From 751edad0edf4b4ba83f9be0e2243c094dc5f9b5c Mon Sep 17 00:00:00 2001
From: Marcus Gartner <marcus@cockroachlabs.com>
Date: Mon, 11 Jan 2021 17:05:10 -0800
Subject: [PATCH] opt: fold Null tuples in FoldColumnAccess

It is possible for the input of a `ColumnAccess` expression to be `Null`
when `FoldIndirection` has folded an `Indirection` expression with an
out-of-bounds index to `Null`. `FoldColumnAccess` now correcty handles
this case rather than erring.

Release note (bug fix): A bug has been fixed that caused errors when
accessing a tuple column (`tuple.column` syntax) of a tuples that could
be statically determined to be null.
---
 pkg/sql/logictest/testdata/logic_test/tuple   | 22 +++++++++++++++++++
 pkg/sql/opt/norm/fold_constants_funcs.go      | 11 ++++++++--
 .../opt/norm/testdata/rules/fold_constants    | 22 +++++++++++++++++++
 3 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/pkg/sql/logictest/testdata/logic_test/tuple b/pkg/sql/logictest/testdata/logic_test/tuple
index 6d3136c2a0d3..60219aabd8d0 100644
--- a/pkg/sql/logictest/testdata/logic_test/tuple
+++ b/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
diff --git a/pkg/sql/opt/norm/fold_constants_funcs.go b/pkg/sql/opt/norm/fold_constants_funcs.go
index 77c1f864b0f7..9b225e416ca0 100644
--- a/pkg/sql/opt/norm/fold_constants_funcs.go
+++ b/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)
 
diff --git a/pkg/sql/opt/norm/testdata/rules/fold_constants b/pkg/sql/opt/norm/testdata/rules/fold_constants
index 469db12f2a2a..3839c3caba0d 100644
--- a/pkg/sql/opt/norm/testdata/rules/fold_constants
+++ b/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
 # --------------------------------------------------