Update certain array elements of a json array in PostgreSQL 9.4

Question

I have a table like this;

CREATE TABLE test (
  id BIGSERIAL PRIMARY KEY,
  data JSONB
);

INSERT INTO test(data) VALUES(\'[1,2,\"a\",4,\"8\",6]\'); -- id =          


        

Answer 1

You cannot manipulate selected elements of a json / jsonb type directly. Functionality for that is still missing in Postgres 9.4 (see @Craig's comment). You have to do 3 steps:

1. Unnest / decompose the JSON value.
2. Manipulate selected elements.
3. Aggregate / compose the value back again.

To replace the 3rd element of the json array (data->3) in the row with id = 1 with a given (new) value ('') in pg 9.4:

UPDATE test t
SET    data = t2.data
FROM  (
   SELECT id, array_to_json(
                 array_agg(CASE WHEN rn = 1 THEN '' ELSE elem END))
              ) AS data
   FROM   test t2
        , json_array_elements_text(t2.data) WITH ORDINALITY x(elem, rn)         
   WHERE  id = 1
   GROUP  BY 1
   ) t2
WHERE  t.id = t2.id
AND    t.data <> t2.data; -- avoid empty updates

About json_array_elements_text():

• How to turn json array into postgres array?

About WITH ORDINALITY:

• PostgreSQL unnest() with element number