Postgres 9.6 - Create unique index for json array

Question

'Sup,

We have a Postgres (9.6) table myTable with a row data jsonb NOT NULL:

DROP TABLE myTable;
CREATE TABLE myTable (data jsonb NOT NULL);

We want to store object containing an array of email addresses:

INSERT INTO myTable (data) VALUES ($${"email": [{"address": "A"}, {"address": "B"}]}$$);
INSERT INTO myTable (data) VALUES ($${"email": [{"address": "C"}]}$$);

We want to create a unique index on address to prevent insertion of row like:

-- "A" NON UNIQUE, SHOULD FAIL
INSERT INTO myTable (data) VALUES ($${"email": [{"address": "A"}]}$$);

-- "C" NON UNIQUE, SHOULD FAIL
INSERT INTO myTable (data) VALUES ($${"email": [{"address": "C"}, {"address": "D"}]}$$);

We tried using:

-- THROW: "ERROR:  index expression cannot return a set"
CREATE UNIQUE INDEX my_index ON myTable(((jsonb_array_elements((data->>'email')::jsonb)->>'address')));

We are looking for a solution using indexes or constraints, without extra table nor views nor triggers is possible. Any comment or idea would be greatly appreciated.

Cheers,