Нашел в доке такое

Whenever a user-defined base data type is created, PostgreSQL automatically creates an associated array type, whose name consists of the base type's name prepended with an underscore. The parser understands this naming convention, and translates requests for columns of type foo[] into requests for type _foo. The implicitly-created array type is variable length and uses the built-in input and output functions array_in and array_out.

CREATE TYPE name AS
( attribute_name data_type [, ... ] )

CREATE TYPE name (
INPUT = input_function,
OUTPUT = output_function
...)

The first form of CREATE TYPE creates a composite type.
The second form of CREATE TYPE creates a new base type (scalar type).

Насколько я понимаю для composite типов массивы создать нельзя...