Postgres 13

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CREATE TABLE namespace_list (id int, namespace_ids json DEFAULT '[]'::json);

INSERT INTO namespace_list VALUES
  (1, '[1,2,3]')
, (2, '[5,9,3]');
2 rows affected

TABLE namespace_list;


id
namespace_ids


1
[1,2,3]

2
[5,9,3]


ALTER TABLE namespace_list
   ALTER COLUMN namespace_ids DROP DEFAULT
,  ALTER COLUMN namespace_ids TYPE INT[] USING translate(namespace_ids::text, '[]','{}')::int[]
,  ALTER COLUMN namespace_ids SET DEFAULT '{}';
TABLE namespace_list;


id
namespace_ids


1
{1,2,3}

2
{5,9,3}