Function get_eclass_for_sort_expr

src/backend/optimizer/path/equivclass.c:620–772

* get_eclass_for_sort_expr * Given an expression and opfamily/collation info, find an existing * equivalence class it is a member of; if none, optionally build a new * single-member EquivalenceClass for it. * * expr is the expression, and nullable_relids is the set of base relids * that are potentially nullable below it. We actually only care about * the set of such relids that are u

 * generating poor (but not incorrect) plans.
 */
EquivalenceClass *
get_eclass_for_sort_expr(PlannerInfo *root,
						 Expr *expr,
						 Relids nullable_relids,
						 List *opfamilies,
						 Oid opcintype,
						 Oid collation,
						 Index sortref,
						 Relids rel,
						 bool create_it)
{
	Relids		expr_relids;
	EquivalenceClass *newec;
	EquivalenceMember *newem;
	ListCell   *lc1;
	MemoryContext oldcontext;

	/*
	 * Ensure the expression exposes the correct type and collation.
	 */
	expr = canonicalize_ec_expression(expr, opcintype, collation);

	/*
	 * Scan through the existing EquivalenceClasses for a match
	 */
	foreach(lc1, root->eq_classes)
	{
		EquivalenceClass *cur_ec = (EquivalenceClass *) lfirst(lc1);
		ListCell   *lc2;

		/*
		 * Never match to a volatile EC, except when we are looking at another
		 * reference to the same volatile SortGroupClause.
		 */
		if (cur_ec->ec_has_volatile &&
			(sortref == 0 || sortref != cur_ec->ec_sortref))
			continue;

		if (collation != cur_ec->ec_collation)
			continue;
		if (!equal(opfamilies, cur_ec->ec_opfamilies))
			continue;

		foreach(lc2, cur_ec->ec_members)
		{
			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);

			/*
			 * Ignore child members unless they match the request.
			 */
			if (cur_em->em_is_child &&
				!bms_equal(cur_em->em_relids, rel))
				continue;

			/*
			 * If below an outer join, don't match constants: they're not as
			 * constant as they look.
			 */