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adbenitez
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this fixed the problem!
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.
…ekeeping Previously transports deleted via sync messages left unused `imap` entries.
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iequidoo
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| // We are sorting outside of SQL to avoid SQLite constructing a query plan | ||
| // that scans the whole `imap` table. Scanning `imap_markseen` is fine | ||
| // as it should not have many items. | ||
| // If you change the SQL query, test it with `EXPLAIN QUERY PLAN`. |
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This should goto STYLE.md i think
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Fixes #8022