Fix slow store_seen_flags_on_imap

src/configure.rs

@@ -243,11 +243,6 @@ impl Context {
                         Ok((id, add_timestamp))
                     },
                 )?;
-                transaction.execute("DELETE FROM imap WHERE transport_id=?", (transport_id,))?;
-                transaction.execute(
-                    "DELETE FROM imap_sync WHERE transport_id=?",
-                    (transport_id,),
-                )?;
 
                 // Removal timestamp should not be lower than addition timestamp
                 // to be accepted by other devices when synced.

src/imap.rs

@@ -1103,15 +1103,22 @@ impl Session {
             return Ok(());
         }
 
+        context
+            .sql
+            .execute(
+                "DELETE FROM imap_markseen WHERE id NOT IN (SELECT imap.id FROM imap)",
+                (),
+            )
+            .await?;
+
         let transport_id = self.transport_id();
-        let rows = context
+        let mut rows = context
             .sql
             .query_map_vec(
                 "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",
+                 AND target = folder",
                 (transport_id,),
                 |row| {
                     let rowid: i64 = row.get(0)?;
@@ -1122,6 +1129,16 @@ impl Session {
             )
             .await?;
 
+        // Number of SQL results is expected to be low as
+        // we usually don't have many messages to mark on IMAP at once.
+        // 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`.
+        rows.sort_unstable_by(|(_rowid1, uid1, folder1), (_rowid2, uid2, folder2)| {
+            (folder1, uid1).cmp(&(folder2, uid2))
+        });
+
         for (folder, rowid_set, uid_set) in UidGrouper::from(rows) {
             let folder_exists = match self.select_with_uidvalidity(context, &folder).await {
                 Err(err) => {

src/sql.rs

@@ -883,6 +883,24 @@ pub async fn housekeeping(context: &Context) -> Result<()> {
         .log_err(context)
         .ok();
 
+    // Cleanup `imap` and `imap_sync` entries for deleted transports.
+    //
+    // Transports may be deleted directly or via sync messages,
+    // so it is easier to cleanup orphaned entries in a single place.
+    context
+        .sql
+        .execute(
+            "DELETE FROM imap WHERE transport_id NOT IN (SELECT transports.id FROM transports)",
+            (),
+        )
+        .await
+        .log_err(context)
+        .ok();
+    context.sql.execute(
+        "DELETE FROM imap_sync WHERE transport_id NOT IN (SELECT transports.id FROM transports)",
+        (),
+    ).await.log_err(context).ok();
+
     // Delete POI locations
     // which don't have corresponding message.
     delete_orphaned_poi_locations(context)