Re: Slow function in queries SELECT clause.

2010/6/19 Davor J. <DavorJ(at)live(dot)com>

> 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
>
>
>
>
Hi,
show us the code of those two functions and explain analyze of those
queries.

regards
Szymon Guz