Fix constant-folding of ROW(...) IS [NOT] NULL with composite fields.

The SQL standard appears to specify that IS [NOT] NULL's tests of field
nullness are non-recursive, ie, we shouldn't consider that a composite
field with value ROW(NULL,NULL) is null for this purpose.
ExecEvalNullTest got this right, but eval_const_expressions did not,
leading to weird inconsistencies depending on whether the expression
was such that the planner could apply constant folding.

Also, adjust the docs to mention that IS [NOT] DISTINCT FROM NULL can be
used as a substitute test if a simple null check is wanted for a rowtype
argument.  That motivated reordering things so that IS [NOT] DISTINCT FROM
is described before IS [NOT] NULL.  In HEAD, I went a bit further and added
a table showing all the comparison-related predicates.

Per bug #14235.  Back-patch to all supported branches, since it's certainly
undesirable that constant-folding should change the semantics.

Report and patch by Andrew Gierth; assorted wordsmithing and revised
regression test cases by me.

Report: <20160708024746.1410.57282@wrigleys.postgresql.org>

2 files changed, 78 insertions, 0 deletions

diff --git a/src/test/regress/expected/rowtypes.out b/src/test/regress/expected/rowtypes.out
index 3630ef49438..2971640b4bd 100644
--- a/src/test/regress/expected/rowtypes.out
+++ b/src/test/regress/expected/rowtypes.out
@@ -657,3 +657,57 @@ select row_to_json(r) from (select q2,q1 from tt1 offset 0) r;
  {"q2":0,"q1":0}
 (3 rows)
 
+--
+-- IS [NOT] NULL should not recurse into nested composites (bug #14235)
+--
+explain (verbose, costs off)
+select r, r is null as isnull, r is not null as isnotnull
+from (values (1,row(1,2)), (1,row(null,null)), (1,null),
+             (null,row(1,2)), (null,row(null,null)), (null,null) ) r(a,b);
+                                                                                           QUERY PLAN                                                                                            
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Values Scan on "*VALUES*"
+   Output: ROW("*VALUES*".column1, "*VALUES*".column2), (("*VALUES*".column1 IS NULL) AND ("*VALUES*".column2 IS NULL)), (("*VALUES*".column1 IS NOT NULL) AND ("*VALUES*".column2 IS NOT NULL))
+(2 rows)
+
+select r, r is null as isnull, r is not null as isnotnull
+from (values (1,row(1,2)), (1,row(null,null)), (1,null),
+             (null,row(1,2)), (null,row(null,null)), (null,null) ) r(a,b);
+      r      | isnull | isnotnull 
+-------------+--------+-----------
+ (1,"(1,2)") | f      | t
+ (1,"(,)")   | f      | t
+ (1,)        | f      | f
+ (,"(1,2)")  | f      | f
+ (,"(,)")    | f      | f
+ (,)         | t      | f
+(6 rows)
+
+explain (verbose, costs off)
+with r(a,b) as
+  (values (1,row(1,2)), (1,row(null,null)), (1,null),
+          (null,row(1,2)), (null,row(null,null)), (null,null) )
+select r, r is null as isnull, r is not null as isnotnull from r;
+                        QUERY PLAN                        
+----------------------------------------------------------
+ CTE Scan on r
+   Output: r.*, (r.* IS NULL), (r.* IS NOT NULL)
+   CTE r
+     ->  Values Scan on "*VALUES*"
+           Output: "*VALUES*".column1, "*VALUES*".column2
+(5 rows)
+
+with r(a,b) as
+  (values (1,row(1,2)), (1,row(null,null)), (1,null),
+          (null,row(1,2)), (null,row(null,null)), (null,null) )
+select r, r is null as isnull, r is not null as isnotnull from r;
+      r      | isnull | isnotnull 
+-------------+--------+-----------
+ (1,"(1,2)") | f      | t
+ (1,"(,)")   | f      | t
+ (1,)        | f      | f
+ (,"(1,2)")  | f      | f
+ (,"(,)")    | f      | f
+ (,)         | t      | f
+(6 rows)
+
diff --git a/src/test/regress/sql/rowtypes.sql b/src/test/regress/sql/rowtypes.sql
index 677d34a62c8..a62dee2ef84 100644
--- a/src/test/regress/sql/rowtypes.sql
+++ b/src/test/regress/sql/rowtypes.sql
@@ -286,3 +286,27 @@ create temp table tt1 as select * from int8_tbl limit 2;
 create temp table tt2 () inherits(tt1);
 insert into tt2 values(0,0);
 select row_to_json(r) from (select q2,q1 from tt1 offset 0) r;
+
+--
+-- IS [NOT] NULL should not recurse into nested composites (bug #14235)
+--
+
+explain (verbose, costs off)
+select r, r is null as isnull, r is not null as isnotnull
+from (values (1,row(1,2)), (1,row(null,null)), (1,null),
+             (null,row(1,2)), (null,row(null,null)), (null,null) ) r(a,b);
+
+select r, r is null as isnull, r is not null as isnotnull
+from (values (1,row(1,2)), (1,row(null,null)), (1,null),
+             (null,row(1,2)), (null,row(null,null)), (null,null) ) r(a,b);
+
+explain (verbose, costs off)
+with r(a,b) as
+  (values (1,row(1,2)), (1,row(null,null)), (1,null),
+          (null,row(1,2)), (null,row(null,null)), (null,null) )
+select r, r is null as isnull, r is not null as isnotnull from r;
+
+with r(a,b) as
+  (values (1,row(1,2)), (1,row(null,null)), (1,null),
+          (null,row(1,2)), (null,row(null,null)), (null,null) )
+select r, r is null as isnull, r is not null as isnotnull from r;