From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | "Sergey E(dot) Koposov" <math(at)sai(dot)msu(dot)ru> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: problem with volatile functions in subselects ? |
Date: | 2006-07-30 20:35:51 |
Message-ID: | 3576.1154291751@sss.pgh.pa.us |
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"Sergey E. Koposov" <math(at)sai(dot)msu(dot)ru> writes:
> I see the very strange behaviour with the following set of queries:
> wsdb=# select na,nb, na::double precision as da, nb::double precision as db from ( select random()::numeric as na,random()::numeric as nb from generate_series(1,2)) as xx;
The planner "flattens" this query to a single level of SELECT, so what
you effectively have is
select random()::numeric as na,
random()::numeric as nb,
random()::numeric::double precision as da,
random()::numeric::double precision as db
from generate_series(1,2) as xx;
There's been some talk about prohibiting flattening if there are any
volatile functions in the subselect's targetlist, but nothing's been
done about that. Disabling flattening is a sufficiently big hit to
the planner's optimization ability that it shouldn't be done lightly.
regards, tom lane
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