Implement GROUP BY DISTINCT

With grouping sets, it's possible that some of the grouping sets are
duplicate.  This is especially common with CUBE and ROLLUP clauses. For
example GROUP BY CUBE (a,b), CUBE (b,c) is equivalent to

  GROUP BY GROUPING SETS (
    (a, b, c),
    (a, b, c),
    (a, b, c),
    (a, b),
    (a, b),
    (a, b),
    (a),
    (a),
    (a),
    (c, a),
    (c, a),
    (c, a),
    (c),
    (b, c),
    (b),
    ()
  )

Some of the grouping sets are calculated multiple times, which is mostly
unnecessary.  This commit implements a new GROUP BY DISTINCT feature, as
defined in the SQL standard, which eliminates the duplicate sets.

Author: Vik Fearing
Reviewed-by: Erik Rijkers, Georgios Kokolatos, Tomas Vondra
Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org

2 files changed, 137 insertions, 0 deletions

diff --git a/src/test/regress/expected/groupingsets.out b/src/test/regress/expected/groupingsets.out
index 7c844c6e09..4c467c1b15 100644
--- a/src/test/regress/expected/groupingsets.out
+++ b/src/test/regress/expected/groupingsets.out
@@ -1929,4 +1929,115 @@ set work_mem to default;
 
 drop table gs_group_1;
 drop table gs_hash_1;
+-- GROUP BY DISTINCT
+-- "normal" behavior...
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by all rollup(a, b), rollup(a, c)
+order by a, b, c;
+ a | b | c 
+---+---+---
+ 1 | 2 | 3
+ 1 | 2 |  
+ 1 | 2 |  
+ 1 |   | 3
+ 1 |   | 3
+ 1 |   |  
+ 1 |   |  
+ 1 |   |  
+ 4 |   | 6
+ 4 |   | 6
+ 4 |   | 6
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 7 | 8 | 9
+ 7 | 8 |  
+ 7 | 8 |  
+ 7 |   | 9
+ 7 |   | 9
+ 7 |   |  
+ 7 |   |  
+ 7 |   |  
+   |   |  
+(25 rows)
+
+-- ...which is also the default
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by rollup(a, b), rollup(a, c)
+order by a, b, c;
+ a | b | c 
+---+---+---
+ 1 | 2 | 3
+ 1 | 2 |  
+ 1 | 2 |  
+ 1 |   | 3
+ 1 |   | 3
+ 1 |   |  
+ 1 |   |  
+ 1 |   |  
+ 4 |   | 6
+ 4 |   | 6
+ 4 |   | 6
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 4 |   |  
+ 7 | 8 | 9
+ 7 | 8 |  
+ 7 | 8 |  
+ 7 |   | 9
+ 7 |   | 9
+ 7 |   |  
+ 7 |   |  
+ 7 |   |  
+   |   |  
+(25 rows)
+
+-- "group by distinct" behavior...
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by distinct rollup(a, b), rollup(a, c)
+order by a, b, c;
+ a | b | c 
+---+---+---
+ 1 | 2 | 3
+ 1 | 2 |  
+ 1 |   | 3
+ 1 |   |  
+ 4 |   | 6
+ 4 |   | 6
+ 4 |   |  
+ 4 |   |  
+ 7 | 8 | 9
+ 7 | 8 |  
+ 7 |   | 9
+ 7 |   |  
+   |   |  
+(13 rows)
+
+-- ...which is not the same as "select distinct"
+select distinct a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by rollup(a, b), rollup(a, c)
+order by a, b, c;
+ a | b | c 
+---+---+---
+ 1 | 2 | 3
+ 1 | 2 |  
+ 1 |   | 3
+ 1 |   |  
+ 4 |   | 6
+ 4 |   |  
+ 7 | 8 | 9
+ 7 | 8 |  
+ 7 |   | 9
+ 7 |   |  
+   |   |  
+(11 rows)
+
 -- end
diff --git a/src/test/regress/sql/groupingsets.sql b/src/test/regress/sql/groupingsets.sql
index 18ae803e9d..3944944704 100644
--- a/src/test/regress/sql/groupingsets.sql
+++ b/src/test/regress/sql/groupingsets.sql
@@ -529,4 +529,30 @@ set work_mem to default;
 drop table gs_group_1;
 drop table gs_hash_1;
 
+-- GROUP BY DISTINCT
+
+-- "normal" behavior...
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by all rollup(a, b), rollup(a, c)
+order by a, b, c;
+
+-- ...which is also the default
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by rollup(a, b), rollup(a, c)
+order by a, b, c;
+
+-- "group by distinct" behavior...
+select a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by distinct rollup(a, b), rollup(a, c)
+order by a, b, c;
+
+-- ...which is not the same as "select distinct"
+select distinct a, b, c
+from (values (1, 2, 3), (4, null, 6), (7, 8, 9)) as t (a, b, c)
+group by rollup(a, b), rollup(a, c)
+order by a, b, c;
+
 -- end