From: | "Davor J(dot)" <DavorJ(at)live(dot)com> |
---|---|
To: | pgsql-performance(at)postgresql(dot)org |
Subject: | Slow function in queries SELECT clause. |
Date: | 2010-06-19 19:38:14 |
Message-ID: | hvj6cg$2dna$1@news.hub.org |
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Thread: | |
Lists: | pgsql-performance |
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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