None of these are Rails specific. Don't stick session data in a relational db if you can put it in memcache or someplace similar. Also, MySQL's query optimizer

None of these are Rails specific. Don't stick session data in a relational db if you can put it in memcache or someplace similar. Also, MySQL's query optimizer and execution engine suck (SQL server, DB2, Oracle, and PostgreSQL all handle in() queries with select statements just fine). The select for update thing, though, sounds like a table type selection issue. Doesn't innodb handle locking at roughly row level (maybe db page)? Seems like that one is for people who stick with MyISAM. If you're doing concurrency sensitive stuff, pick a better storage engine (or more robust db).