Function ExecAppend

src/backend/executor/nodeAppend.c:300–388

---------------------------------------------------------------- * ExecAppend * * Handles iteration over multiple subplans. * ---------------------------------------------------------------- */

 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecAppend(PlanState *pstate)
{
	AppendState *node = castNode(AppendState, pstate);
	TupleTableSlot *result;

	/*
	 * If this is the first call after Init or ReScan, we need to do the
	 * initialization work.
	 */
	if (!node->as_begun)
	{
		Assert(node->as_whichplan == INVALID_SUBPLAN_INDEX);
		Assert(!node->as_syncdone);

		/* Nothing to do if there are no subplans */
		if (node->as_nplans == 0)
			return ExecClearTuple(node->ps.ps_ResultTupleSlot);

		/* If there are any async subplans, begin executing them. */
		if (node->as_nasyncplans > 0)
			ExecAppendAsyncBegin(node);

		/*
		 * If no sync subplan has been chosen, we must choose one before
		 * proceeding.
		 */
		if (!node->choose_next_subplan(node) && node->as_nasyncremain == 0)
			return ExecClearTuple(node->ps.ps_ResultTupleSlot);

		Assert(node->as_syncdone ||
			   (node->as_whichplan >= 0 &&
				node->as_whichplan < node->as_nplans));

		/* And we're initialized. */
		node->as_begun = true;
	}

	for (;;)
	{
		PlanState  *subnode;

		CHECK_FOR_INTERRUPTS();

		/*
		 * try to get a tuple from an async subplan if any
		 */
		if (node->as_syncdone || !bms_is_empty(node->as_needrequest))
		{
			if (ExecAppendAsyncGetNext(node, &result))
				return result;
			Assert(!node->as_syncdone);
			Assert(bms_is_empty(node->as_needrequest));
		}

		/*
		 * figure out which sync subplan we are currently processing
		 */