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.

src/backend/optimizer/path/costsize.c

@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.204 2009/02/06 23:43:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.205 2009/03/21 00:04:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1821,6 +1821,7 @@ cost_hashjoin(HashPath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	int			num_hashclauses = list_length(hashclauses);
 	int			numbuckets;
 	int			numbatches;
+	int			num_skew_mcvs;
 	double		virtualbuckets;
 	Selectivity innerbucketsize;
 	ListCell   *hcl;
@@ -1862,11 +1863,22 @@ cost_hashjoin(HashPath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 		* inner_path_rows;
 	run_cost += cpu_operator_cost * num_hashclauses * outer_path_rows;
 
-	/* Get hash table size that executor would use for inner relation */
+	/*
+	 * Get hash table size that executor would use for inner relation.
+	 *
+	 * XXX for the moment, always assume that skew optimization will be
+	 * performed.  As long as SKEW_WORK_MEM_PERCENT is small, it's not worth
+	 * trying to determine that for sure.
+	 *
+	 * XXX at some point it might be interesting to try to account for skew
+	 * optimization in the cost estimate, but for now, we don't.
+	 */
 	ExecChooseHashTableSize(inner_path_rows,
 							inner_path->parent->width,
+							true,	/* useskew */
 							&numbuckets,
-							&numbatches);
+							&numbatches,
+							&num_skew_mcvs);
 	virtualbuckets = (double) numbuckets *(double) numbatches;
 
 	/*

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;
 
@@ -1902,10 +1910,47 @@ create_hashjoin_plan(PlannerInfo *root,
 	/* We don't want any excess columns in the hashed tuples */
 	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;
 }