Improve statistics estimation for single-column GROUP BY in sub-queries

This commit follows the idea of the 4767bc8ff2.  If sub-query has only one
GROUP BY column, we can consider its output variable as being unique. We can
employ this fact in the statistics to make more precise estimations in the
upper query block.

Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>

src/backend/utils/adt/selfuncs.c

@@ -322,10 +322,10 @@ var_eq_const(VariableStatData *vardata, Oid oproid, Oid collation,
 	}
 
 	/*
-	 * If we matched the var to a unique index or DISTINCT clause, assume
-	 * there is exactly one match regardless of anything else.  (This is
-	 * slightly bogus, since the index or clause's equality operator might be
-	 * different from ours, but it's much more likely to be right than
+	 * If we matched the var to a unique index, DISTINCT or GROUP-BY clause,
+	 * assume there is exactly one match regardless of anything else.  (This
+	 * is slightly bogus, since the index or clause's equality operator might
+	 * be different from ours, but it's much more likely to be right than
 	 * ignoring the information.)
 	 */
 	if (vardata->isunique && vardata->rel && vardata->rel->tuples >= 1.0)
@@ -484,10 +484,10 @@ var_eq_non_const(VariableStatData *vardata, Oid oproid, Oid collation,
 	}
 
 	/*
-	 * If we matched the var to a unique index or DISTINCT clause, assume
-	 * there is exactly one match regardless of anything else.  (This is
-	 * slightly bogus, since the index or clause's equality operator might be
-	 * different from ours, but it's much more likely to be right than
+	 * If we matched the var to a unique index, DISTINCT or GROUP-BY clause,
+	 * assume there is exactly one match regardless of anything else.  (This
+	 * is slightly bogus, since the index or clause's equality operator might
+	 * be different from ours, but it's much more likely to be right than
 	 * ignoring the information.)
 	 */
 	if (vardata->isunique && vardata->rel && vardata->rel->tuples >= 1.0)
@@ -5018,11 +5018,11 @@ ReleaseDummy(HeapTuple tuple)
  *	atttype, atttypmod: actual type/typmod of the "var" expression.  This is
  *		commonly the same as the exposed type of the variable argument,
  *		but can be different in binary-compatible-type cases.
- *	isunique: true if we were able to match the var to a unique index or a
- *		single-column DISTINCT clause, implying its values are unique for
- *		this query.  (Caution: this should be trusted for statistical
- *		purposes only, since we do not check indimmediate nor verify that
- *		the exact same definition of equality applies.)
+ *	isunique: true if we were able to match the var to a unique index, a
+ *		single-column DISTINCT or GROUP-BY clause, implying its values are
+ *		unique for this query.  (Caution: this should be trusted for
+ *		statistical purposes only, since we do not check indimmediate nor
+ *		verify that the exact same definition of equality applies.)
  *	acl_ok: true if current user has permission to read the column(s)
  *		underlying the pg_statistic entry.  This is consulted by
  *		statistic_proc_security_check().
@@ -5680,15 +5680,14 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 		Assert(IsA(subquery, Query));
 
 		/*
-		 * Punt if subquery uses set operations or GROUP BY, as these will
-		 * mash underlying columns' stats beyond recognition.  (Set ops are
-		 * particularly nasty; if we forged ahead, we would return stats
+		 * Punt if subquery uses set operations or grouping sets, as these
+		 * will mash underlying columns' stats beyond recognition.  (Set ops
+		 * are particularly nasty; if we forged ahead, we would return stats
 		 * relevant to only the leftmost subselect...)	DISTINCT is also
 		 * problematic, but we check that later because there is a possibility
 		 * of learning something even with it.
 		 */
 		if (subquery->setOperations ||
-			subquery->groupClause ||
 			subquery->groupingSets)
 			return;
 
@@ -5718,6 +5717,16 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 			return;
 		}
 
+		/* The same idea as with DISTINCT clause works for a GROUP-BY too */
+		if (subquery->groupClause)
+		{
+			if (list_length(subquery->groupClause) == 1 &&
+				targetIsInSortList(ste, InvalidOid, subquery->groupClause))
+				vardata->isunique = true;
+			/* cannot go further */
+			return;
+		}
+
 		/*
 		 * If the sub-query originated from a view with the security_barrier
 		 * attribute, we must not look at the variable's statistics, though it
@@ -5869,11 +5878,11 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
 	}
 
 	/*
-	 * If there is a unique index or DISTINCT clause for the variable, assume
-	 * it is unique no matter what pg_statistic says; the statistics could be
-	 * out of date, or we might have found a partial unique index that proves
-	 * the var is unique for this query.  However, we'd better still believe
-	 * the null-fraction statistic.
+	 * If there is a unique index, DISTINCT or GROUP-BY clause for the
+	 * variable, assume it is unique no matter what pg_statistic says; the
+	 * statistics could be out of date, or we might have found a partial
+	 * unique index that proves the var is unique for this query.  However,
+	 * we'd better still believe the null-fraction statistic.
 	 */
 	if (vardata->isunique)
 		stadistinct = -1.0 * (1.0 - stanullfrac);

src/include/utils/selfuncs.h

@@ -94,7 +94,8 @@ typedef struct VariableStatData
 	Oid			vartype;		/* exposed type of expression */
 	Oid			atttype;		/* actual type (after stripping relabel) */
 	int32		atttypmod;		/* actual typmod (after stripping relabel) */
-	bool		isunique;		/* matches unique index or DISTINCT clause */
+	bool		isunique;		/* matches unique index, DISTINCT or GROUP-BY
+								 * clause */
 	bool		acl_ok;			/* result of ACL check on table or column */
 } VariableStatData;
 

src/test/regress/expected/stats_ext.out

@@ -3368,3 +3368,18 @@ NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to table tststats.priv_test_tbl
 drop cascades to view tststats.priv_test_view
 DROP USER regress_stats_user1;
+CREATE TABLE grouping_unique (x integer);
+INSERT INTO grouping_unique (x) SELECT gs FROM generate_series(1,1000) AS gs;
+ANALYZE grouping_unique;
+-- Optimiser treat GROUP-BY operator as an 'uniqueser' of the input
+SELECT * FROM check_estimated_rows('
+  SELECT * FROM generate_series(1, 1) t1 LEFT JOIN (
+    SELECT x FROM grouping_unique t2 GROUP BY x) AS q1
+  ON t1.t1 = q1.x;
+');
+ estimated | actual 
+-----------+--------
+         1 |      1
+(1 row)
+
+DROP TABLE grouping_unique;

src/test/regress/sql/stats_ext.sql

@@ -1707,3 +1707,15 @@ RESET SESSION AUTHORIZATION;
 DROP TABLE stats_ext_tbl;
 DROP SCHEMA tststats CASCADE;
 DROP USER regress_stats_user1;
+
+CREATE TABLE grouping_unique (x integer);
+INSERT INTO grouping_unique (x) SELECT gs FROM generate_series(1,1000) AS gs;
+ANALYZE grouping_unique;
+
+-- Optimiser treat GROUP-BY operator as an 'uniqueser' of the input
+SELECT * FROM check_estimated_rows('
+  SELECT * FROM generate_series(1, 1) t1 LEFT JOIN (
+    SELECT x FROM grouping_unique t2 GROUP BY x) AS q1
+  ON t1.t1 = q1.x;
+');
+DROP TABLE grouping_unique;