Fix handling of clauses incompatible with extended statistics

Handling of incompatible clauses while applying extended statistics was
a bit confused - while handling a mix of compatible and incompatible
clauses it sometimes incorrectly treated the incompatible clauses as
compatible, resulting in a crash.

Fixed by reworking the code applying the selected statistics object to
make it easier to understand, and adding a proper compatibility check.

Reported-by: David Rowley
Discussion: https://postgr.es/m/CAApHDvpYT10-nkSp8xXe-nbO3jmoaRyRFHbzh-RWMfAJynqgpQ%40mail.gmail.com

src/backend/statistics/extended_stats.c

@@ -1255,6 +1255,10 @@ choose_best_statistics(List *stats, char requiredkind,
 		/*
 		 * Collect attributes and expressions in remaining (unestimated)
 		 * clauses fully covered by this statistic object.
+		 *
+		 * We know already estimated clauses have both clause_attnums and
+		 * clause_exprs set to NULL. We leave the pointers NULL if already
+		 * estimated, or we reset them to NULL after estimating the clause.
 		 */
 		for (i = 0; i < nclauses; i++)
 		{
@@ -1758,19 +1762,44 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
 		/* record which clauses are simple (single column or expression) */
 		simple_clauses = NULL;
 
-		listidx = 0;
+		listidx = -1;
 		foreach(l, clauses)
 		{
+			/* Increment the index before we decide if to skip the clause. */
+			listidx++;
+
 			/*
-			 * If the clause is not already estimated and is compatible with
-			 * the selected statistics object (all attributes and expressions
-			 * covered), mark it as estimated and add it to the list to
-			 * estimate.
+			 * Ignore clauses from which we did not extract any attnums or
+			 * expressions (this needs to be consistent with what we do in
+			 * choose_best_statistics).
+			 *
+			 * This also eliminates already estimated clauses - both those
+			 * estimated before and during applying extended statistics.
+			 *
+			 * XXX This check is needed because both bms_is_subset and
+			 * stat_covers_expressions return true for empty attnums and
+			 * expressions.
 			 */
-			if (!bms_is_member(listidx, *estimatedclauses) &&
-				bms_is_subset(list_attnums[listidx], stat->keys) &&
-				stat_covers_expressions(stat, list_exprs[listidx], NULL))
-			{
+			if (!list_attnums[listidx] && !list_exprs[listidx])
+				continue;
+
+			/*
+			 * The clause was not estimated yet, and we've extracted either
+			 * attnums of expressions from it. Ignore it if it's not fully
+			 * covered by the chosen statistics.
+			 *
+			 * We need to check both attributes and expressions, and reject
+			 * if either is not covered.
+			 */
+			if (!bms_is_subset(list_attnums[listidx], stat->keys) ||
+				!stat_covers_expressions(stat, list_exprs[listidx], NULL))
+				continue;
+
+			/*
+			 * Now we know the clause is compatible (we have either atttnums
+			 * or expressions extracted from it), and was not estimated yet.
+			 */
+
 			/* record simple clauses (single column or expression) */
 			if ((list_attnums[listidx] == NULL &&
 				 list_length(list_exprs[listidx]) == 1) ||
@@ -1779,10 +1808,14 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
 				simple_clauses = bms_add_member(simple_clauses,
 												list_length(stat_clauses));
 
-				/* add clause to list and mark as estimated */
+			/* add clause to list and mark it as estimated */
 			stat_clauses = lappend(stat_clauses, (Node *) lfirst(l));
 			*estimatedclauses = bms_add_member(*estimatedclauses, listidx);
 
+			/*
+			 * Reset the pointers, so that choose_best_statistics knows this
+			 * clause was estimated and does not consider it again.
+			 */
 			bms_free(list_attnums[listidx]);
 			list_attnums[listidx] = NULL;
 
@@ -1790,9 +1823,6 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
 			list_exprs[listidx] = NULL;
 		}
 
-			listidx++;
-		}
-
 		if (is_or)
 		{
 			bool	   *or_matches = NULL;

src/backend/statistics/mcv.c

@@ -1575,6 +1575,8 @@ mcv_match_expression(Node *expr, Bitmapset *keys, List *exprs, Oid *collid)
 			   (idx <= bms_num_members(keys) + list_length(exprs)));
 	}
 
+	Assert((idx >= 0) && (idx < bms_num_members(keys) + list_length(exprs)));
+
 	return idx;
 }
 
@@ -1654,6 +1656,8 @@ mcv_get_match_bitmap(PlannerInfo *root, List *clauses,
 			/* match the attribute/expression to a dimension of the statistic */
 			idx = mcv_match_expression(clause_expr, keys, exprs, &collid);
 
+			Assert((idx >= 0) && (idx < bms_num_members(keys) + list_length(exprs)));
+
 			/*
 			 * Walk through the MCV items and evaluate the current clause. We
 			 * can skip items that were already ruled out, and terminate if

src/test/regress/expected/stats_ext.out

@@ -2938,6 +2938,25 @@ SELECT * FROM check_estimated_rows('SELECT * FROM expr_stats WHERE a = 0 AND (b
 (1 row)
 
 DROP TABLE expr_stats;
+-- test handling of a mix of compatible and incompatible expressions
+CREATE TABLE expr_stats_incompatible_test (
+    c0 double precision,
+    c1 boolean NOT NULL
+);
+CREATE STATISTICS expr_stat_comp_1 ON c0, c1 FROM expr_stats_incompatible_test;
+INSERT INTO expr_stats_incompatible_test VALUES (1234,false), (5678,true);
+ANALYZE expr_stats_incompatible_test;
+SELECT c0 FROM ONLY expr_stats_incompatible_test WHERE
+(
+  upper('x') LIKE ('x'||('[0,1]'::int4range))
+  AND
+  (c0 IN (0, 1) OR c1)
+);
+ c0 
+----
+(0 rows)
+
+DROP TABLE expr_stats_incompatible_test;
 -- Permission tests. Users should not be able to see specific data values in
 -- the extended statistics, if they lack permission to see those values in
 -- the underlying table.

src/test/regress/sql/stats_ext.sql

@@ -1470,6 +1470,25 @@ SELECT * FROM check_estimated_rows('SELECT * FROM expr_stats WHERE a = 0 AND (b
 
 DROP TABLE expr_stats;
 
+-- test handling of a mix of compatible and incompatible expressions
+CREATE TABLE expr_stats_incompatible_test (
+    c0 double precision,
+    c1 boolean NOT NULL
+);
+
+CREATE STATISTICS expr_stat_comp_1 ON c0, c1 FROM expr_stats_incompatible_test;
+
+INSERT INTO expr_stats_incompatible_test VALUES (1234,false), (5678,true);
+ANALYZE expr_stats_incompatible_test;
+
+SELECT c0 FROM ONLY expr_stats_incompatible_test WHERE
+(
+  upper('x') LIKE ('x'||('[0,1]'::int4range))
+  AND
+  (c0 IN (0, 1) OR c1)
+);
+
+DROP TABLE expr_stats_incompatible_test;
 
 -- Permission tests. Users should not be able to see specific data values in
 -- the extended statistics, if they lack permission to see those values in