Comment 9 for bug 234609

What makes this issue even more complicated to handle transparently is the fact that the Dependencies (php5-pgsql | php5-mysql) and (postgresql-client | mysql-client) as well as the Recommends (postgresql | mysql-server) seems to be handled/picked separately. That is, you can have both mysql-server and mysql-client installed and apt will satisfy its dependencies by choosing to install php5-pgsql.

On another note. A quick, and incomplete, solution which might at least help somewhat could be to simply switch the order of the postgres- and mysql-dependencies, putting mysql first. I could be wrong, but I'm guessing that people without any special preferences usually takes the mysql track.