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, 24 insertions, 4 deletions

diff --git a/src/backend/executor/execQual.c b/src/backend/executor/execQual.c
index f2e8ee2f77b..c56a509fc97 100644
--- a/src/backend/executor/execQual.c
+++ b/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;
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index e6d83d4fd93..6093c5419d7 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/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);
 					}