Postgresql - problem using json function in nested queries - hstore

Question

I have a table orders with a field order_detail of type hstore which saves json data.

Now I want to query on the inner objects of my json data. because The query is somehow complicated I'm trying to tell my problem in easier scenario.

I have tested these 4 subqueries:

1. Get the original saved json:

    select order_detail::json as original from orders;

   This query successfully returns the json formatted data.

2. Get the inner object 'transaction' inside the order_details:

    select order_detail::json as original, (order_detail -> 'transaction')::json as transaction from order_details;

   This query also works successfully.

3. Get the id of that transaction:

    select order_detail::json as original, (order_detail -> 'transaction')::json as transaction, ((order_detail -> 'transaction')::json -> 'id')::text as id from order_details;

   The above also works successfully and returns the original json, transaction and id inside the transaction.

4. Select based on the result of query 3 and get one of the results:

    select original from (select order_detail::json as original, (order_detail -> 'transaction')::json as transaction, ((order_detail -> 'transaction')::json -> 'id')::text as id from order_details) s where transaction is null and id is null;

   This query will raise an exception: The exception says that:

   [22P02] ERROR there is a token '=' is invalid**

   Why does this exception occurs only in the 4th query? Can anyone help me on this?

Answer 1

Finally I found the problem. When you want have such a field which is json stored in hstore, if you use a simple query you can use something like this:

order_detail -> 'trasaction'

and this will return the transaction part of hashed-stored data in the order_detail field with no problem.

BUT if you want to use such thing in a nested query, you MUST explicitly declare that the field is json. so you must use something like this instead (in all parts of inner query):

order_detail::json -> 'transaction'