Merge pull request cockroachdb#58899 from mgartner/backport20.1-58747

release-20.1: opt: fold Null tuples in FoldColumnAccess

pkg/sql/logictest/testdata/logic_test/tuple

@@ -933,3 +933,25 @@ query B
 SELECT () = ()
 ----
 true
+
+# Regression tests for #58439. Ensure there are no errors when accessing columns
+# of a null tuple.
+subtest regression_58439
+
+statement ok
+CREATE TABLE t58439 (a INT, b INT);
+INSERT INTO t58439 VALUES (1, 10), (2, 20), (3, 30);
+
+query II
+SELECT (ARRAY[t58439.*][0]).* FROM t58439
+----
+NULL  NULL
+NULL  NULL
+NULL  NULL
+
+query II
+SELECT (ARRAY[t58439.*][2]).* FROM t58439
+----
+NULL  NULL
+NULL  NULL
+NULL  NULL

pkg/sql/opt/norm/fold_constants_funcs.go

@@ -311,12 +311,19 @@ func (c *CustomFuncs) FoldIndirection(input, index opt.ScalarExpr) opt.ScalarExp
 // It returns the referenced tuple field value, or nil if folding is not
 // possible or results in an error.
 func (c *CustomFuncs) FoldColumnAccess(input opt.ScalarExpr, idx memo.TupleOrdinal) opt.ScalarExpr {
-	// Case 1: The input is a static tuple constructor.
+	// Case 1: The input is NULL. This is possible when FoldIndirection has
+	// already folded an Indirection expression with an out-of-bounds index to
+	// Null.
+	if n, ok := input.(*memo.NullExpr); ok {
+		return c.f.ConstructNull(&n.Typ.TupleContents()[idx])
+	}
+
+	// Case 2: The input is a static tuple constructor.
 	if tup, ok := input.(*memo.TupleExpr); ok {
 		return tup.Elems[idx]
 	}
 
-	// Case 2: The input is a constant DTuple.
+	// Case 3: The input is a constant DTuple.
 	if memo.CanExtractConstDatum(input) {
 		datum := memo.ExtractConstDatum(input)
 

pkg/sql/opt/norm/testdata/rules/fold_constants

@@ -947,6 +947,28 @@ values
  ├── fd: ()-->(1)
  └── ('foo',)
 
+# Fold when input is Null. This is possible when FoldIndirection has already
+# folded an Indirection with an out-of-bounds index to Null.
+norm expect=FoldColumnAccess
+SELECT (ARRAY[(('foo', i) AS foo, bar)][0]).foo FROM a
+----
+project
+ ├── columns: foo:7
+ ├── fd: ()-->(7)
+ ├── scan a
+ └── projections
+      └── CAST(NULL AS STRING) [as=foo:7]
+
+norm expect=FoldColumnAccess
+SELECT (ARRAY[(('foo', i) AS foo, bar)][0]).bar FROM a
+----
+project
+ ├── columns: bar:7
+ ├── fd: ()-->(7)
+ ├── scan a
+ └── projections
+      └── CAST(NULL AS INT8) [as=bar:7]
+
 # --------------------------------------------------
 # FoldEqualsAnyNull
 # --------------------------------------------------