Improve estimation of ANDs under ORs using extended statistics.

deanrasheed · deanrasheed · commit 4f5760d4afa9 · 2020-12-08T20:10:11.000Z
Formerly, extended statistics only handled clauses that were RestrictInfos. However, the restrictinfo machinery doesn't create sub-AND RestrictInfos for AND clauses underneath OR clauses. Therefore teach extended statistics to handle bare AND clauses, looking for compatible RestrictInfo clauses underneath them. Dean Rasheed, reviewed by Tomas Vondra. Discussion: https://postgr.es/m/CAEZATCW=J65GUFm50RcPv-iASnS2mTXQbr=CfBvWRVhFLJ_fWA@mail.gmail.com
diff --git a/src/backend/optimizer/path/clausesel.c b/src/backend/optimizer/path/clausesel.c
@@ -538,7 +538,28 @@ find_single_rel_for_clauses(PlannerInfo *root, List *clauses)
 		 * However, currently the extended-stats machinery won't do anything
 		 * with non-RestrictInfo clauses anyway, so there's no point in
 		 * spending extra cycles; just fail if that's what we have.
+		 *
+		 * An exception to that rule is if we have a bare BoolExpr AND clause.
+		 * We treat this as a special case because the restrictinfo machinery
+		 * doesn't build RestrictInfos on top of AND clauses.
 		 */
+		if (is_andclause(rinfo))
+		{
+			RelOptInfo *rel;
+
+			rel = find_single_rel_for_clauses(root,
+											  ((BoolExpr *) rinfo)->args);
+
+			if (rel == NULL)
+				return NULL;
+			if (lastrelid == 0)
+				lastrelid = rel->relid;
+			else if (rel->relid != lastrelid)
+				return NULL;
+
+			continue;
+		}
+
 		if (!IsA(rinfo, RestrictInfo))
 			return NULL;
 
diff --git a/src/backend/statistics/extended_stats.c b/src/backend/statistics/extended_stats.c
@@ -1174,6 +1174,31 @@ statext_is_compatible_clause(PlannerInfo *root, Node *clause, Index relid,
 	RestrictInfo *rinfo = (RestrictInfo *) clause;
 	Oid			userid;
 
+	/*
+	 * Special-case handling for bare BoolExpr AND clauses, because the
+	 * restrictinfo machinery doesn't build RestrictInfos on top of AND
+	 * clauses.
+	 */
+	if (is_andclause(clause))
+	{
+		BoolExpr   *expr = (BoolExpr *) clause;
+		ListCell   *lc;
+
+		/*
+		 * Check that each sub-clause is compatible.  We expect these to be
+		 * RestrictInfos.
+		 */
+		foreach(lc, expr->args)
+		{
+			if (!statext_is_compatible_clause(root, (Node *) lfirst(lc),
+											  relid, attnums))
+				return false;
+		}
+
+		return true;
+	}
+
+	/* Otherwise it must be a RestrictInfo. */
 	if (!IsA(rinfo, RestrictInfo))
 		return false;
 
diff --git a/src/test/regress/expected/stats_ext.out b/src/test/regress/expected/stats_ext.out
@@ -1594,13 +1594,13 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists_partial WHERE a = 0
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists_partial WHERE (a = 0 AND b = 0 AND c = 0) OR (a = 1 AND b = 1 AND c = 1) OR (a = 2 AND b = 2 AND c = 2)');
  estimated | actual 
 -----------+--------
-       300 |    306
+       306 |    306
 (1 row)
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists_partial WHERE (a = 0 AND b = 0) OR (a = 0 AND c = 0) OR (b = 0 AND c = 0)');
  estimated | actual 
 -----------+--------
-       306 |    102
+       108 |    102
 (1 row)
 
 DROP TABLE mcv_lists_partial;
