XMLDB creating new DDL functions - pre 2.0
Before Moodle 1.6, all the DB install and upgrade was developed twice (once to handle MySQL installations and another to handle PostgreSQL installations). This approach, although working, has caused some headaches in the past, mainly because it was really difficult to keep both lines of development 100% on sync. Some developers do they work against one RDBMS and it was complex to develop to the other one (two test environments, skills on both databases, slower development cycle...). And all this was happening with only two supported RDBMS!
One of the main objectives of Moodle 1.7 is to extend the the number of supported RDBMS to other flavours (more exactly, to Oracle and MSSQL). And the old approach (one line of development for each DB) could become an absolute nightmare.