Limit the number of rel sets considered in consider_index_join_outer_rels.

In bug #7626, Brian Dunavant exposes a performance problem created by
commit 3b8968f2: that commit attempted to
consider *all* possible combinations of indexable join clauses, but if said
clauses join to enough different relations, there's an exponential increase
in the number of outer-relation sets considered.

In Brian's example, all the clauses come from the same equivalence class,
which means it's redundant to use more than one of them in an indexscan
anyway.  So we can prevent the problem in this class of cases (which is
probably the majority of real examples) by rejecting combinations that
would only serve to add a known-redundant clause.

But that still leaves us exposed to exponential growth of planning time
when the query has a lot of non-equivalence join clauses that are usable
with the same index.  I chose to prevent such cases by setting an upper
limit on the number of relation sets considered, equal to ten times the
number of index clauses considered so far.  (This sliding limit still
allows new relsets to be added on as we move to additional index columns,
which is probably more important than considering even more combinations of
clauses for the previous column.)  This should keep the amount of work done
roughly linear rather than exponential in the apparent query complexity.
This part of the fix is pretty ad-hoc; but without a clearer idea of
real-world cases for which this would result in markedly inferior plans,
it's hard to see how to do better.

src/backend/optimizer/path/indxpath.c

@@ -92,6 +92,7 @@ static void consider_index_join_outer_rels(PlannerInfo *root, RelOptInfo *rel,
 							   IndexClauseSet *eclauseset,
 							   List **bitindexpaths,
 							   List *indexjoinclauses,
+							   int considered_clauses,
 							   List **considered_relids);
 static void get_join_index_paths(PlannerInfo *root, RelOptInfo *rel,
 					 IndexOptInfo *index,
@@ -101,6 +102,8 @@ static void get_join_index_paths(PlannerInfo *root, RelOptInfo *rel,
 					 List **bitindexpaths,
 					 Relids relids,
 					 List **considered_relids);
+static bool eclass_already_used(EquivalenceClass *parent_ec, Relids oldrelids,
+					List *indexjoinclauses);
 static bool bms_equal_any(Relids relids, List *relids_list);
 static void get_index_paths(PlannerInfo *root, RelOptInfo *rel,
 				IndexOptInfo *index, IndexClauseSet *clauses,
@@ -447,6 +450,7 @@ consider_index_join_clauses(PlannerInfo *root, RelOptInfo *rel,
 							IndexClauseSet *eclauseset,
 							List **bitindexpaths)
 {
+	int			considered_clauses = 0;
 	List	   *considered_relids = NIL;
 	int			indexcol;
 
@@ -460,8 +464,11 @@ consider_index_join_clauses(PlannerInfo *root, RelOptInfo *rel,
 	 * filter (qpqual); which is where an available clause would end up being
 	 * applied if we omit it from the indexquals.
 	 *
-	 * This looks expensive, but in practical cases there won't be very many
-	 * distinct sets of outer rels to consider.
+	 * This looks expensive, but in most practical cases there won't be very
+	 * many distinct sets of outer rels to consider.  As a safety valve when
+	 * that's not true, we use a heuristic: limit the number of outer rel sets
+	 * considered to a multiple of the number of clauses considered.  (We'll
+	 * always consider using each individual join clause, though.)
 	 *
 	 * For simplicity in selecting relevant clauses, we represent each set of
 	 * outer rels as a maximum set of clause_relids --- that is, the indexed
@@ -471,16 +478,20 @@ consider_index_join_clauses(PlannerInfo *root, RelOptInfo *rel,
 	for (indexcol = 0; indexcol < index->ncolumns; indexcol++)
 	{
 		/* Consider each applicable simple join clause */
+		considered_clauses += list_length(jclauseset->indexclauses[indexcol]);
 		consider_index_join_outer_rels(root, rel, index,
 									   rclauseset, jclauseset, eclauseset,
 									   bitindexpaths,
 									   jclauseset->indexclauses[indexcol],
+									   considered_clauses,
 									   &considered_relids);
 		/* Consider each applicable eclass join clause */
+		considered_clauses += list_length(eclauseset->indexclauses[indexcol]);
 		consider_index_join_outer_rels(root, rel, index,
 									   rclauseset, jclauseset, eclauseset,
 									   bitindexpaths,
 									   eclauseset->indexclauses[indexcol],
+									   considered_clauses,
 									   &considered_relids);
 	}
 }
@@ -494,6 +505,7 @@ consider_index_join_clauses(PlannerInfo *root, RelOptInfo *rel,
  * 'rel', 'index', 'rclauseset', 'jclauseset', 'eclauseset', and
  *		'bitindexpaths' as above
  * 'indexjoinclauses' is a list of RestrictInfos for join clauses
+ * 'considered_clauses' is the total number of clauses considered (so far)
  * '*considered_relids' is a list of all relids sets already considered
  */
 static void
@@ -504,6 +516,7 @@ consider_index_join_outer_rels(PlannerInfo *root, RelOptInfo *rel,
 							   IndexClauseSet *eclauseset,
 							   List **bitindexpaths,
 							   List *indexjoinclauses,
+							   int considered_clauses,
 							   List **considered_relids)
 {
 	ListCell   *lc;
@@ -522,7 +535,9 @@ consider_index_join_outer_rels(PlannerInfo *root, RelOptInfo *rel,
 		/*
 		 * Generate the union of this clause's relids set with each
 		 * previously-tried set.  This ensures we try this clause along with
-		 * every interesting subset of previous clauses.
+		 * every interesting subset of previous clauses.  However, to avoid
+		 * exponential growth of planning time when there are many clauses,
+		 * limit the number of relid sets accepted to 10 * considered_clauses.
 		 *
 		 * Note: get_join_index_paths adds entries to *considered_relids, but
 		 * it prepends them to the list, so that we won't visit new entries
@@ -543,6 +558,27 @@ consider_index_join_outer_rels(PlannerInfo *root, RelOptInfo *rel,
 			if (bms_subset_compare(clause_relids, oldrelids) != BMS_DIFFERENT)
 				continue;
 
+			/*
+			 * If this clause was derived from an equivalence class, the
+			 * clause list may contain other clauses derived from the same
+			 * eclass.  We should not consider that combining this clause with
+			 * one of those clauses generates a usefully different
+			 * parameterization; so skip if any clause derived from the same
+			 * eclass would already have been included when using oldrelids.
+			 */
+			if (rinfo->parent_ec &&
+				eclass_already_used(rinfo->parent_ec, oldrelids,
+									indexjoinclauses))
+				continue;
+
+			/*
+			 * If the number of relid sets considered exceeds our heuristic
+			 * limit, stop considering combinations of clauses.  We'll still
+			 * consider the current clause alone, though (below this loop).
+			 */
+			if (list_length(*considered_relids) >= 10 * considered_clauses)
+				break;
+
 			/* OK, try the union set */
 			get_join_index_paths(root, rel, index,
 								 rclauseset, jclauseset, eclauseset,
@@ -647,6 +683,28 @@ get_join_index_paths(PlannerInfo *root, RelOptInfo *rel,
 	*considered_relids = lcons(relids, *considered_relids);
 }
 
+/*
+ * eclass_already_used
+ *		True if any join clause usable with oldrelids was generated from
+ *		the specified equivalence class.
+ */
+static bool
+eclass_already_used(EquivalenceClass *parent_ec, Relids oldrelids,
+					List *indexjoinclauses)
+{
+	ListCell   *lc;
+
+	foreach(lc, indexjoinclauses)
+	{
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+
+		if (rinfo->parent_ec == parent_ec &&
+			bms_is_subset(rinfo->clause_relids, oldrelids))
+			return true;
+	}
+	return false;
+}
+
 /*
  * bms_equal_any
  *		True if relids is bms_equal to any member of relids_list