Database
MediaWiki can use either the MySQL, PostgreSQL or SQLite relational database management system. There is limited support for Oracle Database. A MediaWiki database contains several dozen tables, including a page table that contains page titles, page ids, and other metadata; and a revision table to which is added a new row every time an edit is made, containing the page id, a brief textual summary of the change performed, the user name of the article editor (or its IP address the case of an unregistered user) and a timestamp.
In a 4½ year period, the MediaWiki database had 170 schema versions. Possibly the largest schema change was done in MediaWiki 1.5, when the storage of metadata was separated from that of content, to improve performance flexibility. When this upgrade was applied to Wikipedia, the site was locked for editing, and the schema was converted to the new version in about 22 hours. Some software enhancement proposals, such as a proposal to allow sections of articles to be watched via watchlist, have been rejected because the necessary schema changes would have required excessive Wikipedia downtime.
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