Function begin_partition

src/backend/executor/nodeWindowAgg.c:1335–1494

* begin_partition * Start buffering rows of the next partition. */

 * Start buffering rows of the next partition.
 */
static void
begin_partition(WindowAggState *winstate)
{
	WindowAgg  *node = (WindowAgg *) winstate->ss.ps.plan;
	PlanState  *outerPlan = outerPlanState(winstate);
	int			frameOptions = winstate->frameOptions;
	int			numfuncs = winstate->numfuncs;
	int			i;

	winstate->partition_spooled = false;
	winstate->framehead_valid = false;
	winstate->frametail_valid = false;
	winstate->grouptail_valid = false;
	winstate->spooled_rows = 0;
	winstate->currentpos = 0;
	winstate->frameheadpos = 0;
	winstate->frametailpos = 0;
	winstate->currentgroup = 0;
	winstate->frameheadgroup = 0;
	winstate->frametailgroup = 0;
	winstate->groupheadpos = 0;
	winstate->grouptailpos = -1;	/* see update_grouptailpos */
	ExecClearTuple(winstate->agg_row_slot);
	if (winstate->framehead_slot)
		ExecClearTuple(winstate->framehead_slot);
	if (winstate->frametail_slot)
		ExecClearTuple(winstate->frametail_slot);

	/*
	 * If this is the very first partition, we need to fetch the first input
	 * row to store in first_part_slot.
	 */
	if (TupIsNull(winstate->first_part_slot))
	{
		TupleTableSlot *outerslot = ExecProcNode(outerPlan);

		if (!TupIsNull(outerslot))
			ExecCopySlot(winstate->first_part_slot, outerslot);
		else
		{
			/* outer plan is empty, so we have nothing to do */
			winstate->partition_spooled = true;
			winstate->more_partitions = false;
			return;
		}
	}

	/* Create new tuplestore for this partition */
	winstate->buffer =
		tuplestore_begin_heap(false, false,
							  PlanStateOperatorMemKB((PlanState *) winstate));

	/*
	 * Set up read pointers for the tuplestore.  The current pointer doesn't
	 * need BACKWARD capability, but the per-window-function read pointers do,
	 * and the aggregate pointer does if we might need to restart aggregation.
	 */
	winstate->current_ptr = 0;	/* read pointer 0 is pre-allocated */