Function MJEvalOuterValues

src/backend/executor/nodeMergejoin.c:325–363

* MJEvalOuterValues * * Compute the values of the mergejoined expressions for the current * outer tuple. We also detect whether it's impossible for the current * outer tuple to match anything --- this is true if it yields a NULL * input, since we assume mergejoin operators are strict. If the NULL * is in the first join column, and that column sorts nulls last, then * we can further conclu

 * time we move to a new tuple.
 */
static MJEvalResult
MJEvalOuterValues(MergeJoinState *mergestate)
{
	ExprContext *econtext = mergestate->mj_OuterEContext;
	MJEvalResult result = MJEVAL_MATCHABLE;
	int			i;
	MemoryContext oldContext;

	/* Check for end of outer subplan */
	if (TupIsNull(mergestate->mj_OuterTupleSlot))
		return MJEVAL_ENDOFJOIN;

	ResetExprContext(econtext);

	oldContext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);

	econtext->ecxt_outertuple = mergestate->mj_OuterTupleSlot;

	for (i = 0; i < mergestate->mj_NumClauses; i++)
	{
		MergeJoinClause clause = &mergestate->mj_Clauses[i];

		clause->ldatum = ExecEvalExpr(clause->lexpr, econtext,
									  &clause->lisnull);
		if (clause->lisnull && !clause->notdistinct)
		{
			/* match is impossible; can we end the join early? */
			if (i == 0 && !clause->ssup.ssup_nulls_first &&
				!mergestate->mj_FillOuter)
				result = MJEVAL_ENDOFJOIN;
			else if (result == MJEVAL_MATCHABLE)
				result = MJEVAL_NONMATCHABLE;
		}
	}

	MemoryContextSwitchTo(oldContext);

	return result;
}

/*
 * MJEvalInnerValues