Add a basic regression test for IS DISTINCT FROM, which has spent way too

much time in a broken state for lack of anyone noticing.

src/test/regress/expected/select_distinct.out

@@ -124,3 +124,74 @@ SELECT DISTINCT p.age FROM person* p ORDER BY age using >;
    8
 (20 rows)
 
+--
+-- Also, some tests of IS DISTINCT FROM, which doesn't quite deserve its
+-- very own regression file.
+--
+CREATE TEMP TABLE disttable (f1 integer);
+INSERT INTO DISTTABLE VALUES(1);
+INSERT INTO DISTTABLE VALUES(2);
+INSERT INTO DISTTABLE VALUES(3);
+INSERT INTO DISTTABLE VALUES(NULL);
+-- basic cases
+SELECT f1, f1 IS DISTINCT FROM 2 as "not 2" FROM disttable;
+ f1 | not 2 
+----+-------
+  1 | t
+  2 | f
+  3 | t
+    | t
+(4 rows)
+
+SELECT f1, f1 IS DISTINCT FROM NULL as "not null" FROM disttable;
+ f1 | not null 
+----+----------
+  1 | t
+  2 | t
+  3 | t
+    | f
+(4 rows)
+
+SELECT f1, f1 IS DISTINCT FROM f1 as "false" FROM disttable;
+ f1 | false 
+----+-------
+  1 | f
+  2 | f
+  3 | f
+    | f
+(4 rows)
+
+SELECT f1, f1 IS DISTINCT FROM f1+1 as "not null" FROM disttable;
+ f1 | not null 
+----+----------
+  1 | t
+  2 | t
+  3 | t
+    | f
+(4 rows)
+
+-- check that optimizer constant-folds it properly
+SELECT 1 IS DISTINCT FROM 2 as "yes";
+ yes 
+-----
+ t
+(1 row)
+
+SELECT 2 IS DISTINCT FROM 2 as "no";
+ no 
+----
+ f
+(1 row)
+
+SELECT 2 IS DISTINCT FROM null as "yes";
+ yes 
+-----
+ t
+(1 row)
+
+SELECT null IS DISTINCT FROM null as "no";
+ no 
+----
+ f
+(1 row)
+

src/test/regress/sql/select_distinct.sql

@@ -34,3 +34,25 @@ SELECT DISTINCT two, string4, ten
 --
 SELECT DISTINCT p.age FROM person* p ORDER BY age using >;
 
+--
+-- Also, some tests of IS DISTINCT FROM, which doesn't quite deserve its
+-- very own regression file.
+--
+
+CREATE TEMP TABLE disttable (f1 integer);
+INSERT INTO DISTTABLE VALUES(1);
+INSERT INTO DISTTABLE VALUES(2);
+INSERT INTO DISTTABLE VALUES(3);
+INSERT INTO DISTTABLE VALUES(NULL);
+
+-- basic cases
+SELECT f1, f1 IS DISTINCT FROM 2 as "not 2" FROM disttable;
+SELECT f1, f1 IS DISTINCT FROM NULL as "not null" FROM disttable;
+SELECT f1, f1 IS DISTINCT FROM f1 as "false" FROM disttable;
+SELECT f1, f1 IS DISTINCT FROM f1+1 as "not null" FROM disttable;
+
+-- check that optimizer constant-folds it properly
+SELECT 1 IS DISTINCT FROM 2 as "yes";
+SELECT 2 IS DISTINCT FROM 2 as "no";
+SELECT 2 IS DISTINCT FROM null as "yes";
+SELECT null IS DISTINCT FROM null as "no";