Which database for survey app
PostgreSQL is often overlooked but might be worth your attention.  It supports MVCC for concurrency, can do analytic SQL queries easily, supports indexes, scales up well enough, can do hot backups, supports date functions, and even has JSON data types.  Binary JSON (aka: JSONB, BSON) is being added to newer versions, which will give you the ability to index on JSON content and have improved analytic function &amp; performance.

Check this out article on Phoronix:

Binary JSON Support Added To PostgreSQL, Competes With MongoDB
http://www.phoronix.com/scan.php?page=news_item&amp;px=MTY0MTU