Well, I think this comment can confuse. Basically Postgresql is (was?) a much more feature rich database than MySQL including triggers, a rich sql language and overall a piece of art transaction system that many commercial databases still lack. When compared with Oracle or M$ SQL server on single processors non-clustered machines it's as good or better in many circumstances. I replaced time ago a M$SQL v6 server with Postgresql+cygwin in a J2EE produccion environment 3 years ago and it's still working. On the opposite MySQL started as a light weight sql front-end to Berkley DataBase Library (that is not even a relational library) and its sql syntax has been historically really weak, but since Berkley DB is extremnly fast MySQL has always kept firsts positions in many test when data integrity and advanced features were not important (I have seen MySQL storing video frames from tens IP cammeras in real time using just a normal PC with IDE disks ). MySQL greater popularity was mainly due to the fact that it works on Windows and is easy to install, while Postgresql ran on Windows platforms just with help of Cygwin until recently and was harder to setup. Recently SAP announced its development support for MySQL so thinks could change fast.
it's supposed to be closer feature-wise to oracle/ms sql server than mysql and pgsql (although mysql and pgsql are both catching up nicely) because it's a descendant of interbase.