Avoid creation of useless EquivalenceClasses during planning.

Zoltan Boszormenyi exhibited a test case in which planning time was
dominated by construction of EquivalenceClasses and PathKeys that had no
actual relevance to the query (and in fact got discarded immediately).
This happened because we generated PathKeys describing the sort ordering of
every index on every table in the query, and only after that checked to see
if the sort ordering was relevant.  The EC/PK construction code is O(N^2)
in the number of ECs, which is all right for the intended number of such
objects, but it gets out of hand if there are ECs for lots of irrelevant
indexes.

To fix, twiddle the handling of mergeclauses a little bit to ensure that
every interesting EC is created before we begin path generation.  (This
doesn't cost anything --- in fact I think it's a bit cheaper than before
--- since we always eventually created those ECs anyway.)  Then, if an
index column can't be found in any pre-existing EC, we know that that sort
ordering is irrelevant for the query.  Instead of creating a useless EC,
we can just not build a pathkey for the index column in the first place.
The index will still be considered if it's useful for non-order-related
reasons, but we will think of its output as unsorted.

src/backend/optimizer/README

@@ -632,9 +632,39 @@ sort ordering was important; and so using the same PathKey for both sort
 orderings doesn't create any real problem.
 
 
+Order of processing for EquivalenceClasses and PathKeys
+-------------------------------------------------------
+
+As alluded to above, there is a specific sequence of phases in the
+processing of EquivalenceClasses and PathKeys during planning.  During the
+initial scanning of the query's quals (deconstruct_jointree followed by
+reconsider_outer_join_clauses), we construct EquivalenceClasses based on
+mergejoinable clauses found in the quals.  At the end of this process,
+we know all we can know about equivalence of different variables, so
+subsequently there will be no further merging of EquivalenceClasses.
+At that point it is possible to consider the EquivalenceClasses as
+"canonical" and build canonical PathKeys that reference them.  Before
+we reach that point (actually, before entering query_planner at all)
+we also ensure that we have constructed EquivalenceClasses for all the
+expressions used in the query's ORDER BY and related clauses.  These
+classes might or might not get merged together, depending on what we
+find in the quals.
+
+Because all the EquivalenceClasses are known before we begin path
+generation, we can use them as a guide to which indexes are of interest:
+if an index's column is not mentioned in any EquivalenceClass then that
+index's sort order cannot possibly be helpful for the query.  This allows
+short-circuiting of much of the processing of create_index_paths() for
+irrelevant indexes.
+
+There are some cases where planner.c constructs additional
+EquivalenceClasses and PathKeys after query_planner has completed.
+In these cases, the extra ECs/PKs are needed to represent sort orders
+that were not considered during query_planner.  Such situations should be
+minimized since it is impossible for query_planner to return a plan
+producing such a sort order, meaning a explicit sort will always be needed.
+Currently this happens only for queries involving multiple window functions
+with different orderings, for which extra sorts are needed anyway.
 
-Though Bob Devine <bob.devine@worldnet.att.net> was not involved in the 
-coding of our optimizer, he is available to field questions about
-optimizer topics.
 
 -- bjm & tgl

src/backend/optimizer/path/equivclass.c

@@ -78,6 +78,10 @@ static bool reconsider_full_join_clause(PlannerInfo *root,
  * join.  (This is the reason why we need a failure return.  It's more
  * convenient to check this case here than at the call sites...)
  *
+ * On success return, we have also initialized the clause's left_ec/right_ec
+ * fields to point to the EquivalenceClass representing it.  This saves lookup
+ * effort later.
+ *
  * Note: constructing merged EquivalenceClasses is a standard UNION-FIND
  * problem, for which there exist better data structures than simple lists.
  * If this code ever proves to be a bottleneck then it could be sped up ---
@@ -106,6 +110,10 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 			   *em2;
 	ListCell   *lc1;
 
+	/* Should not already be marked as having generated an eclass */
+	Assert(restrictinfo->left_ec == NULL);
+	Assert(restrictinfo->right_ec == NULL);
+
 	/* Extract info from given clause */
 	Assert(is_opclause(clause));
 	opno = ((OpExpr *) clause)->opno;
@@ -236,8 +244,10 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 		{
 			ec1->ec_sources = lappend(ec1->ec_sources, restrictinfo);
 			ec1->ec_below_outer_join |= below_outer_join;
+			/* mark the RI as associated with this eclass */
+			restrictinfo->left_ec = ec1;
+			restrictinfo->right_ec = ec1;
 			/* mark the RI as usable with this pair of EMs */
-			/* NB: can't set left_ec/right_ec until merging is finished */
 			restrictinfo->left_em = em1;
 			restrictinfo->right_em = em2;
 			return true;
@@ -266,6 +276,9 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 		ec2->ec_relids = NULL;
 		ec1->ec_sources = lappend(ec1->ec_sources, restrictinfo);
 		ec1->ec_below_outer_join |= below_outer_join;
+		/* mark the RI as associated with this eclass */
+		restrictinfo->left_ec = ec1;
+		restrictinfo->right_ec = ec1;
 		/* mark the RI as usable with this pair of EMs */
 		restrictinfo->left_em = em1;
 		restrictinfo->right_em = em2;
@@ -276,6 +289,9 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 		em2 = add_eq_member(ec1, item2, item2_relids, false, item2_type);
 		ec1->ec_sources = lappend(ec1->ec_sources, restrictinfo);
 		ec1->ec_below_outer_join |= below_outer_join;
+		/* mark the RI as associated with this eclass */
+		restrictinfo->left_ec = ec1;
+		restrictinfo->right_ec = ec1;
 		/* mark the RI as usable with this pair of EMs */
 		restrictinfo->left_em = em1;
 		restrictinfo->right_em = em2;
@@ -286,6 +302,9 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 		em1 = add_eq_member(ec2, item1, item1_relids, false, item1_type);
 		ec2->ec_sources = lappend(ec2->ec_sources, restrictinfo);
 		ec2->ec_below_outer_join |= below_outer_join;
+		/* mark the RI as associated with this eclass */
+		restrictinfo->left_ec = ec2;
+		restrictinfo->right_ec = ec2;
 		/* mark the RI as usable with this pair of EMs */
 		restrictinfo->left_em = em1;
 		restrictinfo->right_em = em2;
@@ -311,6 +330,9 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo,
 
 		root->eq_classes = lappend(root->eq_classes, ec);
 
+		/* mark the RI as associated with this eclass */
+		restrictinfo->left_ec = ec;
+		restrictinfo->right_ec = ec;
 		/* mark the RI as usable with this pair of EMs */
 		restrictinfo->left_em = em1;
 		restrictinfo->right_em = em2;
@@ -362,15 +384,19 @@ add_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
 /*
  * get_eclass_for_sort_expr
  *	  Given an expression and opfamily info, find an existing equivalence
- *	  class it is a member of; if none, build a new single-member
+ *	  class it is a member of; if none, optionally build a new single-member
  *	  EquivalenceClass for it.
  *
  * sortref is the SortGroupRef of the originating SortGroupClause, if any,
  * or zero if not.	(It should never be zero if the expression is volatile!)
  *
+ * If create_it is TRUE, we'll build a new EquivalenceClass when there is no
+ * match.  If create_it is FALSE, we just return NULL when no match.
+ *
  * This can be used safely both before and after EquivalenceClass merging;
  * since it never causes merging it does not invalidate any existing ECs
- * or PathKeys.
+ * or PathKeys.  However, ECs added after path generation has begun are
+ * of limited usefulness, so usually it's best to create them beforehand.
  *
  * Note: opfamilies must be chosen consistently with the way
  * process_equivalence() would do; that is, generated from a mergejoinable
@@ -382,7 +408,8 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 						 Expr *expr,
 						 Oid expr_datatype,
 						 List *opfamilies,
-						 Index sortref)
+						 Index sortref,
+						 bool create_it)
 {
 	EquivalenceClass *newec;
 	EquivalenceMember *newem;
@@ -426,8 +453,12 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 		}
 	}
 
+	/* No match; does caller want a NULL result? */
+	if (!create_it)
+		return NULL;
+
 	/*
-	 * No match, so build a new single-member EC
+	 * OK, build a new single-member EC
 	 *
 	 * Here, we must be sure that we construct the EC in the right context. We
 	 * can assume, however, that the passed expr is long-lived.
@@ -1094,8 +1125,8 @@ create_join_clause(PlannerInfo *root,
 	rinfo->parent_ec = parent_ec;
 
 	/*
-	 * We can set these now, rather than letting them be looked up later,
-	 * since this is only used after EC merging is complete.
+	 * We know the correct values for left_ec/right_ec, ie this particular EC,
+	 * so we can just set them directly instead of forcing another lookup.
 	 */
 	rinfo->left_ec = ec;
 	rinfo->right_ec = ec;

src/backend/optimizer/path/joinpath.c

@@ -1041,7 +1041,7 @@ select_mergejoin_clauses(PlannerInfo *root,
 		 * mergejoin is not really all that big a deal, and so it's not clear
 		 * that improving this is important.
 		 */
-		cache_mergeclause_eclasses(root, restrictinfo);
+		update_mergeclause_eclasses(root, restrictinfo);
 
 		if (EC_MUST_BE_REDUNDANT(restrictinfo->left_ec) ||
 			EC_MUST_BE_REDUNDANT(restrictinfo->right_ec))

src/backend/optimizer/path/pathkeys.c

@@ -40,6 +40,7 @@ static PathKey *make_pathkey_from_sortinfo(PlannerInfo *root,
 						   Expr *expr, Oid ordering_op,
 						   bool nulls_first,
 						   Index sortref,
+						   bool create_it,
 						   bool canonicalize);
 static Var *find_indexkey_var(PlannerInfo *root, RelOptInfo *rel,
 				  AttrNumber varattno);
@@ -230,6 +231,10 @@ canonicalize_pathkeys(PlannerInfo *root, List *pathkeys)
  * If the PathKey is being generated from a SortGroupClause, sortref should be
  * the SortGroupClause's SortGroupRef; otherwise zero.
  *
+ * create_it is TRUE if we should create any missing EquivalenceClass
+ * needed to represent the sort key.  If it's FALSE, we return NULL if the
+ * sort key isn't already present in any EquivalenceClass.
+ *
  * canonicalize should always be TRUE after EquivalenceClass merging has
  * been performed, but FALSE if we haven't done EquivalenceClass merging yet.
  */
@@ -238,6 +243,7 @@ make_pathkey_from_sortinfo(PlannerInfo *root,
 						   Expr *expr, Oid ordering_op,
 						   bool nulls_first,
 						   Index sortref,
+						   bool create_it,
 						   bool canonicalize)
 {
 	Oid			opfamily,
@@ -300,9 +306,13 @@ make_pathkey_from_sortinfo(PlannerInfo *root,
 											COERCE_DONTCARE);
 	}
 
-	/* Now find or create a matching EquivalenceClass */
+	/* Now find or (optionally) create a matching EquivalenceClass */
 	eclass = get_eclass_for_sort_expr(root, expr, opcintype, opfamilies,
-									  sortref);
+									  sortref, create_it);
+
+	/* Fail if no EC and !create_it */
+	if (!eclass)
+		return NULL;
 
 	/* And finally we can find or create a PathKey node */
 	if (canonicalize)
@@ -478,8 +488,11 @@ get_cheapest_fractional_path_for_pathkeys(List *paths,
  * The result is canonical, meaning that redundant pathkeys are removed;
  * it may therefore have fewer entries than there are index columns.
  *
- * We generate the full pathkeys list whether or not all are useful for the
- * current query.  Caller should do truncate_useless_pathkeys().
+ * Another reason for stopping early is that we may be able to tell that
+ * an index column's sort order is uninteresting for this query.  However,
+ * that test is just based on the existence of an EquivalenceClass and not
+ * on position in pathkey lists, so it's not complete.  Caller should call
+ * truncate_useless_pathkeys() to possibly remove more pathkeys.
  */
 List *
 build_index_pathkeys(PlannerInfo *root,
@@ -527,14 +540,23 @@ build_index_pathkeys(PlannerInfo *root,
 			indexprs_item = lnext(indexprs_item);
 		}
 
-		/* OK, make a canonical pathkey for this sort key */
+		/* OK, try to make a canonical pathkey for this sort key */
 		cpathkey = make_pathkey_from_sortinfo(root,
 											  indexkey,
 											  sortop,
 											  nulls_first,
 											  0,
+											  false,
 											  true);
 
+		/*
+		 * If the sort key isn't already present in any EquivalenceClass,
+		 * then it's not an interesting sort order for this query.  So
+		 * we can stop now --- lower-order sort keys aren't useful either.
+		 */
+		if (!cpathkey)
+			break;
+
 		/* Add to list unless redundant */
 		if (!pathkey_is_redundant(cpathkey, retval))
 			retval = lappend(retval, cpathkey);
@@ -644,7 +666,14 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
 											 outer_expr,
 											 sub_member->em_datatype,
 											 sub_eclass->ec_opfamilies,
-											 0);
+											 0,
+											 false);
+
+				/*
+				 * If we don't find a matching EC, sub-pathkey isn't
+				 * interesting to the outer query
+				 */
+				if (outer_ec)
 					best_pathkey =
 						make_canonical_pathkey(root,
 											   outer_ec,
@@ -738,7 +767,16 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
 														outer_expr,
 													 sub_member->em_datatype,
 												   sub_eclass->ec_opfamilies,
-														0);
+														0,
+														false);
+
+					/*
+					 * If we don't find a matching EC, this sub-pathkey isn't
+					 * interesting to the outer query
+					 */
+					if (!outer_ec)
+						continue;
+
 					outer_pk = make_canonical_pathkey(root,
 													  outer_ec,
 													sub_pathkey->pk_opfamily,
@@ -859,6 +897,7 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
 											 sortcl->sortop,
 											 sortcl->nulls_first,
 											 sortcl->tleSortGroupRef,
+											 true,
 											 canonicalize);
 
 		/* Canonical form eliminates redundant ordering keys */
@@ -878,27 +917,37 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
  ****************************************************************************/
 
 /*
- * cache_mergeclause_eclasses
- *		Make the cached EquivalenceClass links valid in a mergeclause
- *		restrictinfo.
+ * initialize_mergeclause_eclasses
+ *		Set the EquivalenceClass links in a mergeclause restrictinfo.
  *
  * RestrictInfo contains fields in which we may cache pointers to
  * EquivalenceClasses for the left and right inputs of the mergeclause.
  * (If the mergeclause is a true equivalence clause these will be the
- * same EquivalenceClass, otherwise not.)
+ * same EquivalenceClass, otherwise not.)  If the mergeclause is either
+ * used to generate an EquivalenceClass, or derived from an EquivalenceClass,
+ * then it's easy to set up the left_ec and right_ec members --- otherwise,
+ * this function should be called to set them up.  We will generate new
+ * EquivalenceClauses if necessary to represent the mergeclause's left and
+ * right sides.
+ *
+ * Note this is called before EC merging is complete, so the links won't
+ * necessarily point to canonical ECs.  Before they are actually used for
+ * anything, update_mergeclause_eclasses must be called to ensure that
+ * they've been updated to point to canonical ECs.
  */
 void
-cache_mergeclause_eclasses(PlannerInfo *root, RestrictInfo *restrictinfo)
+initialize_mergeclause_eclasses(PlannerInfo *root, RestrictInfo *restrictinfo)
 {
-	Assert(restrictinfo->mergeopfamilies != NIL);
-
-	/* the cached values should be either both set or both not */
-	if (restrictinfo->left_ec == NULL)
-	{
 	Expr	   *clause = restrictinfo->clause;
 	Oid			lefttype,
 				righttype;
 
+	/* Should be a mergeclause ... */
+	Assert(restrictinfo->mergeopfamilies != NIL);
+	/* ... with links not yet set */
+	Assert(restrictinfo->left_ec == NULL);
+	Assert(restrictinfo->right_ec == NULL);
+
 	/* Need the declared input types of the operator */
 	op_input_types(((OpExpr *) clause)->opno, &lefttype, &righttype);
 
@@ -908,16 +957,41 @@ cache_mergeclause_eclasses(PlannerInfo *root, RestrictInfo *restrictinfo)
 								 (Expr *) get_leftop(clause),
 								 lefttype,
 								 restrictinfo->mergeopfamilies,
-									 0);
+								 0,
+								 true);
 	restrictinfo->right_ec =
 		get_eclass_for_sort_expr(root,
 								 (Expr *) get_rightop(clause),
 								 righttype,
 								 restrictinfo->mergeopfamilies,
-									 0);
-	}
-	else
+								 0,
+								 true);
+}
+
+/*
+ * update_mergeclause_eclasses
+ *		Make the cached EquivalenceClass links valid in a mergeclause
+ *		restrictinfo.
+ *
+ * These pointers should have been set by process_equivalence or
+ * initialize_mergeclause_eclasses, but they might have been set to
+ * non-canonical ECs that got merged later.  Chase up to the canonical
+ * merged parent if so.
+ */
+void
+update_mergeclause_eclasses(PlannerInfo *root, RestrictInfo *restrictinfo)
+{
+	/* Should be a merge clause ... */
+	Assert(restrictinfo->mergeopfamilies != NIL);
+	/* ... with pointers already set */
+	Assert(restrictinfo->left_ec != NULL);
 	Assert(restrictinfo->right_ec != NULL);
+
+	/* Chase up to the top as needed */
+	while (restrictinfo->left_ec->ec_merged)
+		restrictinfo->left_ec = restrictinfo->left_ec->ec_merged;
+	while (restrictinfo->right_ec->ec_merged)
+		restrictinfo->right_ec = restrictinfo->right_ec->ec_merged;
 }
 
 /*
@@ -953,7 +1027,7 @@ find_mergeclauses_for_pathkeys(PlannerInfo *root,
 	{
 		RestrictInfo *rinfo = (RestrictInfo *) lfirst(i);
 
-		cache_mergeclause_eclasses(root, rinfo);
+		update_mergeclause_eclasses(root, rinfo);
 	}
 
 	foreach(i, pathkeys)
@@ -1089,7 +1163,7 @@ select_outer_pathkeys_for_merge(PlannerInfo *root,
 		ListCell   *lc2;
 
 		/* get the outer eclass */
-		cache_mergeclause_eclasses(root, rinfo);
+		update_mergeclause_eclasses(root, rinfo);
 
 		if (rinfo->outer_is_left)
 			oeclass = rinfo->left_ec;
@@ -1250,7 +1324,7 @@ make_inner_pathkeys_for_merge(PlannerInfo *root,
 		EquivalenceClass *ieclass;
 		PathKey    *pathkey;
 
-		cache_mergeclause_eclasses(root, rinfo);
+		update_mergeclause_eclasses(root, rinfo);
 
 		if (rinfo->outer_is_left)
 		{
@@ -1366,7 +1440,7 @@ pathkeys_useful_for_merging(PlannerInfo *root, RelOptInfo *rel, List *pathkeys)
 
 				if (restrictinfo->mergeopfamilies == NIL)
 					continue;
-				cache_mergeclause_eclasses(root, restrictinfo);
+				update_mergeclause_eclasses(root, restrictinfo);
 
 				if (pathkey->pk_eclass == restrictinfo->left_ec ||
 					pathkey->pk_eclass == restrictinfo->right_ec)

src/backend/optimizer/plan/initsplan.c

@@ -1066,6 +1066,12 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 	 *
 	 * If none of the above hold, pass it off to
 	 * distribute_restrictinfo_to_rels().
+	 *
+	 * In all cases, it's important to initialize the left_ec and right_ec
+	 * fields of a mergejoinable clause, so that all possibly mergejoinable
+	 * expressions have representations in EquivalenceClasses.  If
+	 * process_equivalence is successful, it will take care of that;
+	 * otherwise, we have to call initialize_mergeclause_eclasses to do it.
 	 */
 	if (restrictinfo->mergeopfamilies)
 	{
@@ -1073,10 +1079,15 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 		{
 			if (process_equivalence(root, restrictinfo, below_outer_join))
 				return;
-			/* EC rejected it, so pass to distribute_restrictinfo_to_rels */
+			/* EC rejected it, so set left_ec/right_ec the hard way ... */
+			initialize_mergeclause_eclasses(root, restrictinfo);
+			/* ... and fall through to distribute_restrictinfo_to_rels */
 		}
 		else if (maybe_outer_join && restrictinfo->can_join)
 		{
+			/* we need to set up left_ec/right_ec the hard way */
+			initialize_mergeclause_eclasses(root, restrictinfo);
+			/* now see if it should go to any outer-join lists */
 			if (bms_is_subset(restrictinfo->left_relids,
 							  outerjoin_nonnullable) &&
 				!bms_overlap(restrictinfo->right_relids,
@@ -1104,6 +1115,12 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 												  restrictinfo);
 				return;
 			}
+			/* nope, so fall through to distribute_restrictinfo_to_rels */
+		}
+		else
+		{
+			/* we still need to set up left_ec/right_ec */
+			initialize_mergeclause_eclasses(root, restrictinfo);
 		}
 	}
 
@@ -1404,6 +1421,9 @@ process_implied_equality(PlannerInfo *root,
  *
  * This overlaps the functionality of process_implied_equality(), but we
  * must return the RestrictInfo, not push it into the joininfo tree.
+ *
+ * Note: we do not do initialize_mergeclause_eclasses() here.  It is
+ * caller's responsibility that left_ec/right_ec be set as necessary.
  */
 RestrictInfo *
 build_implied_join_equality(Oid opno,

src/include/optimizer/paths.h

@@ -116,7 +116,8 @@ extern EquivalenceClass *get_eclass_for_sort_expr(PlannerInfo *root,
 						 Expr *expr,
 						 Oid expr_datatype,
 						 List *opfamilies,
-						 Index sortref);
+						 Index sortref,
+						 bool create_it);
 extern void generate_base_implied_equalities(PlannerInfo *root);
 extern List *generate_join_implied_equalities(PlannerInfo *root,
 								 RelOptInfo *joinrel,
@@ -172,7 +173,9 @@ extern List *make_pathkeys_for_sortclauses(PlannerInfo *root,
 							  List *sortclauses,
 							  List *tlist,
 							  bool canonicalize);
-extern void cache_mergeclause_eclasses(PlannerInfo *root,
+extern void initialize_mergeclause_eclasses(PlannerInfo *root,
+											RestrictInfo *restrictinfo);
+extern void update_mergeclause_eclasses(PlannerInfo *root,
 						   RestrictInfo *restrictinfo);
 extern List *find_mergeclauses_for_pathkeys(PlannerInfo *root,
 							   List *pathkeys,