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>

src/backend/executor/execQual.c

@@ -3815,6 +3815,21 @@ ExecEvalNullTest(NullTestState *nstate,
 
 	if (ntest->argisrow && !(*isNull))
 	{
+		/*
+		 * The SQL standard defines IS [NOT] NULL for a non-null rowtype
+		 * argument as:
+		 *
+		 * "R IS NULL" is true if every field is the null value.
+		 *
+		 * "R IS NOT NULL" is true if no field is the null value.
+		 *
+		 * This definition is (apparently intentionally) not recursive; so our
+		 * tests on the fields are primitive attisnull tests, not recursive
+		 * checks to see if they are all-nulls or no-nulls rowtypes.
+		 *
+		 * The standard does not consider the possibility of zero-field rows,
+		 * but here we consider them to vacuously satisfy both predicates.
+		 */
 		HeapTupleHeader tuple;
 		Oid			tupType;
 		int32		tupTypmod;

src/backend/optimizer/util/clauses.c

@@ -3273,7 +3273,7 @@ eval_const_expressions_mutator(Node *node,
 
 				arg = eval_const_expressions_mutator((Node *) ntest->arg,
 													 context);
-				if (arg && IsA(arg, RowExpr))
+				if (ntest->argisrow && arg && IsA(arg, RowExpr))
 				{
 					/*
 					 * We break ROW(...) IS [NOT] NULL into separate tests on
@@ -3285,8 +3285,6 @@ eval_const_expressions_mutator(Node *node,
 					List	   *newargs = NIL;
 					ListCell   *l;
 
-					Assert(ntest->argisrow);
-
 					foreach(l, rarg->args)
 					{
 						Node	   *relem = (Node *) lfirst(l);
@@ -3305,10 +3303,17 @@ eval_const_expressions_mutator(Node *node,
 								return makeBoolConst(false, false);
 							continue;
 						}
+
+						/*
+						 * Else, make a scalar (argisrow == false) NullTest
+						 * for this field.  Scalar semantics are required
+						 * because IS [NOT] NULL doesn't recurse; see comments
+						 * in ExecEvalNullTest().
+						 */
 						newntest = makeNode(NullTest);
 						newntest->arg = (Expr *) relem;
 						newntest->nulltesttype = ntest->nulltesttype;
-						newntest->argisrow = type_is_rowtype(exprType(relem));
+						newntest->argisrow = false;
 						newntest->location = ntest->location;
 						newargs = lappend(newargs, newntest);
 					}

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)
+

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;