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Alex Ignatov <a.ignatov@postgrespro.ru> writes:
> In this subquery(below) we have reference to outer variables but it is
> not working as it should(or i dont understand something):
> postgres=# postgres=# select id, ( select string_agg('a','') from
> generate_series(1,trunc(10*random()+1)::int) where id=id) from
> generate_series(1,10) as id;
The inner generate_series() call does not contain any outer references, so
it doesn't get recomputed. There's a comment in ExecReScanFunctionScan
about that:
* Here we have a choice whether to drop the tuplestores (and recompute * the function outputs) or just rescan
them. We must recompute if an * expression contains changed parameters, else we rescan. * * XXX maybe we
shouldrecompute if the function is volatile? But in * general the executor doesn't conditionalize its actions on
that.
So you get some random number of generate_series output rows on the first
call, but then each subsequent run of the subquery just rescans those same
rows. I do not think this is wrong or bad, really; if it was done
differently, examples such as this same generate_series call on the inside
of a nestloop join would behave very strangely.
regards, tom lane