This is not a development question - it should really be posted in the 'general problems' forum instead.
Or, you could take a look at the system requirements page ( http://docs.moodle.org/23/en/Installing_Moodle#Software ) which lists all the databases supported by Moodle.
In fact, I want to develop an activity module in moodle. But mysql can not totally support the fuction of my module. I want to change to another database. Thank you for your link. I want to ask I only can develop the plugin with these 5 kinds of database (MySQL, PostgreSQL, MSSQL, Oracle, SQLite), right?
You're not technically required to use the Moodle database abstraction layer in your own activity module. Just use the php library which supports your database to make a connection and execute SQL queries.
Or, in your case, to execute XPath queries, presumably.
I'd revisit "But mysql can not totally support the fuction of my module." I have a hard time thinking of any data that can't be properly stored in a relational database like MySQL. Since you mentioned XML, I'm going to hazard a guess that you have data which is hierarchical by nature?
If that's the case, a more accurate description of the problem would be "I don't know how to organize hierarchical data in a relational database, and don't care to Google it." If that's so, MySQL actually does have XML functions:
98% of the time that's not the best way to do it, though. Better (and quicker) to spend 30 minutes learning how to store the hierarchy in the normalized tables. Hint: