Looking through the Wrox Press Book "Beginning PHP4 Databases" the comments that it makes on MySQL and PostGres are

MySQL

• Optimised for Reads only apps.
• Doesn't support DRI.
• Doesn't support views or stored procedures.
• Doesn't support transactions or locking

It does make reference to InnoDB http://www.innodb.com which provides an interface to MySQL in an attempt to provide transactions, DRI etc.

Apparently MySQL4 will have these features built in anyway and will include stored procedures.

It waxes lyrical about PostGres

• Object-relational database
• Supports user defined data-types
• Structured Datatypes
• Table inheritance

I'm not sure what all that meant but it sounds interesting.

I've had a brief look at it and there are one or two interesting looking things beyond the above that have me rooting around the documentation.

RedHat ships a version optimised for PostGres .