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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.
select
id,
description,
CASE
WHEN json_array_length(letters) = 0 THEN '"x"'::json
ELSE json_array_elements(letters)
END
from
(
values
(1, 'good', '["a","b", "c"]'::json),
(2, 'bad', '[]'::json)
) as aTable(id,description,letters)
ERROR:  set-returning functions are not allowed in CASE
LINE 6:         ELSE json_array_elements(letters)
                     ^
HINT:  You might be able to move the set-returning function into a LATERAL FROM item.