Fix choose_best_statistics to check clauses individually

When picking the best extended statistics object for a list of clauses,
it's not enough to look at attnums extracted from the clause list as a
whole. Consider for example this query with OR clauses:

   SELECT * FROM t WHERE (t.a = 1) OR (t.b = 1) OR (t.c = 1)

with a statistics defined on columns (a,b). Relying on attnums extracted
from the whole OR clause, we'd consider the statistics usable. That does
not work, as we see the conditions as a single OR-clause, referencing an
attribute not covered by the statistic, leading to empty list of clauses
to be estimated using the statistics and an assert failure.

This changes choose_best_statistics to check which clauses are actually
covered, and only using attributes from the fully covered ones. For the
previous example this means the statistics object will not be considered
as compatible with the OR-clause.

Backpatch to 12, where MCVs were introduced. The issue does not affect
older versions because functional dependencies don't handle OR clauses.

Author: Tomas Vondra
Reviewed-by: Dean Rasheed
Reported-By: Manuel Rigger
Discussion: https://postgr.es/m/CA+u7OA7H5rcE2=8f263w4NZD6ipO_XOrYB816nuLXbmSTH9pQQ@mail.gmail.com
Backpatch-through: 12

Author: tvondra

src/backend/statistics/dependencies.c

@@ -951,14 +951,14 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 	Bitmapset  *clauses_attnums = NULL;
 	StatisticExtInfo *stat;
 	MVDependencies *dependencies;
-	AttrNumber *list_attnums;
+	Bitmapset **list_attnums;
 	int			listidx;
 
 	/* check if there's any stats that might be useful for us. */
 	if (!has_stats_of_kind(rel->statlist, STATS_EXT_DEPENDENCIES))
 		return 1.0;
 
-	list_attnums = (AttrNumber *) palloc(sizeof(AttrNumber) *
+	list_attnums = (Bitmapset **) palloc(sizeof(Bitmapset *) *
 										 list_length(clauses));
 
 	/*
@@ -981,11 +981,11 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 		if (!bms_is_member(listidx, *estimatedclauses) &&
 			dependency_is_compatible_clause(clause, rel->relid, &attnum))
 		{
-			list_attnums[listidx] = attnum;
+			list_attnums[listidx] = bms_make_singleton(attnum);
 			clauses_attnums = bms_add_member(clauses_attnums, attnum);
 		}
 		else
-			list_attnums[listidx] = InvalidAttrNumber;
+			list_attnums[listidx] = NULL;
 
 		listidx++;
 	}
@@ -1002,8 +1002,8 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 	}
 
 	/* find the best suited statistics object for these attnums */
-	stat = choose_best_statistics(rel->statlist, clauses_attnums,
-								  STATS_EXT_DEPENDENCIES);
+	stat = choose_best_statistics(rel->statlist, STATS_EXT_DEPENDENCIES,
+								  list_attnums, list_length(clauses));
 
 	/* if no matching stats could be found then we've nothing to do */
 	if (!stat)
@@ -1043,15 +1043,21 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 		foreach(l, clauses)
 		{
 			Node	   *clause;
+			AttrNumber	attnum;
 
 			listidx++;
 
 			/*
 			 * Skip incompatible clauses, and ones we've already estimated on.
 			 */
-			if (list_attnums[listidx] == InvalidAttrNumber)
+			if (!list_attnums[listidx])
 				continue;
 
+			/*
+			 * We expect the bitmaps ton contain a single attribute number.
+			 */
+			attnum = bms_singleton_member(list_attnums[listidx]);
+
 			/*
 			 * Technically we could find more than one clause for a given
 			 * attnum. Since these clauses must be equality clauses, we choose
@@ -1061,8 +1067,7 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 			 * anyway. If it happens to be compared to the same Const, then
 			 * ignoring the additional clause is just the thing to do.
 			 */
-			if (dependency_implies_attribute(dependency,
-											 list_attnums[listidx]))
+			if (dependency_implies_attribute(dependency, attnum))
 			{
 				clause = (Node *) lfirst(l);
 
@@ -1077,8 +1082,7 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
 				 * We'll want to ignore this when looking for the next
 				 * strongest dependency above.
 				 */
-				clauses_attnums = bms_del_member(clauses_attnums,
-												 list_attnums[listidx]);
+				clauses_attnums = bms_del_member(clauses_attnums, attnum);
 			}
 		}
 

src/backend/statistics/extended_stats.c

@@ -844,15 +844,20 @@ has_stats_of_kind(List *stats, char requiredkind)
  *		there's no match.
  *
  * The current selection criteria is very simple - we choose the statistics
- * object referencing the most of the requested attributes, breaking ties
- * in favor of objects with fewer keys overall.
+ * object referencing the most attributes in covered (and still unestimated
+ * clauses), breaking ties in favor of objects with fewer keys overall.
+ *
+ * The clause_attnums is an array of bitmaps, storing attnums for individual
+ * clauses. A NULL element means the clause is either incompatible or already
+ * estimated.
  *
  * XXX If multiple statistics objects tie on both criteria, then which object
  * is chosen depends on the order that they appear in the stats list. Perhaps
  * further tiebreakers are needed.
  */
 StatisticExtInfo *
-choose_best_statistics(List *stats, Bitmapset *attnums, char requiredkind)
+choose_best_statistics(List *stats, char requiredkind,
+					   Bitmapset **clause_attnums, int nclauses)
 {
 	ListCell   *lc;
 	StatisticExtInfo *best_match = NULL;
@@ -861,17 +866,33 @@ choose_best_statistics(List *stats, Bitmapset *attnums, char requiredkind)
 
 	foreach(lc, stats)
 	{
+		int			i;
 		StatisticExtInfo *info = (StatisticExtInfo *) lfirst(lc);
+		Bitmapset  *matched = NULL;
 		int			num_matched;
 		int			numkeys;
-		Bitmapset  *matched;
 
 		/* skip statistics that are not of the correct type */
 		if (info->kind != requiredkind)
 			continue;
 
-		/* determine how many attributes of these stats can be matched to */
-		matched = bms_intersect(attnums, info->keys);
+		/*
+		 * Collect attributes in remaining (unestimated) clauses fully covered
+		 * by this statistic object.
+		 */
+		for (i = 0; i < nclauses; i++)
+		{
+			/* ignore incompatible/estimated clauses */
+			if (!clause_attnums[i])
+				continue;
+
+			/* ignore clauses that are not covered by this object */
+			if (!bms_is_subset(clause_attnums[i], info->keys))
+				continue;
+
+			matched = bms_add_members(matched, clause_attnums[i]);
+		}
+
 		num_matched = bms_num_members(matched);
 		bms_free(matched);
 
@@ -1233,12 +1254,9 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
 		listidx++;
 	}
 
-	/* We need at least two attributes for multivariate statistics. */
-	if (bms_membership(clauses_attnums) != BMS_MULTIPLE)
-		return 1.0;
-
 	/* find the best suited statistics object for these attnums */
-	stat = choose_best_statistics(rel->statlist, clauses_attnums, STATS_EXT_MCV);
+	stat = choose_best_statistics(rel->statlist, STATS_EXT_MCV,
+								  list_attnums, list_length(clauses));
 
 	/* if no matching stats could be found then we've nothing to do */
 	if (!stat)

src/include/statistics/statistics.h

@@ -118,7 +118,8 @@ extern Selectivity statext_clauselist_selectivity(PlannerInfo *root,
 												  RelOptInfo *rel,
 												  Bitmapset **estimatedclauses);
 extern bool has_stats_of_kind(List *stats, char requiredkind);
-extern StatisticExtInfo *choose_best_statistics(List *stats,
-												Bitmapset *attnums, char requiredkind);
+extern StatisticExtInfo *choose_best_statistics(List *stats, char requiredkind,
+												Bitmapset **clause_attnums,
+												int nclauses);
 
 #endif							/* STATISTICS_H */

src/test/regress/expected/stats_ext.out

@@ -534,6 +534,18 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <
          1 |     50
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
 -- create statistics
 CREATE STATISTICS mcv_lists_stats (mcv) ON a, b, c FROM mcv_lists;
 ANALYZE mcv_lists;
@@ -573,6 +585,19 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <
         50 |     50
 (1 row)
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+ estimated | actual 
+-----------+--------
+       200 |    200
+(1 row)
+
+-- we can't use the statistic for OR clauses that are not fully covered (missing 'd' attribute)
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+ estimated | actual 
+-----------+--------
+       343 |    200
+(1 row)
+
 -- check change of unrelated column type does not reset the MCV statistics
 ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);
 SELECT d.stxdmcv IS NOT NULL

src/test/regress/sql/stats_ext.sql

@@ -342,6 +342,10 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a < 5 AND b <
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <= ''0'' AND c <= 4');
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+
 -- create statistics
 CREATE STATISTICS mcv_lists_stats (mcv) ON a, b, c FROM mcv_lists;
 
@@ -359,6 +363,11 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a < 5 AND b <
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a <= 4 AND b <= ''0'' AND c <= 4');
 
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1');
+
+-- we can't use the statistic for OR clauses that are not fully covered (missing 'd' attribute)
+SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a = 1 OR b = ''1'' OR c = 1 OR d IS NOT NULL');
+
 -- check change of unrelated column type does not reset the MCV statistics
 ALTER TABLE mcv_lists ALTER COLUMN d TYPE VARCHAR(64);