PostgreSQL (9.2+) : How to return an empty json array from an empty resultset.

empty-json-case.sql

/* will return null::json on empty resultset */
SELECT array_to_json(array_agg(row_to_json(t))) FROM t; 

/*
  will return '[]' on empty resultset,
  null::json seems to be managed the same way than sql NULL by COALESCE()
*/
SELECT COALESCE(array_to_json(array_agg(row_to_json(t))), '[]') FROM t;