Function ExecPrefetchQual

src/backend/executor/execUtils.c:1528–1583

* Prefetch JoinQual or NonJoinQual to prevent motion hazard. * * A motion hazard is a deadlock between motions, a classic motion hazard in a * join executor is formed by its inner and outer motions, it can be prevented * by prefetching the inner plan, refer to motion_sanity_check() for details. * * A similar motion hazard can be formed by the outer motion and the join qual * motion(or non j

 * Return true if the JoinQual or NonJoinQual is prefetched.
 */
void
ExecPrefetchQual(JoinState *node, bool isJoinQual)
{
	EState	   *estate = node->ps.state;
	ExprContext *econtext = node->ps.ps_ExprContext;
	PlanState  *inner = innerPlanState(node);
	PlanState  *outer = outerPlanState(node);
	TupleTableSlot *innertuple = econtext->ecxt_innertuple;

	ListCell   *lc = NULL;
	List       *quals = NIL;

	ExprState  *qual;

	if (isJoinQual)
		qual = node->joinqual;
	else
		qual = node->ps.qual;

	Assert(qual);

	/* Outer tuples should not be fetched before us */
	Assert(econtext->ecxt_outertuple == NULL);

	/* Build fake inner & outer tuples */
	econtext->ecxt_innertuple = ExecInitNullTupleSlot(estate,
													  ExecGetResultType(inner),
													  &TTSOpsVirtual);
	econtext->ecxt_outertuple = ExecInitNullTupleSlot(estate,
													  ExecGetResultType(outer),
													  &TTSOpsVirtual);

	if (IsA(node->ps.plan, NestLoop))
	{
		NestLoop *nl = (NestLoop *) (node->ps.plan);
		if (nl->nestParams)
			fake_outer_params(node);
	}

	quals = flatten_logic_exprs((Node *) qual);

	/* Fetch subplan with the fake inner & outer tuples */
	foreach(lc, quals)
	{
		/*
		 * Force every qual is prefech because
		 * our target is to materialize motion node.
		 */
		ExprState  *clause = (ExprState *) lfirst(lc);
		(void) ExecQual(clause, econtext);
	}

	/* Restore previous state */
	econtext->ecxt_innertuple = innertuple;
	econtext->ecxt_outertuple = NULL;
}

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