Is storing redundant data part of the (CouchDB) self-contained data concept

You can store your index as a list of fixed-size offsets into the block containing your key data. For example:

+--------------+
| 3            | number of entries
+--------------+
| 16           | offset of first key data
+--------------+
| 24           | offset of second key data
+--------------+
| 39           | offset of third key data
+--------------+
| key one |
+----------------+
| key number two |
+-----------------------+
| this is the third key |
+-----------------------+

(well, the key data would be sorted in a real example, but you get the idea).

Note that this does not necessarily reflect how index blocks are actually constructed in any database. This is merely an example of how you might organise a block of index data where the key data is of variable length.

I think the answer mainly depends on how much time do you want to spend learning new databases vs how much time to you want to spend learning machine learning.

For example PostgreSQL has lots of GIS stuff built-in which I assume could be very useful for your queries. CouchDB has many useful features with their map/reduce stuff but I find it a bit limited. If you think you're going to add many columns later on based on new algorithms you might want to look into the column-oriented databases like Apache Cassandra.

However, I would recommend splitting the problem into two parts:

• Data gathering
• Data analysis

The first part you seem to have a good plan for already. I would write a super simple app that would just suck in the data from twitter and put it in a table as a BLOB. Or just a file on disk.

When you have that you can go from the raw data and insert it into any kind of backend. Now you can choose backend based on the current problem (algorithm) you're working on, instead of having one solution to fit all your problems. The key point here is to extract exactly the data you need. That way you shouldn't have to consider the fact that the data is nested inside a document from Twitter as you only pick out the parts you need. If you go that way I think an RDBMS would perform very nice as they can run all kinds of queries.

Make sense?