Re: LATERAL and VOLATILE functions

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)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