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With `ORDER BY` statement SQLite searches the `imap` table by `transport_id` and for each found row scans the whole `imap_markseen` table. Number of `imap` entries for each `transport_id` is usually large as we need to know which UIDs to delete on IMAP server when deleting a message. ``` sqlite> EXPLAIN QUERY PLAN SELECT imap.id, uid, folder FROM imap, imap_markseen WHERE imap.id = imap_markseen.id AND imap.transport_id=? AND target = folder ORDER BY folder, uid; QUERY PLAN |--SEARCH imap USING INDEX sqlite_autoindex_imap_1 (transport_id=?) `--SCAN imap_markseen ``` Without `ORDER BY` statement SQLite scans `imap_markseen` table which is expected to be small, and then searches `imap` table by `rowid` for each found result. ``` sqlite> EXPLAIN QUERY PLAN SELECT imap.id, uid, folder FROM imap, imap_markseen WHERE imap.id = imap_markseen.id AND imap.transport_id=? AND target = folder; QUERY PLAN |--SCAN imap_markseen `--SEARCH imap USING INTEGER PRIMARY KEY (rowid=?) ``` Query planning was tested with SQLite 3.52.0. It is possible to explictly make query planner move sorting to the last step with `ORDER +folder, +uid`, but this is not recommended in SQLite documentation (see <https://www.sqlite.org/optoverview.html#uplus>). It is also possible to add indexes, but indexes use space, adding them requires an SQL migration, and each index needs to be updated so it will slow down writes.
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…ekeeping Previously transports deleted via sync messages left unused `imap` entries.
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Fixes #8022