An oversight in ExecInitAgg for grouping sets

I happened to notice $subject. It happens when we build eqfunctions for
each grouping set.

/* for each grouping set */
for (int k = 0; k < phasedata->numsets; k++)
{
int length = phasedata->gset_lengths[k];

if (phasedata->eqfunctions[length - 1] != NULL)
continue;

phasedata->eqfunctions[length - 1] =
execTuplesMatchPrepare(scanDesc,
length,
aggnode->grpColIdx,
aggnode->grpOperators,
aggnode->grpCollations,
(PlanState *) aggstate);
}

If it is an empty grouping set, its length will be zero, and accessing
phasedata->eqfunctions[length - 1] is not right.

I think we can just skip building the eqfunctions for empty grouping
set.

--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -3494,6 +3494,10 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
{
int length = phasedata->gset_lengths[k];

+ /* skip empty grouping set */
+ if (length == 0)
+ continue;
+
if (phasedata->eqfunctions[length - 1] != NULL)
continue;

Thanks
Richard