Function gather_merge_getnext

src/backend/executor/nodeGatherMerge.c:547–591

* Read the next tuple for gather merge. * * Fetch the sorted tuple out of the heap. */

 * Fetch the sorted tuple out of the heap.
 */
static TupleTableSlot *
gather_merge_getnext(GatherMergeState *gm_state)
{
	int			i;

	if (!gm_state->gm_initialized)
	{
		/*
		 * First time through: pull the first tuple from each participant, and
		 * set up the heap.
		 */
		gather_merge_init(gm_state);
	}
	else
	{
		/*
		 * Otherwise, pull the next tuple from whichever participant we
		 * returned from last time, and reinsert that participant's index into
		 * the heap, because it might now compare differently against the
		 * other elements of the heap.
		 */
		i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));

		if (gather_merge_readnext(gm_state, i, false))
			binaryheap_replace_first(gm_state->gm_heap, Int32GetDatum(i));
		else
		{
			/* reader exhausted, remove it from heap */
			(void) binaryheap_remove_first(gm_state->gm_heap);
		}
	}

	if (binaryheap_empty(gm_state->gm_heap))
	{
		/* All the queues are exhausted, and so is the heap */
		gather_merge_clear_tuples(gm_state);
		return NULL;
	}
	else
	{
		/* Return next tuple from whichever participant has the leading one */
		i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
		return gm_state->gm_slots[i];
	}
}

/*
 * Read tuple(s) for given reader in nowait mode, and load into its tuple