postgresql - 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. In Postgres 9.4 or later, cast to jsonb instead. (Or use jsonb to begin with.)

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

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