Hey Justin,
Unfortunately the use of indexes hasn't been well documented. Hoping Lars Breddemann and team will help with this in 2014. I don't have time for a detailed analysis right now, but here's what I found:
- Indexes will almost never help with Analytic Views because they run in the OLAP engine and are used for aggregations.
- Indexes can help with SQL, Attribute Views and Calc Views, in certain situations. This is situations where the default index on a column isn't well sorted to return the information you wanted.
The latter case is what's happening in Leonid Gvirtz's case. If you do something similar and create a large table with individual document IDs, then an INDEX will help with SELECT * performance.
It can equally help where you have joins between two tables and are looking to find individual records. I have never found indexes to help with OLAP-style aggregations.
Hope this helps you.
John