LATERAL and VOLATILE functions

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.

regards, tom lane

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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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