, and not
in the slow one? If so, why not and how can I "force" it? Currently I need
only one function for conversions.
Regards,
Davor
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To: pgsql-performance@postgresql.org
Cc: DavorJ@live.com
Subject: Re: Slow function in queries SELECT clause.
In-Reply-To: <hvj6cg$2dna$1@news.hub.org>
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