Re: Strange result with LATERAL query - Mailing list pgsql-hackers

Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> Something is wrong with the way chgParam is being handled in Agg nodes.
> The code in ExecReScanAgg seems to assume that if the lefttree doesn't
> have any parameter changes then it suffices to re-project the data from
> the existing hashtable; but of course this is nonsense if the parameter
> is in an input to an aggregate function.

It looks like it's sufficient to do this:

diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 1ec2515..f468fad 100644
*** a/src/backend/executor/nodeAgg.c
--- b/src/backend/executor/nodeAgg.c
*************** ExecReScanAgg(AggState *node)
*** 3425,3435 ****
                        return;

                /*
!                * If we do have the hash table and the subplan does not have any
!                * parameter changes, then we can just rescan the existing hash table;
!                * no need to build it again.
                 */
!               if (outerPlan->chgParam == NULL)
                {
                        ResetTupleHashIterator(node->hashtable, &node->hashiter);
                        return;
--- 3425,3436 ----
                        return;

                /*
!                * If we do have the hash table and there are no relevant parameter
!                * changes, then we can just rescan the existing hash table; no need
!                * to build it again.
                 */
!               if (node->ss.ps.chgParam == NULL &&
!                       outerPlan->chgParam == NULL)
                {
                        ResetTupleHashIterator(node->hashtable, &node->hashiter);
                        return;


I'm not sure if it's worth trying to distinguish whether the Param is
inside any aggregate calls or not.  The existing code gets the right
answer for

select array(select x+sum(y) from generate_series(1,3) y group by y)
  from generate_series(1,3) x;

and we'd be losing some efficiency for cases like that if we fix
it as above.  But is it worth the trouble?

                        regards, tom lane

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