When shouldn't you use a relational database?

Question

Apart from the google/bigtable scenario, when shouldn't you use a relational database? Why not, and what should you use? (did you learn 'the hard way'?)

Answer 1

In my experience, you shouldn't use a relational database when any one of these criteria are true:

• your data is structured as a hierarchy or a graph (network) of arbitrary depth,
• the typical access pattern emphasizes reading over writing, or
• there’s no requirement for ad-hoc queries.

Deep hierarchies and graphs do not translate well to relational tables. Even with the assistance of proprietary extensions like Oracle's CONNECT BY, chasing down trees is a mighty pain using SQL.

Relational databases add a lot of overhead for simple read access. Transactional and referential integrity are powerful, but overkill for some applications. So for read-mostly applications, a file metaphor is good enough.

Finally, you simply don’t need a relational database with its full-blown query language if there are no unexpected queries anticipated. If there are no suits asking questions like "how many 5%-discounted blue widgets did we sell in on the east coast grouped by salesperson?", and there never will be, then you, sir, can live free of DB.