On 10/31/2017 10:50 PM, Charles Leifer wrote:
Is the LSM database append-only, as in the file size will always grow/never shrink (even if there are deletions/overwrites)?
An LSM database is basically a series of tree structures on disk. When you write to an LSM database you add the new key to an in-memory tree. Once that tree is full it is flushed out to disk. To query the database for a key, you query each of these trees from newest to oldest until you find a match or run out of trees. To prevent the number of trees on disk from growing indefinitely, two or more old trees are periodically merged together to create a single, larger tree structure.
A DELETE operation is handled like an INSERT, except a special "delete-marker" key is added to the in-memory tree structure.
When two old trees are merged together, if a delete-marker is merged with a real insert-key, both are discarded. So that the new tree contains neither the delete-marker or the insert key.
So if you delete a bunch of data space is reclaimed eventually, but it can take a while to happen. Optimizing the database involves merging all trees on disk together, so this has the effect of reclaiming all unused space:
http://sqlite.org/src4/doc/trunk/www/lsmusr.wiki#database_file_optimization Dan. _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
