Function MJCompare

src/backend/executor/nodeMergejoin.c:422–476

* MJCompare * * Compare the mergejoinable values of the current two input tuples * and return 0 if they are equal (ie, the mergejoin equalities all * succeed), >0 if outer > inner, <0 if outer < inner. * * MJEvalOuterValues and MJEvalInnerValues must already have been called * for the current outer and inner tuples, respectively. */

 * for the current outer and inner tuples, respectively.
 */
static int
MJCompare(MergeJoinState *mergestate)
{
	int			result = 0;
	bool		nulleqnull = false;
	ExprContext *econtext = mergestate->js.ps.ps_ExprContext;
	int			i;
	MemoryContext oldContext;

	/*
	 * Call the comparison functions in short-lived context, in case they leak
	 * memory.
	 */
	ResetExprContext(econtext);

	oldContext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);

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

		/*
		 * Special case for NULL-vs-NULL, else use standard comparison.
		 */
		if (clause->lisnull && clause->risnull)
		{
			nulleqnull = true;	/* NULL "=" NULL */
			continue;
		}

		result = ApplySortComparator(clause->ldatum, clause->lisnull,
									 clause->rdatum, clause->risnull,
									 &clause->ssup);

		if (result != 0)
			break;
	}

	/*
	 * If we had any NULL-vs-NULL inputs, we do not want to report that the
	 * tuples are equal.  Instead, if result is still 0, change it to +1. This
	 * will result in advancing the inner side of the join.
	 *
	 * Likewise, if there was a constant-false joinqual, do not report
	 * equality.  We have to check this as part of the mergequals, else the
	 * rescan logic will do the wrong thing.
	 */
	if (result == 0 &&
		(nulleqnull || mergestate->mj_ConstFalseJoin))
		result = 1;

	MemoryContextSwitchTo(oldContext);

	return result;
}


/*