3 Common Rails + MySQL Mistakes

The first item in the list really bugs me. It's not enough to just tell me not to do something, teach me why it's bad. That way, I can explain to others why this is a mistake instead of just regurgitating the same explanation of 'it's a performance disaster'.

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).