Converting between types of arrays in PostgreSQL

json.sql

BEGIN;

CREATE TABLE json_test (
  id SERIAL PRIMARY KEY,
  label text,
  examples text[] DEFAULT '{}'::text[]
);

INSERT INTO json_test (label,examples) VALUES ('before','{}');
INSERT INTO json_test (label,examples) VALUES ('before','{"[{\"a\":1}]"}');

SELECT * FROM JSON_TEST;

-- This is needed for the conversion to even be attempted by PostgreSQL.
ALTER TABLE json_test ALTER examples DROP DEFAULT;

-- This doesn't work, though it appears to at first. It results in arrays of strings.
-- We might be able to achieve this with a PL/V8 stored procedure as a second step.
-- http://baudehlo.com/2014/04/28/postgresql-converting-text-columns-to-json/
ALTER TABLE json_test ALTER examples TYPE json USING to_json(examples);

-- Also doesn't work.
-- ERROR:  cannot cast type text[] to json
ALTER TABLE json_test ALTER examples TYPE json USING examples::json;

ALTER TABLE json_test ALTER examples SET DEFAULT '[]';

INSERT INTO json_test (label) VALUES ('after');

SELECT * FROM JSON_TEST;

SELECT examples->>0->'a' FROM json_test WHERE id = 2;

ROLLBACK;