The following bug has been logged on the website:
Bug reference: 7620
Logged by: Greg Hazel
Email address: ghazel(at)gmail(dot)com
PostgreSQL version: 9.2.1
Operating system: Amazon Linux
Description:
array_to_json(ARRAY['foo', 100, true]) complains because arrays can't have
mixed types, but json arrays can.
So, it's not possible to form a heterogeneous json array, when this is often
desired.