root@localhost:26257/defaultdb> select s, s || s from manystrings limit 3;
    s   |  ?column?
--------+-------------
        |
  NULL  | NULL
  hello | hellohello
(3 rows)


Time: 1ms total (execution 1ms / network 0ms)

root@localhost:26257/defaultdb> create changefeed with schema_change_policy='stop' as select s || s from manystrings;
{"key":"[807033528558616577]","table":"manystrings","value":"{\"column_1\": \"\"}"}
(error encountered after some results were delivered)
ERROR: while evaluating projection: SELECT s || s FROM [122 AS manystrings]: ambiguous binary operator: <unknown> || <unknown>