So how does PostGreSQL compare to MySQL?

Postgresql's sequences are lock-free and don't block other transactions. They allow high INSERT concurrency, but don't guarantee gapless sequences (ie: on ROLLBACK or statement failure, you might have a "missing" number, which you shouldn't care about for a primary key).

Looking at some MySQL documentation, though, it looks like for recent versions of InnoDB AUTO_INCREMENT works much like PostgreSQL's sequences. I don't know enough about the MySQL side to go into depth on any differences. It looks like InnoDB's AUTO_INCREMENT these days may behave very like a sequence if InnoDB has the configuration param "innodb_autoinc_lock_mode = 2" set, but that's about all I can say.

http://dev.mysql.com/doc/refman/5.1/en/innodb-auto-increm...

( PostgreSQL doesn't support statement level replication, so the statement-replication concerns about auto-increment don't apply ).