Optimize multi-batch hash joins when the outer relation has a nonuniform

distribution, by creating a special fast path for the (first few) most common
values of the outer relation.  Tuples having hashvalues matching the MCVs
are effectively forced to be in the first batch, so that we never write
them out to the batch temp files.

Bryce Cutt and Ramon Lawrence, with some editorialization by me.

1 files changed, 58 insertions, 4 deletions

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index a243ca80d2d..be4d79f1bf2 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.255 2009/01/01 17:23:44 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.256 2009/03/21 00:04:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -112,7 +112,11 @@ static HashJoin *make_hashjoin(List *tlist,
 			  List *hashclauses,
 			  Plan *lefttree, Plan *righttree,
 			  JoinType jointype);
-static Hash *make_hash(Plan *lefttree);
+static Hash *make_hash(Plan *lefttree,
+					   Oid skewTable,
+					   AttrNumber skewColumn,
+					   Oid skewColType,
+					   int32 skewColTypmod);
 static MergeJoin *make_mergejoin(List *tlist,
 			   List *joinclauses, List *otherclauses,
 			   List *mergeclauses,
@@ -1864,6 +1868,10 @@ create_hashjoin_plan(PlannerInfo *root,
 	List	   *joinclauses;
 	List	   *otherclauses;
 	List	   *hashclauses;
+	Oid			skewTable = InvalidOid;
+	AttrNumber	skewColumn = InvalidAttrNumber;
+	Oid			skewColType = InvalidOid;
+	int32		skewColTypmod = -1;
 	HashJoin   *join_plan;
 	Hash	   *hash_plan;
 
@@ -1903,9 +1911,46 @@ create_hashjoin_plan(PlannerInfo *root,
 	disuse_physical_tlist(inner_plan, best_path->jpath.innerjoinpath);
 
 	/*
+	 * If there is a single join clause and we can identify the outer
+	 * variable as a simple column reference, supply its identity for
+	 * possible use in skew optimization.  (Note: in principle we could
+	 * do skew optimization with multiple join clauses, but we'd have to
+	 * be able to determine the most common combinations of outer values,
+	 * which we don't currently have enough stats for.)
+	 */
+	if (list_length(hashclauses) == 1)
+	{
+		OpExpr	   *clause = (OpExpr *) linitial(hashclauses);
+		Node	   *node;
+
+		Assert(is_opclause(clause));
+		node = (Node *) linitial(clause->args);
+		if (IsA(node, RelabelType))
+			node = (Node *) ((RelabelType *) node)->arg;
+		if (IsA(node, Var))
+		{
+			Var	   *var = (Var *) node;
+			RangeTblEntry *rte;
+
+			rte = root->simple_rte_array[var->varno];
+			if (rte->rtekind == RTE_RELATION)
+			{
+				skewTable = rte->relid;
+				skewColumn = var->varattno;
+				skewColType = var->vartype;
+				skewColTypmod = var->vartypmod;
+			}
+		}
+	}
+
+	/*
 	 * Build the hash node and hash join node.
 	 */
-	hash_plan = make_hash(inner_plan);
+	hash_plan = make_hash(inner_plan,
+						  skewTable,
+						  skewColumn,
+						  skewColType,
+						  skewColTypmod);
 	join_plan = make_hashjoin(tlist,
 							  joinclauses,
 							  otherclauses,
@@ -2713,7 +2758,11 @@ make_hashjoin(List *tlist,
 }
 
 static Hash *
-make_hash(Plan *lefttree)
+make_hash(Plan *lefttree,
+		  Oid skewTable,
+		  AttrNumber skewColumn,
+		  Oid skewColType,
+		  int32 skewColTypmod)
 {
 	Hash	   *node = makeNode(Hash);
 	Plan	   *plan = &node->plan;
@@ -2730,6 +2779,11 @@ make_hash(Plan *lefttree)
 	plan->lefttree = lefttree;
 	plan->righttree = NULL;
 
+	node->skewTable = skewTable;
+	node->skewColumn = skewColumn;
+	node->skewColType = skewColType;
+	node->skewColTypmod = skewColTypmod;
+
 	return node;
 }