Function spool_tuples

src/backend/executor/nodeWindowAgg.c:1500–1566

* Read tuples from the outer node, up to and including position 'pos', and * store them into the tuplestore. If pos is -1, reads the whole partition. */

 * store them into the tuplestore. If pos is -1, reads the whole partition.
 */
static void
spool_tuples(WindowAggState *winstate, int64 pos)
{
	WindowAgg  *node = (WindowAgg *) winstate->ss.ps.plan;
	PlanState  *outerPlan;
	TupleTableSlot *outerslot;
	MemoryContext oldcontext;

	if (!winstate->buffer)
		return;					/* just a safety check */
	if (winstate->partition_spooled)
		return;					/* whole partition done already */

	/*
	 * If the tuplestore has spilled to disk, alternate reading and writing
	 * becomes quite expensive due to frequent buffer flushes.  It's cheaper
	 * to force the entire partition to get spooled in one go.
	 *
	 * XXX this is a horrid kluge --- it'd be better to fix the performance
	 * problem inside tuplestore.  FIXME
	 */
	if (!tuplestore_in_memory(winstate->buffer))
		pos = -1;

	outerPlan = outerPlanState(winstate);

	/* Must be in query context to call outerplan */
	oldcontext = MemoryContextSwitchTo(winstate->ss.ps.ps_ExprContext->ecxt_per_query_memory);

	while (winstate->spooled_rows <= pos || pos == -1)
	{
		outerslot = ExecProcNode(outerPlan);
		if (TupIsNull(outerslot))
		{
			/* reached the end of the last partition */
			winstate->partition_spooled = true;
			winstate->more_partitions = false;
			break;
		}

		if (node->partNumCols > 0)
		{
			ExprContext *econtext = winstate->tmpcontext;

			econtext->ecxt_innertuple = winstate->first_part_slot;
			econtext->ecxt_outertuple = outerslot;

			/* Check if this tuple still belongs to the current partition */
			if (!ExecQualAndReset(winstate->partEqfunction, econtext))
			{
				/*
				 * end of partition; copy the tuple for the next cycle.
				 */
				ExecCopySlot(winstate->first_part_slot, outerslot);
				winstate->partition_spooled = true;
				winstate->more_partitions = true;
				break;
			}