Is it possible to select distinct values in a json document using PostgreSQL?
Assuming a JSON array in a Postgres 9.4 jsonb column, this would do the job:

SELECT DISTINCT ON (doc-&gt;'s') doc
FROM  (
   SELECT '[
    {
      "id":1,
      "s":"foo"
    },
    {
      "id":2,
      "s":"bar"
    },
    {
      "id":3,
      "s":"foo"
    }]'::jsonb AS j
   ) t
   , jsonb_array_elements(t.j) WITH ORDINALITY t1(doc, rn)
ORDER  BY doc-&gt;'s', rn;


Or, unless s is a nested object, it's probably cheaper to fold on the text value instead of the jsonb (sub-)record. Just use the operator  -&gt;&gt; instead of -&gt; in this case. The result is the same:

 doc
----------------------
'{"s": "bar", "id": 2}'
'{"s": "foo", "id": 1}'


Replace the subquery t with your actual table.

Key elements are jsonb_array_elements() (or json_array_elements()) in a LATERAL join with WITH ORDINALITY and then the Postgres-specific DISTINCT ON.

Related, with more explanation:


How to remove object from json array?
How to turn JSON array into Postgres array?
How to preserve the original order of elements in an unnested array?
Select first row in each GROUP BY group?