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>

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -289,6 +289,32 @@
     a nonempty range is always implied.
    </para>
 
+   <para>
+    <indexterm>
+     <primary>IS DISTINCT FROM</primary>
+    </indexterm>
+    <indexterm>
+     <primary>IS NOT DISTINCT FROM</primary>
+    </indexterm>
+    Ordinary comparison operators yield null (signifying <quote>unknown</>),
+    not true or false, when either input is null.  For example,
+    <literal>7 = NULL</> yields null, as does <literal>7 &lt;&gt; NULL</>.  When
+    this behavior is not suitable, use the
+    <literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
+<synopsis>
+<replaceable>a</replaceable> IS DISTINCT FROM <replaceable>b</replaceable>
+<replaceable>a</replaceable> IS NOT DISTINCT FROM <replaceable>b</replaceable>
+</synopsis>
+    For non-null inputs, <literal>IS DISTINCT FROM</literal> is
+    the same as the <literal>&lt;&gt;</> operator.  However, if both
+    inputs are null it returns false, and if only one input is
+    null it returns true.  Similarly, <literal>IS NOT DISTINCT
+    FROM</literal> is identical to <literal>=</literal> for non-null
+    inputs, but it returns true when both inputs are null, and false when only
+    one input is null. Thus, these constructs effectively act as though null
+    were a normal data value, rather than <quote>unknown</>.
+   </para>
+
    <para>
     <indexterm>
      <primary>IS NULL</primary>
@@ -320,8 +346,7 @@
     <literal><replaceable>expression</replaceable> = NULL</literal>
     because <literal>NULL</> is not <quote>equal to</quote>
     <literal>NULL</>.  (The null value represents an unknown value,
-    and it is not known whether two unknown values are equal.) This
-    behavior conforms to the SQL standard.
+    and it is not known whether two unknown values are equal.)
    </para>
 
   <tip>
@@ -338,47 +363,20 @@
    </para>
   </tip>
 
-  <note>
    <para>
     If the <replaceable>expression</replaceable> is row-valued, then
     <literal>IS NULL</> is true when the row expression itself is null
     or when all the row's fields are null, while
     <literal>IS NOT NULL</> is true when the row expression itself is non-null
     and all the row's fields are non-null.  Because of this behavior,
     <literal>IS NULL</> and <literal>IS NOT NULL</> do not always return
-    inverse results for row-valued expressions, i.e., a row-valued
-    expression that contains both NULL and non-null values will return false
-    for both tests.
-    This definition conforms to the SQL standard, and is a change from the
-    inconsistent behavior exhibited by <productname>PostgreSQL</productname>
-    versions prior to 8.2.
-   </para>
-  </note>
-
-   <para>
-    <indexterm>
-     <primary>IS DISTINCT FROM</primary>
-    </indexterm>
-    <indexterm>
-     <primary>IS NOT DISTINCT FROM</primary>
-    </indexterm>
-    Ordinary comparison operators yield null (signifying <quote>unknown</>),
-    not true or false, when either input is null.  For example,
-    <literal>7 = NULL</> yields null, as does <literal>7 &lt;&gt; NULL</>.  When
-    this behavior is not suitable, use the
-    <literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
-<synopsis>
-<replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>
-<replaceable>expression</replaceable> IS NOT DISTINCT FROM <replaceable>expression</replaceable>
-</synopsis>
-    For non-null inputs, <literal>IS DISTINCT FROM</literal> is
-    the same as the <literal>&lt;&gt;</> operator.  However, if both
-    inputs are null it returns false, and if only one input is
-    null it returns true.  Similarly, <literal>IS NOT DISTINCT
-    FROM</literal> is identical to <literal>=</literal> for non-null
-    inputs, but it returns true when both inputs are null, and false when only
-    one input is null. Thus, these constructs effectively act as though null
-    were a normal data value, rather than <quote>unknown</>.
+    inverse results for row-valued expressions; in particular, a row-valued
+    expression that contains both null and non-null fields will return false
+    for both tests.  In some cases, it may be preferable to
+    write <replaceable>row</replaceable> <literal>IS DISTINCT FROM NULL</>
+    or <replaceable>row</replaceable> <literal>IS NOT DISTINCT FROM NULL</>,
+    which will simply check whether the overall row value is null without any
+    additional tests on the row fields.
    </para>
 
    <para>

src/backend/executor/execQual.c

@@ -3793,6 +3793,21 @@ 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

@@ -3330,7 +3330,7 @@ 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
@@ -3342,8 +3342,6 @@ eval_const_expressions_mutator(Node *node,
 					List	   *newargs = NIL;
 					ListCell   *l;
 
-					Assert(ntest->argisrow);
-
 					foreach(l, rarg->args)
 					{
 						Node	   *relem = (Node *) lfirst(l);
@@ -3362,10 +3360,17 @@ 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;