Added FAQ and db dependencies to docs

docs/faq.rst

@@ -0,0 +1,34 @@
+FAQ
+===
+
+
+Can I query/join multiple tables at one time?
+---------------------------------------------
+Not directly no. A Caravel SQLAlchemy datasource can only be a single table
+or a view.
+
+When working with tables, the solution would be to materialize
+a table that contains all the fields needed for your analysis, most likely
+through some scheduled batch process.
+
+A view is a simple logical layer that abstract an arbitrary SQL queries as
+a virtual table. This can allow you to join and union multiple tables, and
+to apply some transformation using arbitrary SQL expressions. The limitation
+there is your database performance as Caravel effectively will run a query
+on top of your query (view). A good practice may be to limit yourself to
+joining your main large table to one or many small tables only, and avoid
+using ``GROUP BY`` where possible as Caravel will do its own ``GROUP BY`` and
+doing the work twice might slow down performance.
+
+Whether you use a table or a view, the important factor is whether your
+database is fast enough to serve it in an interactive fashion to provide
+a good user experience in Caravel.
+
+
+How BIG can my data source be?
+------------------------------
+
+It can be gigantic! As mentioned above, the main criteria is whether your
+database can execute queries and return results in a time frame that is
+acceptable to your users. Many distributed databases out there can execute
+queries that scan through terabytes in an interactive fashion.

docs/index.rst

@@ -34,6 +34,7 @@ Contents
     tutorial
     videos
     gallery
+    faq
 
 
 Indices and tables

docs/installation.rst

@@ -121,6 +121,40 @@ the `Flask App Builder Documentation
 <http://flask-appbuilder.readthedocs.org/en/latest/config.html>`_
 for more information on how to configure Caravel.
 
+Database dependencies
+---------------------
+
+Caravel does not ship bundled with connectivity to databases, except
+for Sqlite, which is part of the Python standard library.
+You'll need to install the required packages for the database you
+want to use as your metadata database as well as the packages needed to
+connect to the databases you want to access through Caravel.
+
+Here's a list of some of the recommended packages.
+
++---------------+-------------------------------------+-------------------------------------------------+
+| database      | pypi package                        | SQLAlchemy URI prefix                           |
++===============+=====================================+=================================================+
+|  MySQL        | ``pip install mysqlclient``         | ``mysql://``                                    |
++---------------+-------------------------------------+-------------------------------------------------+
+|  Postgres     | ``pip install psycopg2``            | ``postgresql+psycopg2://``                      |
++---------------+-------------------------------------+-------------------------------------------------+
+|  Presto       | ``pip install pyhive``              | ``presto://``                                   |
++---------------+-------------------------------------+-------------------------------------------------+
+|  Oracle       | ``pip install cx_Oracle``           | ``oracle://``                                   |
++---------------+-------------------------------------+-------------------------------------------------+
+|  sqlite       |                                     | ``sqlite://``                                   |
++---------------+-------------------------------------+-------------------------------------------------+
+|  Redshift     | ``pip install sqlalchemy-redshift`` | ``redshift+psycopg2://``                        |
++---------------+-------------------------------------+-------------------------------------------------+
+|  MSSQL        | ``pip install pymssql``             | ``mssql://``                                    |
++---------------+-------------------------------------+-------------------------------------------------+
+
+Note that many other database are supported, the main criteria being the
+existence of a functional SqlAlchemy dialect and Python driver. Googling
+the keyword ``sqlalchemy`` in addition of a keyword that describes the
+database you want to connect to should get you to the right place.
+
 
 Caching
 -------
@@ -147,7 +181,7 @@ parameters exposed by SQLAlchemy. In the ``Database`` edit view, you will
 find an ``extra`` field as a ``JSON`` blob.
 
 .. image:: _static/img/tutorial/add_db.png
-   :scale: 50 %
+   :scale: 30 %
 
 This JSON string contains extra configuration elements. The ``engine_params``
 object gets unpacked into the