There probably ought to be an implementation of the Store interface that talks to DBI. That would be the simplest way of supporting a database back end. The thorny problem is how to handle searching; the massive assumptions made by TWiki about the structure of the data store make it difficult (embedded meta-data).
See also:
ContentAccessSyntax
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Contributors: CrawfordCurrie
Discussion
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CrawfordCurrie - 15 Feb 2006