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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 236832a2ca7..3a81d4f2670 100644
--- a/src/include/nodes/parsenodes.h
+++ b/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 (...), ... */