Fix planning of star-schema-style queries.

tglsfdc · tglsfdc · commit 6f419958a611 · 2015-02-28T12:43:04.000-05:00
Part of the intent of the parameterized-path mechanism was to handle star-schema queries efficiently, but some overly-restrictive search limiting logic added in commit e2fa76d prevented such cases from working as desired. Fix that and add a regression test about it. Per gripe from Marc Cousin. This is arguably a bug rather than a new feature, so back-patch to 9.2 where parameterized paths were introduced.
diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README
@@ -705,17 +705,37 @@ intermediate layers of joins, for example:
 			-> Index Scan using C_Z_IDX on C
 				Index Condition: C.Z = A.X
 
-If all joins are plain inner joins then this is unnecessary, because
-it's always possible to reorder the joins so that a parameter is used
+If all joins are plain inner joins then this is usually unnecessary,
+because it's possible to reorder the joins so that a parameter is used
 immediately below the nestloop node that provides it.  But in the
-presence of outer joins, join reordering may not be possible, and then
-this option can be critical.  Before version 9.2, Postgres used ad-hoc
-methods for planning and executing such queries, and those methods could
-not handle passing parameters down through multiple join levels.
+presence of outer joins, such join reordering may not be possible.
+
+Also, the bottom-level scan might require parameters from more than one
+other relation.  In principle we could join the other relations first
+so that all the parameters are supplied from a single nestloop level.
+But if those other relations have no join clause in common (which is
+common in star-schema queries for instance), the planner won't consider
+joining them directly to each other.  In such a case we need to be able
+to create a plan like
+
+    NestLoop
+        -> Seq Scan on SmallTable1 A
+        NestLoop
+            -> Seq Scan on SmallTable2 B
+            NestLoop
+                -> Index Scan using XYIndex on LargeTable C
+                      Index Condition: C.X = A.AID and C.Y = B.BID
+
+so we should be willing to pass down A.AID through a join even though
+there is no join order constraint forcing the plan to look like this.
+
+Before version 9.2, Postgres used ad-hoc methods for planning and
+executing nestloop queries of this kind, and those methods could not
+handle passing parameters down through multiple join levels.
 
 To plan such queries, we now use a notion of a "parameterized path",
 which is a path that makes use of a join clause to a relation that's not
-scanned by the path.  In the example just above, we would construct a
+scanned by the path.  In the example two above, we would construct a
 path representing the possibility of doing this:
 
 	-> Index Scan using C_Z_IDX on C
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
@@ -113,13 +113,14 @@ add_paths_to_joinrel(PlannerInfo *root,
 	/*
 	 * Decide whether it's sensible to generate parameterized paths for this
 	 * joinrel, and if so, which relations such paths should require.  There
-	 * is no need to create a parameterized result path unless there is a join
-	 * order restriction that prevents joining one of our input rels directly
-	 * to the parameter source rel instead of joining to the other input rel.
-	 * This restriction reduces the number of parameterized paths we have to
-	 * deal with at higher join levels, without compromising the quality of
-	 * the resulting plan.  We express the restriction as a Relids set that
-	 * must overlap the parameterization of any proposed join path.
+	 * is usually no need to create a parameterized result path unless there
+	 * is a join order restriction that prevents joining one of our input rels
+	 * directly to the parameter source rel instead of joining to the other
+	 * input rel.  (But see exception in try_nestloop_path.)  This restriction
+	 * reduces the number of parameterized paths we have to deal with at
+	 * higher join levels, without compromising the quality of the resulting
+	 * plan.  We express the restriction as a Relids set that must overlap the
+	 * parameterization of any proposed join path.
 	 */
 	foreach(lc, root->join_info_list)
 	{
@@ -227,9 +228,29 @@ try_nestloop_path(PlannerInfo *root,
 	if (required_outer &&
 		!bms_overlap(required_outer, param_source_rels))
 	{
-		/* Waste no memory when we reject a path here */
-		bms_free(required_outer);
-		return;
+		/*
+		 * We override the param_source_rels heuristic to accept nestloop
+		 * paths in which the outer rel satisfies some but not all of the
+		 * inner path's parameterization.  This is necessary to get good plans
+		 * for star-schema scenarios, in which a parameterized path for a
+		 * large table may require parameters from multiple small tables that
+		 * will not get joined directly to each other.  We can handle that by
+		 * stacking nestloops that have the small tables on the outside; but
+		 * this breaks the rule the param_source_rels heuristic is based on,
+		 * namely that parameters should not be passed down across joins
+		 * unless there's a join-order-constraint-based reason to do so.  So
+		 * ignore the param_source_rels restriction when this case applies.
+		 */
+		Relids		outerrelids = outer_path->parent->relids;
+		Relids		innerparams = PATH_REQ_OUTER(inner_path);
+
+		if (!(bms_overlap(innerparams, outerrelids) &&
+			  bms_nonempty_difference(innerparams, outerrelids)))
+		{
+			/* Waste no memory when we reject a path here */
+			bms_free(required_outer);
+			return;
+		}
 	}
 
 	/*
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
@@ -2789,6 +2789,25 @@ where thousand = (q1 + q2);
          ->  Seq Scan on int4_tbl
 (12 rows)
 
+--
+-- test ability to generate a suitable plan for a star-schema query
+--
+explain (costs off)
+select * from
+  tenk1, int8_tbl a, int8_tbl b
+where thousand = a.q1 and tenthous = b.q1 and a.q2 = 1 and b.q2 = 2;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Nested Loop
+   ->  Seq Scan on int8_tbl b
+         Filter: (q2 = 2)
+   ->  Nested Loop
+         ->  Seq Scan on int8_tbl a
+               Filter: (q2 = 1)
+         ->  Index Scan using tenk1_thous_tenthous on tenk1
+               Index Cond: ((thousand = a.q1) AND (tenthous = b.q1))
+(8 rows)
+
 --
 -- test placement of movable quals in a parameterized join tree
 --
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
@@ -763,6 +763,15 @@ select * from
   int4(sin(0)) q2
 where thousand = (q1 + q2);
 
+--
+-- test ability to generate a suitable plan for a star-schema query
+--
+
+explain (costs off)
+select * from
+  tenk1, int8_tbl a, int8_tbl b
+where thousand = a.q1 and tenthous = b.q1 and a.q2 = 1 and b.q2 = 2;
+
 --
 -- test placement of movable quals in a parameterized join tree
 --
