Fix distinctness check for queries with grouping sets

query_is_distinct_for() is intended to determine whether a query never
returns duplicates of the specified columns.  For queries using
grouping sets, if there are no grouping expressions, the query may
contain one or more empty grouping sets.  The goal is to detect
whether there is exactly one empty grouping set, in which case the
query would return a single row and thus be distinct.

The previous logic in query_is_distinct_for() was incomplete because
the check was insufficiently thorough and could return false when it
could have returned true.  It failed to consider cases where the
DISTINCT clause is used on the GROUP BY, in which case duplicate empty
grouping sets are removed, leaving only one.  It also did not
correctly handle all possible structures of GroupingSet nodes that
represent a single empty grouping set.

To fix, add a check for the groupDistinct flag, and expand the query's
groupingSets tree into a flat list, then verify that the expanded list
contains only one element.

No backpatch as this could result in plan changes.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAMbWs480Z04NtP8-O55uROq2Zego309+h3hhaZhz6ztmgWLEBw@mail.gmail.com

src/backend/optimizer/plan/analyzejoins.c

@@ -31,6 +31,7 @@
 #include "optimizer/placeholder.h"
 #include "optimizer/planmain.h"
 #include "optimizer/restrictinfo.h"
+#include "parser/parse_agg.h"
 #include "rewrite/rewriteManip.h"
 #include "utils/lsyscache.h"
 
@@ -1175,6 +1176,8 @@ query_is_distinct_for(Query *query, List *colnos, List *opids)
 	}
 	else if (query->groupingSets)
 	{
+		List	   *gsets;
+
 		/*
 		 * If we have grouping sets with expressions, we probably don't have
 		 * uniqueness and analysis would be hard. Punt.
@@ -1184,15 +1187,17 @@ query_is_distinct_for(Query *query, List *colnos, List *opids)
 
 		/*
 		 * If we have no groupClause (therefore no grouping expressions), we
-		 * might have one or many empty grouping sets. If there's just one,
-		 * then we're returning only one row and are certainly unique. But
-		 * otherwise, we know we're certainly not unique.
+		 * might have one or many empty grouping sets.  If there's just one,
+		 * or if the DISTINCT clause is used on the GROUP BY, then we're
+		 * returning only one row and are certainly unique.  But otherwise, we
+		 * know we're certainly not unique.
 		 */
-		if (list_length(query->groupingSets) == 1 &&
-			((GroupingSet *) linitial(query->groupingSets))->kind == GROUPING_SET_EMPTY)
+		if (query->groupDistinct)
 			return true;
-		else
-			return false;
+
+		gsets = expand_grouping_sets(query->groupingSets, false, -1);
+
+		return (list_length(gsets) == 1);
 	}
 	else
 	{

src/test/regress/expected/join.out

@@ -6161,6 +6161,40 @@ select d.* from d left join (select * from b group by b.id, b.c_id) s
  Seq Scan on d
 (1 row)
 
+-- check that join removal works for a left join when joining a subquery
+-- that is guaranteed to be unique by GROUPING SETS
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by ()) s
+  on d.a = s.x;
+  QUERY PLAN   
+---------------
+ Seq Scan on d
+(1 row)
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets(())) s
+  on d.a = s.x;
+  QUERY PLAN   
+---------------
+ Seq Scan on d
+(1 row)
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets(()), grouping sets(())) s
+  on d.a = s.x;
+  QUERY PLAN   
+---------------
+ Seq Scan on d
+(1 row)
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by distinct grouping sets((), ())) s
+  on d.a = s.x;
+  QUERY PLAN   
+---------------
+ Seq Scan on d
+(1 row)
+
 -- similarly, but keying off a DISTINCT clause
 explain (costs off)
 select d.* from d left join (select distinct * from b) s
@@ -6189,6 +6223,55 @@ select d.* from d left join (select * from b group by b.id, b.c_id) s
          ->  Seq Scan on d
 (8 rows)
 
+-- join removal is not possible when the GROUP BY contains non-empty grouping
+-- sets or multiple empty grouping sets
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by rollup(x)) s
+  on d.a = s.x;
+           QUERY PLAN            
+---------------------------------
+ Hash Left Join
+   Hash Cond: (d.a = (1))
+   ->  Seq Scan on d
+   ->  Hash
+         ->  MixedAggregate
+               Hash Key: 1
+               Group Key: ()
+               ->  Seq Scan on b
+(8 rows)
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets((), ())) s
+  on d.a = s.x;
+               QUERY PLAN                
+-----------------------------------------
+ Hash Left Join
+   Hash Cond: (d.a = (1))
+   ->  Seq Scan on d
+   ->  Hash
+         ->  Append
+               ->  Result
+                     Replaces: Aggregate
+               ->  Result
+                     Replaces: Aggregate
+(9 rows)
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets((), grouping sets(()))) s
+  on d.a = s.x;
+               QUERY PLAN                
+-----------------------------------------
+ Hash Left Join
+   Hash Cond: (d.a = (1))
+   ->  Seq Scan on d
+   ->  Hash
+         ->  Append
+               ->  Result
+                     Replaces: Aggregate
+               ->  Result
+                     Replaces: Aggregate
+(9 rows)
+
 -- similarly, but keying off a DISTINCT clause
 explain (costs off)
 select d.* from d left join (select distinct * from b) s

src/test/regress/sql/join.sql

@@ -2232,6 +2232,24 @@ explain (costs off)
 select d.* from d left join (select * from b group by b.id, b.c_id) s
   on d.a = s.id and d.b = s.c_id;
 
+-- check that join removal works for a left join when joining a subquery
+-- that is guaranteed to be unique by GROUPING SETS
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by ()) s
+  on d.a = s.x;
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets(())) s
+  on d.a = s.x;
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets(()), grouping sets(())) s
+  on d.a = s.x;
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by distinct grouping sets((), ())) s
+  on d.a = s.x;
+
 -- similarly, but keying off a DISTINCT clause
 explain (costs off)
 select d.* from d left join (select distinct * from b) s
@@ -2245,6 +2263,20 @@ explain (costs off)
 select d.* from d left join (select * from b group by b.id, b.c_id) s
   on d.a = s.id;
 
+-- join removal is not possible when the GROUP BY contains non-empty grouping
+-- sets or multiple empty grouping sets
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by rollup(x)) s
+  on d.a = s.x;
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets((), ())) s
+  on d.a = s.x;
+
+explain (costs off)
+select d.* from d left join (select 1 as x from b group by grouping sets((), grouping sets(()))) s
+  on d.a = s.x;
+
 -- similarly, but keying off a DISTINCT clause
 explain (costs off)
 select d.* from d left join (select distinct * from b) s