// src\backend\executor\nodeAgg.c:

static TupleTableSlot *
ExecAgg(PlanState *pstate)
{
	AggState   *node = castNode(AggState, pstate);
	TupleTableSlot *result = NULL;

	CHECK_FOR_INTERRUPTS();

	if (!node->agg_done)
	{
		/* Dispatch based on strategy */
		switch (node->phase->aggstrategy)
		{
			case AGG_HASHED:
				if (!node->table_filled)
					agg_fill_hash_table(node);
				/* FALLTHROUGH */
			case AGG_MIXED:
				result = agg_retrieve_hash_table(node);
				break;
			case AGG_PLAIN:
			case AGG_SORTED:
				result = agg_retrieve_direct(node);
				break;
		}

		if (!TupIsNull(result))
			return result;
	}

	return NULL;
}