release-21.1: sql: fix ColumnAccessExpr.Eval with NULL inner expression

pkg/sql/logictest/testdata/logic_test/tuple

@@ -978,3 +978,35 @@ CREATE TABLE t74729 AS SELECT g % 2 = 1 AS _bool FROM generate_series(1, 5) AS g
 
 statement ok
 SELECT CASE WHEN _bool THEN (1, ('a', 2)) ELSE (3, NULL) END FROM t74729
+
+# Regression test for #78159. Column access of NULL should not cause an internal
+# error.
+subtest 78159
+
+statement ok
+CREATE TABLE t78159 (b BOOL)
+
+statement ok
+INSERT INTO t78159 VALUES (false)
+
+query B
+SELECT (CASE WHEN b THEN ((ROW(1) AS a)) ELSE NULL END).a from t78159
+----
+NULL
+
+# Regression test for #78515. Propagate tuple labels when type-checking
+# expressions with multiple matching tuple types.
+subtest 78515
+
+statement ok
+SELECT (CASE WHEN false THEN (ROW(1) AS a) ELSE (ROW(2) AS a) END).a;
+
+# The label of the first tuple is used in the type of the CASE expression. This
+# is similar to Postgres, but not exactly the same - Postgres uses the labels of
+# the last tuple. This difference should be addressed in the future when we try
+# to adhere more closely to Postgres's type conversion behavior (see #75101).
+statement ok
+SELECT (CASE WHEN false THEN (ROW(1) AS a) ELSE (ROW(2) AS b) END).a;
+
+statement error could not identify column \"b\" in tuple{int AS a}
+SELECT (CASE WHEN false THEN (ROW(1) AS a) ELSE (ROW(2) AS b) END).b;

pkg/sql/sem/tree/eval.go

@@ -3927,6 +3927,9 @@ func (expr *ColumnAccessExpr) Eval(ctx *EvalContext) (Datum, error) {
 	if err != nil {
 		return nil, err
 	}
+	if d == DNull {
+		return d, nil
+	}
 	return d.(*DTuple).D[expr.ColIndex], nil
 }
 

pkg/sql/sem/tree/type_check.go

@@ -2397,7 +2397,7 @@ func typeCheckSameTypedTupleExprs(
 	}
 
 	// All expressions within tuples at the same indexes must be the same type.
-	resTypes := types.MakeTuple(make([]*types.T, firstLen))
+	resTypes := types.MakeLabeledTuple(make([]*types.T, firstLen), first.Labels)
 	sameTypeExprs := make([]Expr, 0, len(exprs))
 	// We will be skipping nulls, so we need to keep track at which indices in
 	// exprs are the non-null tuples.