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You should try to use the new Redis backend always. It is by far the fastest backend and easiest to maintain, very little moving parts and it will handle autoexpiring by itself.
https://spamassassin.apache.org/full/3.4.x/doc/Mail_SpamAssassin_BayesStore_Redis.html
See: http://svn.apache.org/repos/asf/spamassassin/trunk/contrib/HOWTO.Bayes-Redis/
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