just nitpicking, but the json data type was added with 9.2 where it was not much more than syntax checked text plus one or two functions to convert from records and arrays into json.

In 9.3 we got many more useful functions that allow to query into json documents, though if you wanted to select rows based on the content of a json document, you'd have to resort to functional indexes.

9.4 (which is what the above post is talking about) will provide index support for arbitrary queries and a much more efficient format to store JSON data which will not involve reparsing it whenever you use any of the above functions.

You don't need an index to select rows based on the contents of a json document in 9.3. You may want or need them depending on your queries. (I use them sparingly, the most common pattern for me is that an indexed non-json column filters a query down to a smaller set of rows, and then a further index on the attributes referenced in the query isn't necessary.)

And if you do want to create an index now on a JSON attribute in 9.3, a function isn't necessary.

Am I right in thinking this is only good for reads though?

Do writes still require loading the field into memory, parsing, updating and the writing back?

Yes. You are right. At the moment there's no exposed functions for altering that binary data structure.

Because of the way how Postgres works though, the row will always have to be rewritten in the future (all updates to a row will cause a new copy to be written - rows are immutable in Postgres). What we might gain in the future is a way to skip the parsing process, but the document will always have to be rewritten.