Added doc notes about how to avoid JSON parsing

Added FAQ too as it has bitten more than one user
(see tickets #172, #190).

doc/src/extras.rst

@@ -189,6 +189,20 @@ the `Json` adapter::
 Reading from the database, |pgjson| values will be automatically converted to
 Python objects.
 
+.. note::
+
+    If you are using the PostgreSQL :sql:`json` data type but you want to read
+    it as string in Python instead of having it parsed, your can either cast
+    the column to :sql:`text` in the query (it is an efficient operation, that
+    doesn't involve a copy)::
+
+        cur.execute("select jsondata::text from mytable")
+
+    or you can register a no-op `!loads()` function with
+    `register_default_json()`::
+
+        psycopg2.extras.register_default_json(loads=lambda x: x)
+
 .. note::
 
     You can use `~psycopg2.extensions.register_adapter()` to adapt any Python
@@ -204,7 +218,7 @@ Python objects.
     effects.
 
 If you want to customize the adaptation from Python to PostgreSQL you can
-either provide a custom `!dumps()` function to `!Json`::
+either provide a custom `!dumps()` function to `Json`::
 
     curs.execute("insert into mytable (jsondata) values (%s)",
         [Json({'a': 100}, dumps=simplejson.dumps)])

doc/src/faq.rst

@@ -125,6 +125,18 @@ Psycopg converts :sql:`decimal`\/\ :sql:`numeric` database types into Python `!D
     `!psycopg2._psycopg.DECIMAL` instead.
 
 
+.. _faq-json-adapt:
+.. cssclass:: faq
+
+Psycopg automatically converts PostgreSQL :sql:`json` data into Python objects. How can I receive strings instead?
+    The easiest way to avoid JSON parsing is to register a no-op function with
+    `~psycopg2.extras.register_default_json()`::
+
+        psycopg2.extras.register_default_json(loads=lambda x: x)
+
+    See :ref:`adapt-json` for further details.
+
+
 .. _faq-bytea-9.0:
 .. cssclass:: faq