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

doc/src/sgml/queries.sgml

@@ -1372,6 +1372,55 @@ GROUP BY GROUPING SETS (
 </programlisting>
    </para>
 
+   <para>
+    <indexterm zone="queries-grouping-sets">
+     <primary>ALL</primary>
+     <secondary>GROUP BY ALL</secondary>
+    </indexterm>
+    <indexterm zone="queries-grouping-sets">
+     <primary>DISTINCT</primary>
+     <secondary>GROUP BY DISTINCT</secondary>
+    </indexterm>
+    When specifying multiple grouping items together, the final set of grouping
+    sets might contain duplicates. For example:
+<programlisting>
+GROUP BY ROLLUP (a, b), ROLLUP (a, c)
+</programlisting>
+    is equivalent to
+<programlisting>
+GROUP BY GROUPING SETS (
+    (a, b, c),
+    (a, b),
+    (a, b),
+    (a, c),
+    (a),
+    (a),
+    (a, c),
+    (a),
+    ()
+)
+</programlisting>
+    If these duplicates are undesirable, they can be removed using the
+    <literal>DISTINCT</literal> clause directly on the <literal>GROUP BY</literal>.
+    Therefore:
+<programlisting>
+GROUP BY <emphasis>DISTINCT</emphasis> ROLLUP (a, b), ROLLUP (a, c)
+</programlisting>
+    is equivalent to
+<programlisting>
+GROUP BY GROUPING SETS (
+    (a, b, c),
+    (a, b),
+    (a, c),
+    (a),
+    ()
+)
+</programlisting>
+    This is not the same as using <literal>SELECT DISTINCT</literal> because the output
+    rows may still contain duplicates.  If any of the ungrouped columns contains NULL,
+    it will be indistinguishable from the NULL used when that same column is grouped.
+   </para>
+
   <note>
    <para>
     The construct <literal>(a, b)</literal> is normally recognized in expressions as
@@ -1560,8 +1609,13 @@ SELECT a "from", b + c AS sum FROM ...
   <sect2 id="queries-distinct">
    <title><literal>DISTINCT</literal></title>
 
+   <indexterm zone="queries-distinct">
+    <primary>ALL</primary>
+    <secondary>SELECT ALL</secondary>
+   </indexterm>
    <indexterm zone="queries-distinct">
     <primary>DISTINCT</primary>
+    <secondary>SELECT DISTINCT</secondary>
    </indexterm>
 
    <indexterm zone="queries-distinct">

doc/src/sgml/ref/select.sgml

@@ -37,7 +37,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac
     [ * | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
     [ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
     [ WHERE <replaceable class="parameter">condition</replaceable> ]
-    [ GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...] ]
+    [ GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...] ]
     [ HAVING <replaceable class="parameter">condition</replaceable> ]
     [ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...] ]
     [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] <replaceable class="parameter">select</replaceable> ]
@@ -778,7 +778,7 @@ WHERE <replaceable class="parameter">condition</replaceable>
    <para>
     The optional <literal>GROUP BY</literal> clause has the general form
 <synopsis>
-GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...]
+GROUP BY [ ALL | DISTINCT ] <replaceable class="parameter">grouping_element</replaceable> [, ...]
 </synopsis>
    </para>
 
@@ -802,7 +802,10 @@ GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...]
     independent <replaceable>grouping sets</replaceable>.  The effect of this is
     equivalent to constructing a <literal>UNION ALL</literal> between
     subqueries with the individual grouping sets as their
-    <literal>GROUP BY</literal> clauses.  For further details on the handling
+    <literal>GROUP BY</literal> clauses.  The optional <literal>DISTINCT</literal>
+    clause removes duplicate sets before processing; it does <emphasis>not</emphasis>
+    transform the <literal>UNION ALL</literal> into a <literal>UNION DISTINCT</literal>.
+    For further details on the handling
     of grouping sets see <xref linkend="queries-grouping-sets"/>.
    </para>
 

src/backend/catalog/sql_features.txt

@@ -482,7 +482,7 @@ T351	Bracketed SQL comments (/*...*/ comments)			YES
 T431	Extended grouping capabilities			YES	
 T432	Nested and concatenated GROUPING SETS			YES	
 T433	Multiargument GROUPING function			YES	
-T434	GROUP BY DISTINCT			NO	
+T434	GROUP BY DISTINCT			YES	
 T441	ABS and MOD functions			YES	
 T461	Symmetric BETWEEN predicate			YES	
 T471	Result sets return value			NO	

src/backend/nodes/copyfuncs.c

@@ -3135,6 +3135,7 @@ _copyQuery(const Query *from)
 	COPY_NODE_FIELD(onConflict);
 	COPY_NODE_FIELD(returningList);
 	COPY_NODE_FIELD(groupClause);
+	COPY_SCALAR_FIELD(groupDistinct);
 	COPY_NODE_FIELD(groupingSets);
 	COPY_NODE_FIELD(havingQual);
 	COPY_NODE_FIELD(windowClause);
@@ -3221,6 +3222,7 @@ _copySelectStmt(const SelectStmt *from)
 	COPY_NODE_FIELD(fromClause);
 	COPY_NODE_FIELD(whereClause);
 	COPY_NODE_FIELD(groupClause);
+	COPY_SCALAR_FIELD(groupDistinct);
 	COPY_NODE_FIELD(havingClause);
 	COPY_NODE_FIELD(windowClause);
 	COPY_NODE_FIELD(valuesLists);

src/backend/nodes/equalfuncs.c

@@ -977,6 +977,7 @@ _equalQuery(const Query *a, const Query *b)
 	COMPARE_NODE_FIELD(onConflict);
 	COMPARE_NODE_FIELD(returningList);
 	COMPARE_NODE_FIELD(groupClause);
+	COMPARE_SCALAR_FIELD(groupDistinct);
 	COMPARE_NODE_FIELD(groupingSets);
 	COMPARE_NODE_FIELD(havingQual);
 	COMPARE_NODE_FIELD(windowClause);
@@ -1053,6 +1054,7 @@ _equalSelectStmt(const SelectStmt *a, const SelectStmt *b)
 	COMPARE_NODE_FIELD(fromClause);
 	COMPARE_NODE_FIELD(whereClause);
 	COMPARE_NODE_FIELD(groupClause);
+	COMPARE_SCALAR_FIELD(groupDistinct);
 	COMPARE_NODE_FIELD(havingClause);
 	COMPARE_NODE_FIELD(windowClause);
 	COMPARE_NODE_FIELD(valuesLists);

src/backend/nodes/list.c

@@ -1506,6 +1506,22 @@ list_sort(List *list, list_sort_comparator cmp)
 		qsort(list->elements, len, sizeof(ListCell), (qsort_comparator) cmp);
 }
 
+/*
+ * list_sort comparator for sorting a list into ascending int order.
+ */
+int
+list_int_cmp(const ListCell *p1, const ListCell *p2)
+{
+	int			v1 = lfirst_int(p1);
+	int			v2 = lfirst_int(p2);
+
+	if (v1 < v2)
+		return -1;
+	if (v1 > v2)
+		return 1;
+	return 0;
+}
+
 /*
  * list_sort comparator for sorting a list into ascending OID order.
  */

src/backend/nodes/outfuncs.c

@@ -2771,6 +2771,7 @@ _outSelectStmt(StringInfo str, const SelectStmt *node)
 	WRITE_NODE_FIELD(fromClause);
 	WRITE_NODE_FIELD(whereClause);
 	WRITE_NODE_FIELD(groupClause);
+	WRITE_BOOL_FIELD(groupDistinct);
 	WRITE_NODE_FIELD(havingClause);
 	WRITE_NODE_FIELD(windowClause);
 	WRITE_NODE_FIELD(valuesLists);
@@ -2996,6 +2997,7 @@ _outQuery(StringInfo str, const Query *node)
 	WRITE_NODE_FIELD(onConflict);
 	WRITE_NODE_FIELD(returningList);
 	WRITE_NODE_FIELD(groupClause);
+	WRITE_BOOL_FIELD(groupDistinct);
 	WRITE_NODE_FIELD(groupingSets);
 	WRITE_NODE_FIELD(havingQual);
 	WRITE_NODE_FIELD(windowClause);

src/backend/nodes/readfuncs.c

@@ -271,6 +271,7 @@ _readQuery(void)
 	READ_NODE_FIELD(onConflict);
 	READ_NODE_FIELD(returningList);
 	READ_NODE_FIELD(groupClause);
+	READ_BOOL_FIELD(groupDistinct);
 	READ_NODE_FIELD(groupingSets);
 	READ_NODE_FIELD(havingQual);
 	READ_NODE_FIELD(windowClause);

src/backend/optimizer/plan/planner.c

@@ -2442,7 +2442,7 @@ preprocess_grouping_sets(PlannerInfo *root)
 	ListCell   *lc_set;
 	grouping_sets_data *gd = palloc0(sizeof(grouping_sets_data));
 
-	parse->groupingSets = expand_grouping_sets(parse->groupingSets, -1);
+	parse->groupingSets = expand_grouping_sets(parse->groupingSets, parse->groupDistinct, -1);
 
 	gd->any_hashable = false;
 	gd->unhashable_refs = NULL;

src/backend/parser/analyze.c

@@ -1264,6 +1264,7 @@ transformSelectStmt(ParseState *pstate, SelectStmt *stmt)
 											qry->sortClause,
 											EXPR_KIND_GROUP_BY,
 											false /* allow SQL92 rules */ );
+	qry->groupDistinct = stmt->groupDistinct;
 
 	if (stmt->distinctClause == NIL)
 	{

src/backend/parser/gram.y

@@ -134,6 +134,13 @@ typedef struct SelectLimit
 	LimitOption limitOption;
 } SelectLimit;
 
+/* Private struct for the result of group_clause production */
+typedef struct GroupClause
+{
+	bool	distinct;
+	List   *list;
+} GroupClause;
+
 /* ConstraintAttributeSpec yields an integer bitmask of these flags: */
 #define CAS_NOT_DEFERRABLE			0x01
 #define CAS_DEFERRABLE				0x02
@@ -250,6 +257,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	PartitionBoundSpec	*partboundspec;
 	RoleSpec			*rolespec;
 	struct SelectLimit	*selectlimit;
+	SetQuantifier	 setquantifier;
+	struct GroupClause  *groupclause;
 }
 
 %type <node>	stmt schema_stmt
@@ -405,7 +414,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 				target_list opt_target_list insert_column_list set_target_list
 				set_clause_list set_clause
 				def_list operator_def_list indirection opt_indirection
-				reloption_list group_clause TriggerFuncArgs opclass_item_list opclass_drop_list
+				reloption_list TriggerFuncArgs opclass_item_list opclass_drop_list
 				opclass_purpose opt_opfamily transaction_mode_list_or_empty
 				OptTableFuncElementList TableFuncElementList opt_type_modifiers
 				prep_type_clause
@@ -418,6 +427,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 				vacuum_relation_list opt_vacuum_relation_list
 				drop_option_list
 
+%type <groupclause> group_clause
 %type <list>	group_by_list
 %type <node>	group_by_item empty_grouping_set rollup_clause cube_clause
 %type <node>	grouping_sets_clause
@@ -443,7 +453,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <node>	for_locking_item
 %type <list>	for_locking_clause opt_for_locking_clause for_locking_items
 %type <list>	locked_rels_list
-%type <boolean>	all_or_distinct
+%type <setquantifier> set_quantifier
 
 %type <node>	join_qual
 %type <jtype>	join_type
@@ -11294,7 +11304,8 @@ simple_select:
 					n->intoClause = $4;
 					n->fromClause = $5;
 					n->whereClause = $6;
-					n->groupClause = $7;
+					n->groupClause = ($7)->list;
+					n->groupDistinct = ($7)->distinct;
 					n->havingClause = $8;
 					n->windowClause = $9;
 					$$ = (Node *)n;
@@ -11309,7 +11320,8 @@ simple_select:
 					n->intoClause = $4;
 					n->fromClause = $5;
 					n->whereClause = $6;
-					n->groupClause = $7;
+					n->groupClause = ($7)->list;
+					n->groupDistinct = ($7)->distinct;
 					n->havingClause = $8;
 					n->windowClause = $9;
 					$$ = (Node *)n;
@@ -11334,17 +11346,17 @@ simple_select:
 					n->fromClause = list_make1($2);
 					$$ = (Node *)n;
 				}
-			| select_clause UNION all_or_distinct select_clause
+			| select_clause UNION set_quantifier select_clause
 				{
-					$$ = makeSetOp(SETOP_UNION, $3, $1, $4);
+					$$ = makeSetOp(SETOP_UNION, $3 == SET_QUANTIFIER_ALL, $1, $4);
 				}
-			| select_clause INTERSECT all_or_distinct select_clause
+			| select_clause INTERSECT set_quantifier select_clause
 				{
-					$$ = makeSetOp(SETOP_INTERSECT, $3, $1, $4);
+					$$ = makeSetOp(SETOP_INTERSECT, $3 == SET_QUANTIFIER_ALL, $1, $4);
 				}
-			| select_clause EXCEPT all_or_distinct select_clause
+			| select_clause EXCEPT set_quantifier select_clause
 				{
-					$$ = makeSetOp(SETOP_EXCEPT, $3, $1, $4);
+					$$ = makeSetOp(SETOP_EXCEPT, $3 == SET_QUANTIFIER_ALL, $1, $4);
 				}
 		;
 
@@ -11542,10 +11554,10 @@ opt_table:	TABLE
 			| /*EMPTY*/
 		;
 
-all_or_distinct:
-			ALL										{ $$ = true; }
-			| DISTINCT								{ $$ = false; }
-			| /*EMPTY*/								{ $$ = false; }
+set_quantifier:
+			ALL										{ $$ = SET_QUANTIFIER_ALL; }
+			| DISTINCT								{ $$ = SET_QUANTIFIER_DISTINCT; }
+			| /*EMPTY*/								{ $$ = SET_QUANTIFIER_DEFAULT; }
 		;
 
 /* We use (NIL) as a placeholder to indicate that all target expressions
@@ -11771,8 +11783,20 @@ first_or_next: FIRST_P								{ $$ = 0; }
  * GroupingSet node of some type.
  */
 group_clause:
-			GROUP_P BY group_by_list				{ $$ = $3; }
-			| /*EMPTY*/								{ $$ = NIL; }
+			GROUP_P BY set_quantifier group_by_list
+				{
+					GroupClause *n = (GroupClause *) palloc(sizeof(GroupClause));
+					n->distinct = $3 == SET_QUANTIFIER_DISTINCT;
+					n->list = $4;
+					$$ = n;
+				}
+			| /*EMPTY*/
+				{
+					GroupClause *n = (GroupClause *) palloc(sizeof(GroupClause));
+					n->distinct = false;
+					n->list = NIL;
+					$$ = n;
+				}
 		;
 
 group_by_list:
@@ -15145,7 +15169,8 @@ PLpgSQL_Expr: opt_distinct_clause opt_target_list
 					n->targetList = $2;
 					n->fromClause = $3;
 					n->whereClause = $4;
-					n->groupClause = $5;
+					n->groupClause = ($5)->list;
+					n->groupDistinct = ($5)->distinct;
 					n->havingClause = $6;
 					n->windowClause = $7;
 					n->sortClause = $8;

src/backend/parser/parse_agg.c

@@ -1071,7 +1071,7 @@ parseCheckAggregates(ParseState *pstate, Query *qry)
 		 * The limit of 4096 is arbitrary and exists simply to avoid resource
 		 * issues from pathological constructs.
 		 */
-		List	   *gsets = expand_grouping_sets(qry->groupingSets, 4096);
+		List	   *gsets = expand_grouping_sets(qry->groupingSets, qry->groupDistinct, 4096);
 
 		if (!gsets)
 			ereport(ERROR,
@@ -1735,6 +1735,33 @@ cmp_list_len_asc(const ListCell *a, const ListCell *b)
 	return (la > lb) ? 1 : (la == lb) ? 0 : -1;
 }
 
+/* list_sort comparator to sort sub-lists by length and contents */
+static int
+cmp_list_len_contents_asc(const ListCell *a, const ListCell *b)
+{
+	int		res = cmp_list_len_asc(a, b);
+
+	if (res == 0)
+	{
+		List		   *la = (List *) lfirst(a);
+		List		   *lb = (List *) lfirst(b);
+		ListCell	   *lca;
+		ListCell	   *lcb;
+
+		forboth(lca, la, lcb, lb)
+		{
+			int		va = intVal(lca);
+			int		vb = intVal(lcb);
+			if (va > vb)
+				return 1;
+			if (va < vb)
+				return -1;
+		}
+	}
+
+	return res;
+}
+
 /*
  * Expand a groupingSets clause to a flat list of grouping sets.
  * The returned list is sorted by length, shortest sets first.
@@ -1743,7 +1770,7 @@ cmp_list_len_asc(const ListCell *a, const ListCell *b)
  * some consistency checks.
  */
 List *
-expand_grouping_sets(List *groupingSets, int limit)
+expand_grouping_sets(List *groupingSets, bool groupDistinct, int limit)
 {
 	List	   *expanded_groups = NIL;
 	List	   *result = NIL;
@@ -1801,8 +1828,31 @@ expand_grouping_sets(List *groupingSets, int limit)
 		result = new_result;
 	}
 
-	/* Now sort the lists by length */
+	/* Now sort the lists by length and deduplicate if necessary */
+	if (!groupDistinct || list_length(result) < 2)
 		list_sort(result, cmp_list_len_asc);
+	else
+	{
+		ListCell   *cell;
+		List	   *prev;
+
+		/* Sort each groupset individually */
+		foreach(cell, result)
+			list_sort(lfirst(cell), list_int_cmp);
+
+		/* Now sort the list of groupsets by length and contents */
+		list_sort(result, cmp_list_len_contents_asc);
+
+		/* Finally, remove duplicates */
+		prev = list_nth_node(List, result, 0);
+		for_each_from(cell, result, 1)
+		{
+			if (equal(lfirst(cell), prev))
+				foreach_delete_current(result, cell);
+			else
+				prev = lfirst(cell);
+		}
+	}
 
 	return result;
 }

src/backend/utils/adt/ruleutils.c

@@ -5517,6 +5517,8 @@ get_basic_select_query(Query *query, deparse_context *context,
 
 		appendContextKeyword(context, " GROUP BY ",
 							 -PRETTYINDENT_STD, PRETTYINDENT_STD, 1);
+		if (query->groupDistinct)
+			appendStringInfoString(buf, "DISTINCT ");
 
 		save_exprkind = context->special_exprkind;
 		context->special_exprkind = EXPR_KIND_GROUP_BY;

src/include/nodes/parsenodes.h

@@ -62,6 +62,14 @@ typedef enum SortByNulls
 	SORTBY_NULLS_LAST
 } SortByNulls;
 
+/* Options for [ ALL | DISTINCT ] */
+typedef enum SetQuantifier
+{
+	SET_QUANTIFIER_DEFAULT,
+	SET_QUANTIFIER_ALL,
+	SET_QUANTIFIER_DISTINCT
+} SetQuantifier;
+
 /*
  * Grantable rights are encoded so that we can OR them together in a bitmask.
  * The present representation of AclItem limits us to 16 distinct rights,
@@ -146,6 +154,7 @@ typedef struct Query
 	List	   *returningList;	/* return-values list (of TargetEntry) */
 
 	List	   *groupClause;	/* a list of SortGroupClause's */
+	bool		groupDistinct;	/* is the group by clause distinct? */
 
 	List	   *groupingSets;	/* a list of GroupingSet's if present */
 
@@ -1629,6 +1638,7 @@ typedef struct SelectStmt
 	List	   *fromClause;		/* the FROM clause */
 	Node	   *whereClause;	/* WHERE qualification */
 	List	   *groupClause;	/* GROUP BY clauses */
+	bool		groupDistinct;	/* Is this GROUP BY DISTINCT? */
 	Node	   *havingClause;	/* HAVING conditional-expression */
 	List	   *windowClause;	/* WINDOW window_name AS (...), ... */
 

src/include/nodes/pg_list.h

@@ -604,6 +604,7 @@ extern pg_nodiscard List *list_copy_deep(const List *oldlist);
 typedef int (*list_sort_comparator) (const ListCell *a, const ListCell *b);
 extern void list_sort(List *list, list_sort_comparator cmp);
 
+extern int	list_int_cmp(const ListCell *p1, const ListCell *p2);
 extern int	list_oid_cmp(const ListCell *p1, const ListCell *p2);
 
 #endif							/* PG_LIST_H */

src/include/parser/parse_agg.h

@@ -26,7 +26,7 @@ extern void transformWindowFuncCall(ParseState *pstate, WindowFunc *wfunc,
 
 extern void parseCheckAggregates(ParseState *pstate, Query *qry);
 
-extern List *expand_grouping_sets(List *groupingSets, int limit);
+extern List *expand_grouping_sets(List *groupingSets, bool groupDistinct, int limit);
 
 extern int	get_aggregate_argtypes(Aggref *aggref, Oid *inputTypes);
 

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

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