Postgres 14

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create table if not exists testing (val jsonb);
insert into testing values('{"a":"b"}'::jsonb);
insert into testing values('{"a":"b", "errors":["a", "b"]}'::jsonb);

CREATE TABLE

INSERT 0 1

INSERT 0 1

update testing set val = jsonb_set(val, '{errors}'::text[], coalesce(val->'errors', '[]'::jsonb) || '["a","b"]'::jsonb) where val->>'a'='b'

UPDATE 2

select * from testing

val

{"a": "b", "errors": ["a", "b"]}

{"a": "b", "errors": ["a", "b", "a", "b"]}

SELECT 2