Disallow UNION/INTERSECT/EXCEPT over no columns.

tglsfdc · tglsfdc · commit bd29bc417e71 · 2017-12-22T12:08:28.000-05:00
Since 9.4, we've allowed the syntax "select union select" and variants of that. However, the planner wasn't expecting a no-column set operation and ended up treating the set operation as if it were UNION ALL. Pre-v10, there seem to be some executor issues that would need to be fixed to support such cases, and it doesn't really seem worth expending much effort on. Just disallow it, instead. Per report from Victor Yegorov. Discussion: https://postgr.es/m/CAGnEbojGJrRSOgJwNGM7JSJZpVAf8xXcVPbVrGdhbVEHZ-BUMw@mail.gmail.com
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
@@ -697,9 +697,13 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
+	/* punt if nothing to group on (not worth fixing in back branches) */
 	if (groupList == NIL)
-		return path;
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 /* translator: %s is UNION, INTERSECT, or EXCEPT */
+				 errmsg("%s over no columns is not supported",
+						(op->op == SETOP_INTERSECT) ? "INTERSECT" : "EXCEPT")));
 
 	/*
 	 * Estimate number of distinct groups that we'll need hashtable entries
@@ -850,9 +854,12 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
+	/* punt if nothing to group on (not worth fixing in back branches) */
 	if (groupList == NIL)
-		return path;
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 /* translator: %s is UNION, INTERSECT, or EXCEPT */
+				 errmsg("%s over no columns is not supported", "UNION")));
 
 	/*
 	 * XXX for the moment, take the number of distinct groups as equal to the
