Re: LATERAL and VOLATILE functions - Mailing list pgsql-hackers

From Pavel Stehule
Subject Re: LATERAL and VOLATILE functions
Date
Msg-id CAFj8pRBco+4-67Hvvw0vm=dKjVGUZXpPpMApWYPgY8Vn9OJoAQ@mail.gmail.com
Whole thread Raw
In response to Re: LATERAL and VOLATILE functions  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers
2012/12/15 Tom Lane <tgl@sss.pgh.pa.us>:
> Pavel Stehule <pavel.stehule@gmail.com> writes:
>> Is this behave expected?
>
>> -- unexpected
>> postgres=# select * from generate_series(1,3) g(v), LATERAL (SELECT random()) x;
>> ;
>>  v        random
>> ---+------------------
>>  1   0.63025646051392
>>  2   0.63025646051392
>>  3   0.63025646051392
>> (3 rows)
>
> The LATERAL keyword is a no-op since x doesn't contain any
> side-reference to g(v).  So you get a plain join between g and
> a single-row relation x.
>
> If the SQL standard actually specified what LATERAL means, we could
> argue about whether that's a correct interpretation or not.  I haven't
> been able to find anyplace where the spec defines the semantics though.
>
> And I'm fairly certain that we *don't* want it to mean "recompute
> for every row generated to the left of the keyword, whether there is
> a variable reference or not".  Consider for example
>
>         select ... from a, b, c join lateral d on ...
>
> If the D item only contains references to C, it's unlikely that the
> programmer wants it to be re-evaluated again for each possible row
> in A*B.

Stable and immutable functions should be recalculated once time, but
for volatile functions is recalculation probably more natural
(expected). Every time is strange, when function random() returns same
numbers. I am not sure if this behave can be problem in real usage -
probably it can be a surprise for someone who use random() for some
testing.

Regards

Pavel

>
>                         regards, tom lane



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