How to query a json column for empty objects?

Question

Looking to find all rows where a certain json column contains an empty object, {}. This is possible with JSON arrays, or if I am looking for a specific key in the object. But I just want to know if the object is empty. Can't seem to find an operator that will do this.

 dev=# \d test
     Table "public.test"
  Column | Type | Modifiers
 --------+------+-----------
  foo    | json |

 dev=# select * from test;
    foo
 ---------
  {"a":1}
  {"b":1}
  {}
 (3 rows)

 dev=# select * from test where foo != '{}';
 ERROR:  operator does not exist: json <> unknown
 LINE 1: select * from test where foo != '{}';
                                      ^
 HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
 dev=# select * from test where foo != to_json('{}'::text);
 ERROR:  operator does not exist: json <> json
 LINE 1: select * from test where foo != to_json('{}'::text);
                                      ^
 HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
 dwv=# select * from test where foo != '{}'::json;
 ERROR:  operator does not exist: json <> json
 LINE 1: select * from test where foo != '{}'::json;
                                      ^
 HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

Answer 1

There is no equality (or inequality) operator for the data type json as a whole, because equality is hard to establish. Consider jsonb in Postgres 9.4 or later, where this is possible. More details in this related answer on dba.SE (last chapter):

• How to remove known elements from a JSON[] array in PostgreSQL?

SELECT DISTINCT json_column ... or ... GROUP BY json_column fail for the same reason (no equality operator).

Casting both sides of the expression to text allows = or <> operators, but that's not normally reliable as there are many possible text representations for the same JSON value.

However, for this particular case (empty object) it works just fine:

select * from test where foo::text <> '{}'::text;