Best database for web backend on Linux?

Now that we got all sorts of commercial and free databases available for Linux, I don't know what to use. I currently use PostgreSQL, but its SQL implementation is quite incomplete (no foreign key, alter table add constraint, etc.), and there's no row-level locking. I want to try something else but don't have much experience with other databases. Can someone summarize the pros and cons of each database as a web backend? I'm considering PostgreSQL, MySQL, Solid, Oracle, Sybase, and Informix. The features I'm most interested in are transaction support, general SQL support, good Perl DBI/DBD implementation, row-level locking, online backup, max size of VARCHAR, and persistent connection. Oh, also price. (I know Sybase for Linux is free, but what's the price for Oracle as a web backend?) Thanks, Yun