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How do I know when a project is just to big for MySQL and I should use something with a better reputation for scalability?
Is there a max database size for MySQL before degradation of performance occurs? What factors contribute to MySQL not being a viable option compared to a commercial DBMS like Oracle or SQL Server?
Mostly it is table size.
I am assuming here that you will use the Oracle innoDB plugin for mysql as your engine. If you do not, that probably means you’re using a commercial engine such as infiniDB, InfoBright for Tokutek, in which case your questions should be sent to them.
InnoDB gets a bit nasty with very large tables. You are advised to partition your tables if at all possible with very large instances. Essentially, if your (frequently used) indexes don’t all fit into ram, inserts will be very slow as they need to touch a lot of pages not in ram. This cannot be worked around.
You can use the MySQL 5.1 partitioning feature if it does what you want, or partition your tables at the application level if it does not. If you can get your tables’ indexes to fit in ram, and only load one table at a time, then you’re on a winner.
You can use the plugin’s compression to make your ram go a bit further (as the pages are compressed in ram as well as on disc) but it cannot beat the fundamental limtation.
If your table’s indexes don’t all (or at least MOSTLY – if you have a few indexes which are NULL in 99.99% of cases you might get away without those ones) fit in ram, insert speed will suck.
Database size is not a major issue, provided your tables individually fit in ram while you’re doing bulk loading (and of course, you only load one at once).
These limitations really happen with most row-based databases. If you need more, consider a column database.
Infobright and Infinidb both use a mysql-based core and are column based engines which can handle very large tables.
Tokutek is quite interesting too – you may want to contact them for an evaluation.
When you evaluate the engine’s suitability, be sure to load it with very large data on production-grade hardware. There’s no point in testing it with a (e.g.) 10G database, that won’t prove anything.