sql: add tests for IS (NOT) NULL with empty tuples

pkg/sql/opt/norm/fold_constants_funcs.go

@@ -88,9 +88,10 @@ func (c *CustomFuncs) HasNullElement(input opt.ScalarExpr) bool {
 }
 
 // HasAllNullElements returns true if the input tuple has only constant, null
-// elements. Note that it only returns true if all elements are known to be
-// null. For example, given the tuple (NULL, x), it will return false because x
-// is not guaranteed to be null.
+// elements, or if the tuple is empty (has 0 elements). Note that it only
+// returns true if all elements are known to be null. For example, given the
+// tuple (NULL, x), it will return false because x is not guaranteed to be
+// null.
 func (c *CustomFuncs) HasAllNullElements(input opt.ScalarExpr) bool {
 	tup := input.(*memo.TupleExpr)
 	for _, e := range tup.Elems {
@@ -122,9 +123,10 @@ func (c *CustomFuncs) HasNonNullElement(input opt.ScalarExpr) bool {
 }
 
 // HasAllNonNullElements returns true if the input tuple has all constant,
-// non-null elements. Note that it only returns true if all elements are known
-// to be non-null. For example, given the tuple (1, x), it will return false
-// because x is not guaranteed to be non-null.
+// non-null elements, or if the tuple is empty (has 0 elements). Note that it
+// only returns true if all elements are known to be non-null. For example,
+// given the tuple (1, x), it will return false because x is not guaranteed to
+// be non-null.
 func (c *CustomFuncs) HasAllNonNullElements(input opt.ScalarExpr) bool {
 	tup := input.(*memo.TupleExpr)
 	for _, e := range tup.Elems {

pkg/sql/opt/norm/testdata/rules/comp

@@ -363,6 +363,16 @@ values
  ├── fd: ()-->(1)
  └── (true,)
 
+norm expect=FoldNullTupleIsTupleNull
+SELECT () IS NULL AS r
+----
+values
+ ├── columns: r:1!null
+ ├── cardinality: [1 - 1]
+ ├── key: ()
+ ├── fd: ()-->(1)
+ └── (true,)
+
 norm expect-not=FoldNullTupleIsTupleNull
 SELECT (k, NULL) IS NULL FROM a
 ----
@@ -535,6 +545,16 @@ values
  ├── fd: ()-->(1)
  └── (true,)
 
+norm expect=FoldNonNullTupleIsTupleNotNull
+SELECT () IS NOT NULL AS r
+----
+values
+ ├── columns: r:1!null
+ ├── cardinality: [1 - 1]
+ ├── key: ()
+ ├── fd: ()-->(1)
+ └── (true,)
+
 norm expect-not=FoldNonNullTupleIsTupleNotNull
 SELECT (1, k) IS NOT NULL FROM a
 ----

pkg/sql/sem/tree/testdata/eval/is

@@ -524,3 +524,15 @@ eval
 NOT ((NULL, NULL) IS DISTINCT FROM NULL)
 ----
 false
+
+# Empty tuples.
+
+eval
+() IS NULL
+----
+true
+
+eval
+() IS NOT NULL
+----
+true