Function update_frameheadpos

src/backend/executor/nodeWindowAgg.c:1725–1980

* update_frameheadpos * make frameheadpos valid for the current row * * Note that frameheadpos is computed without regard for any window exclusion * clause; the current row and/or its peers are considered part of the frame * for this purpose even if they must be excluded later. * * May clobber winstate->temp_slot_2. */

 * May clobber winstate->temp_slot_2.
 */
static void
update_frameheadpos(WindowAggState *winstate)
{
	WindowAgg  *node = (WindowAgg *) winstate->ss.ps.plan;
	int			frameOptions = winstate->frameOptions;
	MemoryContext oldcontext;

	if (winstate->framehead_valid)
		return;					/* already known for current row */

	/* We may be called in a short-lived context */
	oldcontext = MemoryContextSwitchTo(winstate->ss.ps.ps_ExprContext->ecxt_per_query_memory);

	compute_start_end_offsets(winstate);

	if (frameOptions & FRAMEOPTION_START_UNBOUNDED_PRECEDING)
	{
		/* In UNBOUNDED PRECEDING mode, frame head is always row 0 */
		winstate->frameheadpos = 0;
		winstate->framehead_valid = true;
	}
	else if (frameOptions & FRAMEOPTION_START_CURRENT_ROW)
	{
		if (frameOptions & FRAMEOPTION_ROWS)
		{
			/* In ROWS mode, frame head is the same as current */
			winstate->frameheadpos = winstate->currentpos;
			winstate->framehead_valid = true;
		}
		else if (frameOptions & (FRAMEOPTION_RANGE | FRAMEOPTION_GROUPS))
		{
			/* If no ORDER BY, all rows are peers with each other */
			if (node->ordNumCols == 0)
			{
				winstate->frameheadpos = 0;
				winstate->framehead_valid = true;
				MemoryContextSwitchTo(oldcontext);
				return;
			}

			/*
			 * In RANGE or GROUPS START_CURRENT_ROW mode, frame head is the
			 * first row that is a peer of current row.  We keep a copy of the
			 * last-known frame head row in framehead_slot, and advance as
			 * necessary.  Note that if we reach end of partition, we will
			 * leave frameheadpos = end+1 and framehead_slot empty.
			 */
			tuplestore_select_read_pointer(winstate->buffer,
										   winstate->framehead_ptr);
			if (winstate->frameheadpos == 0 &&
				TupIsNull(winstate->framehead_slot))
			{
				/* fetch first row into framehead_slot, if we didn't already */
				if (!tuplestore_gettupleslot(winstate->buffer, true, true,
											 winstate->framehead_slot))
					elog(ERROR, "unexpected end of tuplestore");
			}