demo@127.0.0.1:26257/defaultdb> create table t (a int);
CREATE TABLE
demo@127.0.0.1:26257/defaultdb> create function f() returns int language sql as 'select * from t';
CREATE FUNCTION
demo@127.0.0.1:26257/defaultdb> alter table t add column b int; 
ALTER TABLE
demo@127.0.0.1:26257/defaultdb> select f();
   f
--------
  NULL
(1 row)
demo@127.0.0.1:26257/defaultdb> insert into t values(1);
INSERT 0 1
demo@127.0.0.1:26257/defaultdb> select f();
*
* ERROR: a SQL panic has occurred while executing the following statement:
* SELECT f()
*
*
* ERROR: a panic has occurred!
* runtime error: index out of range [1] with length 1
* (1) attached stack trace
*   -- stack trace:
*   | github.com/cockroachdb/cockroach/pkg/sql.(*Server).ServeConn.func1
*   |   github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:806
*   | [...repeated from below...]
* Wraps: (2) while executing: SELECT f()
* Wraps: (3) attached stack trace
*   -- stack trace:
*   | github.com/cockroachdb/cockroach/pkg/sql.(*Server).ServeConn.func1
*   |   github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:806
*   | runtime.gopanic
*   |   GOROOT/src/runtime/panic.go:838
*   | github.com/cockroachdb/cockroach/pkg/sql/colexecerror.CatchVectorizedRuntimeError.func1
*   |   github.com/cockroachdb/cockroach/pkg/sql/colexecerror/error.go:58
*   | runtime.gopanic
*   |   GOROOT/src/runtime/panic.go:838
*   | runtime.goPanicIndex
*   |   GOROOT/src/runtime/panic.go:89
*   | github.com/cockroachdb/cockroach/pkg/sql.(*rowContainerHelper).AddRow
*   |   github.com/cockroachdb/cockroach/pkg/sql/buffer_util.go:90
*   | github.com/cockroachdb/cockroach/pkg/sql.(*RowResultWriter).AddRow
*   |   github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:908
*   | github.com/cockroachdb/cockroach/pkg/sql.(*DistSQLReceiver).Push
*   |   github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:1241
*   | github.com/cockroachdb/cockroach/pkg/sql/execinfra.Run
*   |   github.com/cockroachdb/cockroach/pkg/sql/execinfra/base.go:189
*   | github.com/cockroachdb/cockroach/pkg/sql/execinfra.(*ProcessorBaseNoHelper).Run
*   |   github.com/cockroachdb/cockroach/pkg/sql/execinfra/processorsbase.go:722

jordan=> select f();
ERROR:  return type mismatch in function declared to return integer
DETAIL:  Final statement must return exactly one column.
CONTEXT:  SQL function "f" during startup