Add an at-least-marginally-plausible method of estimating the number

of groups produced by GROUP BY.  This improves the accuracy of planning
estimates for grouped subselects, and is needed to check whether a
hashed aggregation plan risks memory overflow.

src/backend/executor/nodeAgg.c

@@ -45,7 +45,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeAgg.c,v 1.95 2002/11/13 00:39:47 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/executor/nodeAgg.c,v 1.96 2002/11/19 23:21:57 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -619,6 +619,9 @@ lookup_hash_entry(Agg *node, TupleTableSlot *slot)
 		Datum		attr;
 		bool		isNull;
 
+		/* rotate hashkey left 1 bit at each step */
+		hashkey = (hashkey << 1) | ((hashkey & 0x80000000) ? 1 : 0);
+
 		attr = heap_getattr(tuple, att, tupdesc, &isNull);
 		if (isNull)
 			continue;			/* treat nulls as having hash key 0 */

src/backend/nodes/copyfuncs.c

@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/nodes/copyfuncs.c,v 1.218 2002/11/15 02:50:06 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/nodes/copyfuncs.c,v 1.219 2002/11/19 23:21:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1865,8 +1865,8 @@ _copyQuery(Query *from)
 
 	/*
 	 * We do not copy the planner internal fields: base_rel_list,
-	 * other_rel_list, join_rel_list, equi_key_list, query_pathkeys. Not
-	 * entirely clear if this is right?
+	 * other_rel_list, join_rel_list, equi_key_list, query_pathkeys,
+	 * hasJoinRTEs.  Not entirely clear if this is right?
 	 */
 
 	return newnode;

src/backend/nodes/equalfuncs.c

@@ -20,7 +20,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/nodes/equalfuncs.c,v 1.164 2002/11/15 02:50:06 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/nodes/equalfuncs.c,v 1.165 2002/11/19 23:21:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -628,9 +628,9 @@ _equalQuery(Query *a, Query *b)
 
 	/*
 	 * We do not check the internal-to-the-planner fields: base_rel_list,
-	 * other_rel_list, join_rel_list, equi_key_list, query_pathkeys. They
-	 * might not be set yet, and in any case they should be derivable from
-	 * the other fields.
+	 * other_rel_list, join_rel_list, equi_key_list, query_pathkeys,
+	 * hasJoinRTEs.  They might not be set yet, and in any case they should
+	 * be derivable from the other fields.
 	 */
 	return true;
 }

src/backend/optimizer/plan/createplan.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/createplan.c,v 1.122 2002/11/15 02:36:53 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/createplan.c,v 1.123 2002/11/19 23:21:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1684,7 +1684,8 @@ make_material(List *tlist, Plan *lefttree)
 
 Agg *
 make_agg(List *tlist, List *qual, AggStrategy aggstrategy,
-		 int ngrp, AttrNumber *grpColIdx, Plan *lefttree)
+		 int ngrp, AttrNumber *grpColIdx, long numGroups, int numAggs,
+		 Plan *lefttree)
 {
 	Agg		   *node = makeNode(Agg);
 	Plan	   *plan = &node->plan;
@@ -1692,6 +1693,7 @@ make_agg(List *tlist, List *qual, AggStrategy aggstrategy,
 	node->aggstrategy = aggstrategy;
 	node->numCols = ngrp;
 	node->grpColIdx = grpColIdx;
+	node->numGroups = numGroups;
 
 	copy_plan_costsize(plan, lefttree);
 
@@ -1699,15 +1701,11 @@ make_agg(List *tlist, List *qual, AggStrategy aggstrategy,
 	 * Charge one cpu_operator_cost per aggregate function per input
 	 * tuple.
 	 */
-	plan->total_cost += cpu_operator_cost * plan->plan_rows *
-		(length(pull_agg_clause((Node *) tlist)) +
-		 length(pull_agg_clause((Node *) qual)));
+	plan->total_cost += cpu_operator_cost * plan->plan_rows * numAggs;
 
 	/*
 	 * We will produce a single output tuple if not grouping,
-	 * and a tuple per group otherwise.  For now, estimate the number of
-	 * groups as 10% of the number of tuples --- bogus, but how to do
-	 * better?
+	 * and a tuple per group otherwise.
 	 */
 	if (aggstrategy == AGG_PLAIN)
 	{
@@ -1716,10 +1714,7 @@ make_agg(List *tlist, List *qual, AggStrategy aggstrategy,
 	}
 	else
 	{
-		plan->plan_rows *= 0.1;
-		if (plan->plan_rows < 1)
-			plan->plan_rows = 1;
-		node->numGroups = (long) plan->plan_rows;
+		plan->plan_rows = numGroups;
 	}
 
 	plan->state = (EState *) NULL;
@@ -1735,6 +1730,7 @@ Group *
 make_group(List *tlist,
 		   int ngrp,
 		   AttrNumber *grpColIdx,
+		   double numGroups,
 		   Plan *lefttree)
 {
 	Group	   *node = makeNode(Group);
@@ -1748,13 +1744,8 @@ make_group(List *tlist,
 	 */
 	plan->total_cost += cpu_operator_cost * plan->plan_rows * ngrp;
 
-	/*
-	 * Estimate the number of groups as 10% of the number of tuples
-	 * --- bogus, but how to do better?
-	 */
-	plan->plan_rows *= 0.1;
-	if (plan->plan_rows < 1)
-		plan->plan_rows = 1;
+	/* One output tuple per estimated result group */
+	plan->plan_rows = numGroups;
 
 	plan->state = (EState *) NULL;
 	plan->qual = NULL;
@@ -1786,17 +1777,16 @@ make_unique(List *tlist, Plan *lefttree, List *distinctList)
 
 	/*
 	 * Charge one cpu_operator_cost per comparison per input tuple. We
-	 * assume all columns get compared at most of the tuples.
+	 * assume all columns get compared at most of the tuples.  (XXX probably
+	 * this is an overestimate.)
 	 */
 	plan->total_cost += cpu_operator_cost * plan->plan_rows * numCols;
 
 	/*
-	 * As for Group, we make the unsupported assumption that there will be
-	 * 10% as many tuples out as in.
+	 * plan->plan_rows is left as a copy of the input subplan's plan_rows;
+	 * ie, we assume the filter removes nothing.  The caller must alter this
+	 * if he has a better idea.
 	 */
-	plan->plan_rows *= 0.1;
-	if (plan->plan_rows < 1)
-		plan->plan_rows = 1;
 
 	plan->state = (EState *) NULL;
 	plan->targetlist = tlist;
@@ -1850,8 +1840,8 @@ make_setop(SetOpCmd cmd, List *tlist, Plan *lefttree,
 	plan->total_cost += cpu_operator_cost * plan->plan_rows * numCols;
 
 	/*
-	 * As for Group, we make the unsupported assumption that there will be
-	 * 10% as many tuples out as in.
+	 * We make the unsupported assumption that there will be 10% as many
+	 * tuples out as in.  Any way to do better?
 	 */
 	plan->plan_rows *= 0.1;
 	if (plan->plan_rows < 1)

src/backend/optimizer/plan/initsplan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/initsplan.c,v 1.75 2002/09/04 20:31:21 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/initsplan.c,v 1.76 2002/11/19 23:21:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -784,6 +784,71 @@ process_implied_equality(Query *root, Node *item1, Node *item2,
 							pull_varnos((Node *) clause));
 }
 
+/*
+ * vars_known_equal
+ *	  Detect whether two Vars are known equal due to equijoin clauses.
+ *
+ * This is not completely accurate since we avoid adding redundant restriction
+ * clauses to individual base rels (see qual_is_redundant).  However, after
+ * the implied-equality-deduction phase, it is complete for Vars of different
+ * rels; that's sufficient for planned uses.
+ */
+bool
+vars_known_equal(Query *root, Var *var1, Var *var2)
+{
+	Index		irel1;
+	Index		irel2;
+	RelOptInfo *rel1;
+	List	   *restrictlist;
+	List	   *itm;
+
+	/*
+	 * Would need more work here if we wanted to check for known equality
+	 * of general clauses: there might be multiple base rels involved.
+	 */
+	Assert(IsA(var1, Var));
+	irel1 = var1->varno;
+	Assert(IsA(var2, Var));
+	irel2 = var2->varno;
+
+	/*
+	 * If both vars belong to same rel, we need to look at that rel's
+	 * baserestrictinfo list.  If different rels, each will have a
+	 * joininfo node for the other, and we can scan either list.
+	 */
+	rel1 = find_base_rel(root, irel1);
+	if (irel1 == irel2)
+		restrictlist = rel1->baserestrictinfo;
+	else
+	{
+		JoinInfo   *joininfo = find_joininfo_node(rel1,
+												  makeListi1(irel2));
+
+		restrictlist = joininfo->jinfo_restrictinfo;
+	}
+
+	/*
+	 * Scan to see if equality is known.
+	 */
+	foreach(itm, restrictlist)
+	{
+		RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(itm);
+		Node	   *left,
+				   *right;
+
+		if (restrictinfo->mergejoinoperator == InvalidOid)
+			continue;			/* ignore non-mergejoinable clauses */
+		/* We now know the restrictinfo clause is a binary opclause */
+		left = (Node *) get_leftop(restrictinfo->clause);
+		right = (Node *) get_rightop(restrictinfo->clause);
+		if ((equal(var1, left) && equal(var2, right)) ||
+			(equal(var2, left) && equal(var1, right)))
+			return true;		/* found a matching clause */
+	}
+
+	return false;
+}
+
 /*
  * qual_is_redundant
  *	  Detect whether an implied-equality qual that turns out to be a

src/backend/optimizer/plan/planner.c

@@ -8,14 +8,17 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.128 2002/11/14 19:00:36 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.129 2002/11/19 23:21:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 
 #include "postgres.h"
 
+#include <limits.h>
+
 #include "catalog/pg_type.h"
+#include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #ifdef OPTIMIZER_DEBUG
 #include "nodes/print.h"
@@ -35,6 +38,7 @@
 #include "parser/parse_expr.h"
 #include "rewrite/rewriteManip.h"
 #include "utils/lsyscache.h"
+#include "utils/selfuncs.h"
 
 
 /* Expression kind codes for preprocess_expression */
@@ -160,6 +164,23 @@ subquery_planner(Query *parse, double tuple_fraction)
 	parse->jointree = (FromExpr *)
 		preprocess_jointree(parse, (Node *) parse->jointree);
 
+	/*
+	 * Detect whether any rangetable entries are RTE_JOIN kind; if not,
+	 * we can avoid the expense of doing flatten_join_alias_vars().
+	 * This must be done after we have done pull_up_subqueries, of course.
+	 */
+	parse->hasJoinRTEs = false;
+	foreach(lst, parse->rtable)
+	{
+		RangeTblEntry *rte = (RangeTblEntry *) lfirst(lst);
+
+		if (rte->rtekind == RTE_JOIN)
+		{
+			parse->hasJoinRTEs = true;
+			break;
+		}
+	}
+
 	/*
 	 * Do expression preprocessing on targetlist and quals.
 	 */
@@ -694,9 +715,6 @@ preprocess_jointree(Query *parse, Node *jtnode)
 static Node *
 preprocess_expression(Query *parse, Node *expr, int kind)
 {
-	bool		has_join_rtes;
-	List	   *rt;
-
 	/*
 	 * Simplify constant expressions.
 	 *
@@ -737,22 +755,8 @@ preprocess_expression(Query *parse, Node *expr, int kind)
 	 * with base-relation variables, to allow quals to be pushed down. We
 	 * must do this after sublink processing, since it does not recurse
 	 * into sublinks.
-	 *
-	 * The flattening pass is expensive enough that it seems worthwhile to
-	 * scan the rangetable to see if we can avoid it.
 	 */
-	has_join_rtes = false;
-	foreach(rt, parse->rtable)
-	{
-		RangeTblEntry *rte = lfirst(rt);
-
-		if (rte->rtekind == RTE_JOIN)
-		{
-			has_join_rtes = true;
-			break;
-		}
-	}
-	if (has_join_rtes)
+	if (parse->hasJoinRTEs)
 		expr = flatten_join_alias_vars(expr, parse->rtable, false);
 
 	return expr;
@@ -931,6 +935,9 @@ grouping_planner(Query *parse, double tuple_fraction)
 		AttrNumber *groupColIdx = NULL;
 		Path	   *cheapest_path;
 		Path	   *sorted_path;
+		double		dNumGroups = 0;
+		long		numGroups = 0;
+		int			numAggs = 0;
 		bool		use_hashed_grouping = false;
 
 		/* Preprocess targetlist in case we are inside an INSERT/UPDATE. */
@@ -1006,6 +1013,19 @@ grouping_planner(Query *parse, double tuple_fraction)
 		sort_pathkeys = make_pathkeys_for_sortclauses(parse->sortClause,
 													  tlist);
 
+		/*
+		 * Will need actual number of aggregates for estimating costs.
+		 * Also, it's possible that optimization has eliminated all
+		 * aggregates, and we may as well check for that here.
+		 */
+		if (parse->hasAggs)
+		{
+			numAggs = length(pull_agg_clause((Node *) tlist)) +
+				length(pull_agg_clause(parse->havingQual));
+			if (numAggs == 0)
+				parse->hasAggs = false;
+		}
+
 		/*
 		 * Figure out whether we need a sorted result from query_planner.
 		 *
@@ -1215,6 +1235,14 @@ grouping_planner(Query *parse, double tuple_fraction)
 		 */
 		if (parse->groupClause)
 		{
+			/*
+			 * Always estimate the number of groups.
+			 */
+			dNumGroups = estimate_num_groups(parse,
+											 parse->groupClause,
+											 cheapest_path->parent->rows);
+			numGroups = (long) Min(dNumGroups, (double) LONG_MAX);
+
 			/*
 			 * Executor doesn't support hashed aggregation with DISTINCT
 			 * aggregates.  (Doing so would imply storing *all* the input
@@ -1226,12 +1254,32 @@ grouping_planner(Query *parse, double tuple_fraction)
 				use_hashed_grouping = false;
 			else
 			{
-#if 0							/* much more to do here */
+				/*
+				 * Use hashed grouping if (a) we think we can fit the
+				 * hashtable into SortMem, *and* (b) the estimated cost
+				 * is no more than doing it the other way.  While avoiding
+				 * the need for sorted input is usually a win, the fact
+				 * that the output won't be sorted may be a loss; so we
+				 * need to do an actual cost comparison.
+				 *
+				 * In most cases we have no good way to estimate the size of
+				 * the transition value needed by an aggregate; arbitrarily
+				 * assume it is 100 bytes.  Also set the overhead per hashtable
+				 * entry at 64 bytes.
+				 */
+				int		hashentrysize = cheapest_path->parent->width + 64 +
+					numAggs * 100;
+
+				if (hashentrysize * dNumGroups <= SortMem * 1024L)
+				{
+					/* much more to do here */
+#if 0
 					/* TEMPORARY HOTWIRE FOR TESTING */
 					use_hashed_grouping = true;
 #endif
 				}
 			}
+		}
 
 		/*
 		 * Select the best path and create a plan to execute it.
@@ -1319,6 +1367,8 @@ grouping_planner(Query *parse, double tuple_fraction)
 											AGG_HASHED,
 											length(parse->groupClause),
 											groupColIdx,
+											numGroups,
+											numAggs,
 											result_plan);
 			/* Hashed aggregation produces randomly-ordered results */
 			current_pathkeys = NIL;
@@ -1356,6 +1406,8 @@ grouping_planner(Query *parse, double tuple_fraction)
 											aggstrategy,
 											length(parse->groupClause),
 											groupColIdx,
+											numGroups,
+											numAggs,
 											result_plan);
 		}
 		else
@@ -1387,6 +1439,7 @@ grouping_planner(Query *parse, double tuple_fraction)
 				result_plan = (Plan *) make_group(tlist,
 												  length(parse->groupClause),
 												  groupColIdx,
+												  dNumGroups,
 												  result_plan);
 			}
 		}
@@ -1410,6 +1463,16 @@ grouping_planner(Query *parse, double tuple_fraction)
 	{
 		result_plan = (Plan *) make_unique(tlist, result_plan,
 										   parse->distinctClause);
+		/*
+		 * If there was grouping or aggregation, leave plan_rows as-is
+		 * (ie, assume the result was already mostly unique).  If not,
+		 * it's reasonable to assume the UNIQUE filter has effects
+		 * comparable to GROUP BY.
+		 */
+		if (!parse->groupClause && !parse->hasAggs)
+			result_plan->plan_rows = estimate_num_groups(parse,
+														 parse->distinctClause,
+														 result_plan->plan_rows);
 	}
 
 	/*

src/backend/optimizer/plan/setrefs.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/setrefs.c,v 1.81 2002/09/04 20:31:21 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/plan/setrefs.c,v 1.82 2002/11/19 23:21:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -439,7 +439,14 @@ join_references_mutator(Node *node,
 			return (Node *) newvar;
 		}
 
-		/* Perhaps it's a join alias that can be resolved to input vars? */
+		/* Return the Var unmodified, if it's for acceptable_rel */
+		if (var->varno == context->acceptable_rel)
+			return (Node *) copyObject(var);
+
+		/*
+		 * Perhaps it's a join alias that can be resolved to input vars?
+		 * We try this last since it's relatively slow.
+		 */
 		newnode = flatten_join_alias_vars((Node *) var,
 										  context->rtable,
 										  true);
@@ -450,13 +457,8 @@ join_references_mutator(Node *node,
 			return newnode;
 		}
 
-		/*
-		 * No referent found for Var --- either raise an error, or return
-		 * the Var unmodified if it's for acceptable_rel.
-		 */
-		if (var->varno != context->acceptable_rel)
+		/* No referent found for Var */
 		elog(ERROR, "join_references: variable not in subplan target lists");
-		return (Node *) copyObject(var);
 	}
 	return expression_tree_mutator(node,
 								   join_references_mutator,

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/selfuncs.c,v 1.120 2002/11/08 20:23:57 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/utils/adt/selfuncs.c,v 1.121 2002/11/19 23:21:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -85,7 +85,10 @@
 #include "optimizer/cost.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/plancat.h"
+#include "optimizer/planmain.h"
 #include "optimizer/prep.h"
+#include "optimizer/tlist.h"
+#include "optimizer/var.h"
 #include "parser/parse_func.h"
 #include "parser/parse_oper.h"
 #include "parser/parsetree.h"
@@ -1809,6 +1812,251 @@ mergejoinscansel(Query *root, Node *clause,
 		*rightscan = 1.0;
 }
 
+/*
+ * estimate_num_groups		- Estimate number of groups in a grouped query
+ *
+ * Given a query having a GROUP BY clause, estimate how many groups there
+ * will be --- ie, the number of distinct combinations of the GROUP BY
+ * expressions.
+ *
+ * This routine is also used to estimate the number of rows emitted by
+ * a DISTINCT filtering step; that is an isomorphic problem.  (Note:
+ * actually, we only use it for DISTINCT when there's no grouping or
+ * aggregation ahead of the DISTINCT.)
+ *
+ * Inputs:
+ *	root - the query
+ *	groupClauses - list of GroupClauses (or SortClauses for the DISTINCT
+ *		case, but those are equivalent structs)
+ *	input_rows - number of rows estimated to arrive at the group/unique
+ *		filter step
+ *
+ * Given the lack of any cross-correlation statistics in the system, it's
+ * impossible to do anything really trustworthy with GROUP BY conditions
+ * involving multiple Vars.  We should however avoid assuming the worst
+ * case (all possible cross-product terms actually appear as groups) since
+ * very often the grouped-by Vars are highly correlated.  Our current approach
+ * is as follows:
+ *	1.	Reduce the given expressions to a list of unique Vars used.  For
+ *		example, GROUP BY a, a + b is treated the same as GROUP BY a, b.
+ *		It is clearly correct not to count the same Var more than once.
+ *		It is also reasonable to treat f(x) the same as x: f() cannot
+ *		increase the number of distinct values (unless it is volatile,
+ *		which we consider unlikely for grouping), but it probably won't
+ *		reduce the number of distinct values much either.
+ *	2.	If the list contains Vars of different relations that are known equal
+ *		due to equijoin clauses, then drop all but one of the Vars from each
+ *		known-equal set, keeping the one with smallest estimated # of values
+ *		(since the extra values of the others can't appear in joined rows).
+ *		Note the reason we only consider Vars of different relations is that
+ *		if we considered ones of the same rel, we'd be double-counting the
+ *		restriction selectivity of the equality in the next step.
+ *	3.	For Vars within a single source rel, we multiply together the numbers
+ *		of values, clamp to the number of rows in the rel, and then multiply
+ *		by the selectivity of the restriction clauses for that rel.  The
+ *		initial product is probably too high (it's the worst case) but since
+ *		we can clamp to the rel's rows it won't be hugely bad.  Multiplying
+ *		by the restriction selectivity is effectively assuming that the
+ *		restriction clauses are independent of the grouping, which is a crummy
+ *		assumption, but it's hard to do better.
+ *	4.	If there are Vars from multiple rels, we repeat step 3 for each such
+ *		rel, and multiply the results together.
+ * Note that rels not containing grouped Vars are ignored completely, as are
+ * join clauses other than the equijoin clauses used in step 2.  Such rels
+ * cannot increase the number of groups, and we assume such clauses do not
+ * reduce the number either (somewhat bogus, but we don't have the info to
+ * do better).
+ */
+double
+estimate_num_groups(Query *root, List *groupClauses, double input_rows)
+{
+	List	   *allvars = NIL;
+	List	   *varinfos = NIL;
+	double		numdistinct;
+	List	   *l;
+	typedef struct {			/* varinfos is a List of these */
+		Var	   *var;
+		double	ndistinct;
+	} MyVarInfo;
+
+	/* We should not be called unless query has GROUP BY (or DISTINCT) */
+	Assert(groupClauses != NIL);
+
+	/* Step 1: get the unique Vars used */
+	foreach(l, groupClauses)
+	{
+		GroupClause *grpcl = (GroupClause *) lfirst(l);
+		Node	   *groupexpr = get_sortgroupclause_expr(grpcl,
+														 root->targetList);
+		List	   *varshere;
+
+		varshere = pull_var_clause(groupexpr, false);
+		/*
+		 * Replace any JOIN alias Vars with the underlying Vars.  (This
+		 * is not really right for FULL JOIN ...)
+		 */
+		if (root->hasJoinRTEs)
+		{
+			varshere = (List *) flatten_join_alias_vars((Node *) varshere,
+														root->rtable,
+														true);
+			varshere = pull_var_clause((Node *) varshere, false);
+		}
+		/*
+		 * If we find any variable-free GROUP BY item, then either it is
+		 * a constant (and we can ignore it) or it contains a volatile
+		 * function; in the latter case we punt and assume that each input
+		 * row will yield a distinct group.
+		 */
+		if (varshere == NIL)
+		{
+			if (contain_volatile_functions(groupexpr))
+				return input_rows;
+			continue;
+		}
+		allvars = nconc(allvars, varshere);
+	}
+
+	/* If now no Vars, we must have an all-constant GROUP BY list. */
+	if (allvars == NIL)
+		return 1.0;
+
+	/* Use set_union() to discard duplicates */
+	allvars = set_union(NIL, allvars);
+
+	/*
+	 * Step 2: acquire statistical estimate of number of distinct values
+	 * of each Var (total in its table, without regard for filtering).
+	 * Also, detect known-equal Vars and discard the ones we don't want.
+	 */
+	foreach(l, allvars)
+	{
+		Var	   *var = (Var *) lfirst(l);
+		Oid		relid = getrelid(var->varno, root->rtable);
+		HeapTuple	statsTuple = NULL;
+		Form_pg_statistic stats = NULL;
+		double ndistinct;
+		bool	keep = true;
+		List   *l2;
+
+		if (OidIsValid(relid))
+		{
+			statsTuple = SearchSysCache(STATRELATT,
+										ObjectIdGetDatum(relid),
+										Int16GetDatum(var->varattno),
+										0, 0);
+			if (HeapTupleIsValid(statsTuple))
+				stats = (Form_pg_statistic) GETSTRUCT(statsTuple);
+		}
+		ndistinct = get_att_numdistinct(root, var, stats);
+		if (HeapTupleIsValid(statsTuple))
+			ReleaseSysCache(statsTuple);
+
+		foreach(l2, varinfos)
+		{
+			MyVarInfo  *varinfo = (MyVarInfo *) lfirst(l2);
+
+			if (var->varno != varinfo->var->varno &&
+				vars_known_equal(root, var, varinfo->var))
+			{
+				/* Found a match */
+				if (varinfo->ndistinct <= ndistinct)
+				{
+					/* Keep older item, forget new one */
+					keep = false;
+					break;
+				}
+				else
+				{
+					/*
+					 * Delete the older item.  We assume lremove() will not
+					 * break the lnext link of the item...
+					 */
+					varinfos = lremove(varinfo, varinfos);
+				}
+			}
+		}
+
+		if (keep)
+		{
+			MyVarInfo  *varinfo = (MyVarInfo *) palloc(sizeof(MyVarInfo));
+
+			varinfo->var = var;
+			varinfo->ndistinct = ndistinct;
+			varinfos = lcons(varinfo, varinfos);
+		}
+	}
+
+	/*
+	 * Steps 3/4: group Vars by relation and estimate total numdistinct.
+	 *
+	 * For each iteration of the outer loop, we process the frontmost
+	 * Var in varinfos, plus all other Vars in the same relation.  We
+	 * remove these Vars from the newvarinfos list for the next iteration.
+	 * This is the easiest way to group Vars of same rel together.
+	 */
+	Assert(varinfos != NIL);
+	numdistinct = 1.0;
+
+	do
+	{
+		MyVarInfo  *varinfo1 = (MyVarInfo *) lfirst(varinfos);
+		RelOptInfo *rel = find_base_rel(root, varinfo1->var->varno);
+		double	reldistinct = varinfo1->ndistinct;
+		List   *newvarinfos = NIL;
+
+		/*
+		 * Get the largest numdistinct estimate of the Vars for this rel.
+		 * Also, construct new varinfos list of remaining Vars.
+		 */
+		foreach(l, lnext(varinfos))
+		{
+			MyVarInfo  *varinfo2 = (MyVarInfo *) lfirst(l);
+
+			if (varinfo2->var->varno == varinfo1->var->varno)
+			{
+				reldistinct *= varinfo2->ndistinct;
+			}
+			else
+			{
+				/* not time to process varinfo2 yet */
+				newvarinfos = lcons(varinfo2, newvarinfos);
+			}
+		}
+
+		/*
+		 * Clamp to size of rel, multiply by restriction selectivity.
+		 */
+		Assert(rel->reloptkind == RELOPT_BASEREL);
+		if (reldistinct > rel->tuples)
+			reldistinct = rel->tuples;
+		reldistinct *= rel->rows / rel->tuples;
+
+		/*
+		 * Update estimate of total distinct groups.
+		 */
+		numdistinct *= reldistinct;
+
+		varinfos = newvarinfos;
+	} while (varinfos != NIL);
+
+	/* Guard against out-of-range answers */
+	if (numdistinct > input_rows)
+		numdistinct = input_rows;
+	if (numdistinct < 1.0)
+		numdistinct = 1.0;
+
+	return numdistinct;
+}
+
+
+/*-------------------------------------------------------------------------
+ *
+ * Support routines
+ *
+ *-------------------------------------------------------------------------
+ */
+
 /*
  * get_var_maximum
  *		Estimate the maximum value of the specified variable.
@@ -3271,7 +3519,7 @@ pattern_selectivity(Const *patt, Pattern_Type ptype)
 
 
 /*
- * We want test whether the database's LC_COLLATE setting is safe for
+ * We want to test whether the database's LC_COLLATE setting is safe for
  * LIKE/regexp index optimization.
  *
  * The key requirement here is that given a prefix string, say "foo",
@@ -3284,7 +3532,7 @@ pattern_selectivity(Const *patt, Pattern_Type ptype)
  *
  * (In theory, locales other than C may be LIKE-safe so this function
  * could be different from lc_collate_is_c(), but in a different
- * theory, non-C locales are completely unpredicable so it's unlikely
+ * theory, non-C locales are completely unpredictable so it's unlikely
  * to happen.)
  *
  * Be sure to maintain the correspondence with the code in initdb.

src/include/nodes/parsenodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: parsenodes.h,v 1.215 2002/11/15 03:09:39 momjian Exp $
+ * $Id: parsenodes.h,v 1.216 2002/11/19 23:21:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -102,6 +102,7 @@ typedef struct Query
 	List	   *equi_key_list;	/* list of lists of equijoined
 								 * PathKeyItems */
 	List	   *query_pathkeys; /* desired pathkeys for query_planner() */
+	bool		hasJoinRTEs;	/* true if any RTEs are RTE_JOIN kind */
 } Query;
 
 

src/include/optimizer/planmain.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: planmain.h,v 1.61 2002/11/06 00:00:45 tgl Exp $
+ * $Id: planmain.h,v 1.62 2002/11/19 23:22:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -35,8 +35,11 @@ extern Sort *make_sort(Query *root, List *tlist,
 extern Sort *make_sort_from_pathkeys(Query *root, List *tlist,
 						Plan *lefttree, List *pathkeys);
 extern Agg *make_agg(List *tlist, List *qual, AggStrategy aggstrategy,
-					 int ngrp, AttrNumber *grpColIdx, Plan *lefttree);
-extern Group *make_group(List *tlist, int ngrp, AttrNumber *grpColIdx,
+					 int ngrp, AttrNumber *grpColIdx,
+					 long numGroups, int numAggs,
+					 Plan *lefttree);
+extern Group *make_group(List *tlist,
+						 int ngrp, AttrNumber *grpColIdx, double numGroups,
 						 Plan *lefttree);
 extern Material *make_material(List *tlist, Plan *lefttree);
 extern Unique *make_unique(List *tlist, Plan *lefttree, List *distinctList);
@@ -54,6 +57,7 @@ extern void build_base_rel_tlists(Query *root, List *tlist);
 extern Relids distribute_quals_to_rels(Query *root, Node *jtnode);
 extern void process_implied_equality(Query *root, Node *item1, Node *item2,
 						 Oid sortop1, Oid sortop2);
+extern bool vars_known_equal(Query *root, Var *var1, Var *var2);
 
 /*
  * prototypes for plan/setrefs.c

src/include/utils/selfuncs.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: selfuncs.h,v 1.9 2002/10/19 02:56:16 tgl Exp $
+ * $Id: selfuncs.h,v 1.10 2002/11/19 23:22:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -75,6 +75,9 @@ extern void mergejoinscansel(Query *root, Node *clause,
 				 Selectivity *leftscan,
 				 Selectivity *rightscan);
 
+extern double estimate_num_groups(Query *root, List *groupClauses,
+								  double input_rows);
+
 extern Datum btcostestimate(PG_FUNCTION_ARGS);
 extern Datum rtcostestimate(PG_FUNCTION_ARGS);
 extern Datum hashcostestimate(PG_FUNCTION_ARGS);