Function find_hash_columns

src/backend/executor/nodeAgg.c:1624–1749

* Compute columns that actually need to be stored in hashtable entries. The * incoming tuples from the child plan node will contain grouping columns, * other columns referenced in our targetlist and qual, columns used to * compute the aggregate functions, and perhaps just junk columns we don't use * at all. Only columns of the first two types need to be stored in the * hashtable, and gettin

 * the per-query context (unlike the hash table itself).
 */
static void
find_hash_columns(AggState *aggstate)
{
	Bitmapset  *base_colnos;
	Bitmapset  *aggregated_colnos;
	TupleDesc	scanDesc = aggstate->ss.ss_ScanTupleSlot->tts_tupleDescriptor;
	List	   *outerTlist = outerPlanState(aggstate)->plan->targetlist;
	int			numHashes = aggstate->num_hashes;
	EState	   *estate = aggstate->ss.ps.state;
	int			j;

	/* Find Vars that will be needed in tlist and qual */
	find_cols(aggstate, &aggregated_colnos, &base_colnos);
	aggstate->colnos_needed = bms_union(base_colnos, aggregated_colnos);
	aggstate->max_colno_needed = 0;
	aggstate->all_cols_needed = true;

	for (int i = 0; i < scanDesc->natts; i++)
	{
		int			colno = i + 1;

		if (bms_is_member(colno, aggstate->colnos_needed))
			aggstate->max_colno_needed = colno;
		else
			aggstate->all_cols_needed = false;
	}

	for (j = 0; j < numHashes; ++j)
	{
		AggStatePerHash perhash = &aggstate->perhash[j];
		Bitmapset  *colnos = bms_copy(base_colnos);
		AttrNumber *grpColIdx = perhash->aggnode->grpColIdx;
		List	   *hashTlist = NIL;
		TupleDesc	hashDesc;
		int			maxCols;
		int			i;

		perhash->largestGrpColIdx = 0;

		/*
		 * If we're doing grouping sets, then some Vars might be referenced in
		 * tlist/qual for the benefit of other grouping sets, but not needed
		 * when hashing; i.e. prepare_projection_slot will null them out, so
		 * there'd be no point storing them.  Use prepare_projection_slot's
		 * logic to determine which.
		 */
		if (aggstate->phases[0].grouped_cols)
		{
			Bitmapset  *grouped_cols = aggstate->phases[0].grouped_cols[j];
			ListCell   *lc;

			foreach(lc, aggstate->all_grouped_cols)
			{
				int			attnum = lfirst_int(lc);

				if (!bms_is_member(attnum, grouped_cols))
					colnos = bms_del_member(colnos, attnum);
			}