Add DatabaseCleanup.md.

Technotes/DatabaseCleanup.md

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+# Database Cleanup
+
+Old articles and statuses are removed after a certain period of time.
+
+(This applies to the OnMyMac account only. Other syncing systems have their own rules, and the app should follow them.)
+
+We do this so that the database doesn’t just grow forever. Because the bigger it gets, the slower it gets.
+
+The cleanup process is run at app launch, before other reads and writes. It should be fast enough that it goes unnoticed by the user.
+
+Articles are deleted first, then statuses.
+
+## Articles
+
+All articles whose feed is no longer in the subscriptions list are deleted.
+
+A cut-off date of 4 * 31 days ago is calculated.
+
+For the remaining articles, we go feed-by-feed.
+
+If a feed has 20 or fewer articles, it’s left alone.
+
+For each article (oldest first) delete until:
+  The feed’s count of articles is down to 20,
+  or the article is newer than the cut-off date.
+
+The reason we preserve some articles that would otherwise be deleted: you want to be able to select a feed that hasn’t been updated in a year (or whatever long period of time) and still articles from that feed.
+
+## Statuses
+
+Statuses older than five months (see statuses.dateArrived) with no matching articleID in articles are deleted.
+