json select question

All;

I'm working with a client running postgres 9.2, they have a table with a
"json_data_string" column of type json

the data looks something like this with lots of rows for each (i.e. lots
of json_data_string->book_name rows, lots of
json_data_string->catalog_name rows, etc:

|'{ "book_name": "Book the Fourth", "author": { "first_name": "Charles",
"last_name": "Davis" } }' ||'{ "catalog_name": "Catalog the Fourth", "author": { "first_name":
"Charles", "last_name": "Davis" } }'|
|'{ "magazine_name": "mag4", "author": { "first_name": "Charles",
"last_name": "Davis" } }' |||'{ "category_name": "region", "author": { "first_name": "Charles",
"last_name": "Davis" } }'||

How can i pull a unique list of all json column names? such as book_name, catalog_name, etc

Thanks in advance