Q:

Postgres coalesce to empty JSONB array

How can I coalesce a null column into an empty JSONB array? This doesn't work:
SELECT jsonb_array_elements(coalesce(null_column, '{}'::jsonb))
FROM table
WHERE id = 13;

-- ERROR:  cannot extract elements from an object

Neither this:
SELECT jsonb_array_elements(coalesce(null_column, '[]'::jsonb))
FROM table
WHERE id = 13;

-- ERROR:  cannot extract elements from a scalar

A:

{} is an object but jsonb_array_elements expects an array, so replace {} with []
Make sure that both arguments return a jsonb array. For example, if your column is an integer, you can use concat to add the square brackets and ::jsonb for the conversion
SELECT jsonb_array_elements(coalesce(concat('[',my_column,']')::jsonb,'[]'::jsonb))

