Ignore nullingrels when looking up statistics

When looking up statistical data about an expression, we do not need
to concern ourselves with the outer joins that could null the
Vars/PHVs contained in the expression.  Accounting for nullingrels in
the expression could cause estimate_num_groups to count the same Var
multiple times if it's marked with different nullingrels.  This is
incorrect, and could lead to "ERROR:  corrupt MVNDistinct entry" when
searching for multivariate n-distinct.

Furthermore, the nullingrels could prevent us from matching an
expression to expressional index columns or to the expressions in
extended statistics, leading to inaccurate estimates.

To fix, strip out all the nullingrels from the expression before we
look up statistical data about it.  There is one ensuing plan change
in the regression tests, but it looks reasonable and does not
compromise its original purpose.

This patch could result in plan changes, but it fixes an actual bug,
so back-patch to v16 where the outer-join-aware-Var infrastructure was
introduced.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-2Z4k+nFTiZe0Qbu5n8juUWenDAtMzi98bAZQtwHx0-w@mail.gmail.com

src/backend/utils/adt/selfuncs.c

@@ -121,6 +121,7 @@
 #include "parser/parse_clause.h"
 #include "parser/parse_relation.h"
 #include "parser/parsetree.h"
+#include "rewrite/rewriteManip.h"
 #include "statistics/statistics.h"
 #include "storage/bufmgr.h"
 #include "utils/acl.h"
@@ -3306,6 +3307,15 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
 
 	ndistinct = get_variable_numdistinct(vardata, &isdefault);
 
+	/*
+	 * The nullingrels bits within the var could cause the same var to be
+	 * counted multiple times if it's marked with different nullingrels.  They
+	 * could also prevent us from matching the var to the expressions in
+	 * extended statistics (see estimate_multivariate_ndistinct).  So strip
+	 * them out first.
+	 */
+	var = remove_nulling_relids(var, root->outer_join_rels, NULL);
+
 	foreach(lc, varinfos)
 	{
 		varinfo = (GroupVarInfo *) lfirst(lc);
@@ -5017,6 +5027,7 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 {
 	Node	   *basenode;
 	Relids		varnos;
+	Relids		basevarnos;
 	RelOptInfo *onerel;
 
 	/* Make sure we don't return dangling pointers in vardata */
@@ -5058,10 +5069,11 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 	 * relation are considered "real" vars.
 	 */
 	varnos = pull_varnos(root, basenode);
+	basevarnos = bms_difference(varnos, root->outer_join_rels);
 
 	onerel = NULL;
 
-	if (bms_is_empty(varnos))
+	if (bms_is_empty(basevarnos))
 	{
 		/* No Vars at all ... must be pseudo-constant clause */
 	}
@@ -5069,7 +5081,8 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 	{
 		int			relid;
 
-		if (bms_get_singleton_member(varnos, &relid))
+		/* Check if the expression is in vars of a single base relation */
+		if (bms_get_singleton_member(basevarnos, &relid))
 		{
 			if (varRelid == 0 || varRelid == relid)
 			{
@@ -5099,7 +5112,7 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 		}
 	}
 
-	bms_free(varnos);
+	bms_free(basevarnos);
 
 	vardata->var = node;
 	vardata->atttype = exprType(node);
@@ -5124,6 +5137,14 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 		ListCell   *slist;
 		Oid			userid;
 
+		/*
+		 * The nullingrels bits within the expression could prevent us from
+		 * matching it to expressional index columns or to the expressions in
+		 * extended statistics.  So strip them out first.
+		 */
+		if (bms_overlap(varnos, root->outer_join_rels))
+			node = remove_nulling_relids(node, root->outer_join_rels, NULL);
+
 		/*
 		 * Determine the user ID to use for privilege checks: either
 		 * onerel->userid if it's set (e.g., in case we're accessing the table
@@ -5394,6 +5415,8 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
 			}
 		}
 	}
+
+	bms_free(varnos);
 }
 
 /*

src/test/regress/expected/join.out

@@ -2517,10 +2517,11 @@ where t1.f1 = coalesce(t2.f1, 1);
                ->  Materialize
                      ->  Seq Scan on int4_tbl t2
                            Filter: (f1 > 1)
-         ->  Seq Scan on int4_tbl t3
+         ->  Materialize
+               ->  Seq Scan on int4_tbl t3
    ->  Materialize
          ->  Seq Scan on int4_tbl t4
-(13 rows)
+(14 rows)
 
 explain (costs off)
 select * from int4_tbl t1
@@ -8014,3 +8015,24 @@ SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS
 
 RESET enable_indexonlyscan;
 RESET enable_seqscan;
+-- Test that we do not account for nullingrels when looking up statistics
+CREATE TABLE group_tbl (a INT, b INT);
+INSERT INTO group_tbl SELECT 1, 1;
+CREATE STATISTICS group_tbl_stat (ndistinct) ON a, b FROM group_tbl;
+ANALYZE group_tbl;
+EXPLAIN (COSTS OFF)
+SELECT 1 FROM group_tbl t1
+    LEFT JOIN (SELECT a c1, COALESCE(a) c2 FROM group_tbl t2) s ON TRUE
+GROUP BY s.c1, s.c2;
+                 QUERY PLAN                 
+--------------------------------------------
+ Group
+   Group Key: t2.a, (COALESCE(t2.a))
+   ->  Sort
+         Sort Key: t2.a, (COALESCE(t2.a))
+         ->  Nested Loop Left Join
+               ->  Seq Scan on group_tbl t1
+               ->  Seq Scan on group_tbl t2
+(7 rows)
+
+DROP TABLE group_tbl;

src/test/regress/sql/join.sql

@@ -2952,3 +2952,16 @@ SELECT t1.a FROM skip_fetch t1 LEFT JOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS
 
 RESET enable_indexonlyscan;
 RESET enable_seqscan;
+
+-- Test that we do not account for nullingrels when looking up statistics
+CREATE TABLE group_tbl (a INT, b INT);
+INSERT INTO group_tbl SELECT 1, 1;
+CREATE STATISTICS group_tbl_stat (ndistinct) ON a, b FROM group_tbl;
+ANALYZE group_tbl;
+
+EXPLAIN (COSTS OFF)
+SELECT 1 FROM group_tbl t1
+    LEFT JOIN (SELECT a c1, COALESCE(a) c2 FROM group_tbl t2) s ON TRUE
+GROUP BY s.c1, s.c2;
+
+DROP TABLE group_tbl;