[Bug 290393] New: moving the contents of a mail folder is extremely slow

[Bruno Haible]

https://bugs.kde.org/show_bug.cgi?id=290393

           Summary: moving the contents of a mail folder is extremely slow
           Product: kontact
           Version: 4.7.2
          Platform: openSUSE RPMs
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: mail
        AssignedTo: kdepim-bugs at kde.org
        ReportedBy: bruno at clisp.org


Version:           4.7.2 (using KDE 4.7.3) 
OS:                Linux

kmail 4.7.4, akonadi 4.7.4

Inside a mixedmaildir resource, I moved the entire contents of a mail folder to
an empty mail folder. Both mail folders are in mbox format, on a local disk.

Time it takes to move 139 mails: 12 sec.
Time it takes to move 263 mails: 45 sec.
Time it takes to move 4462 mails: 32340 sec.

As you can see, the time needed is O(N^2) (or more), where N is the number of
emails to be moved.

More precisely, when moving the 4462 mails:
- Moving the mails to the new folder on disk took 41 minutes (on a slow USB 1.1
disk),
- The size of the old folder then went down to 0,
- For 485 minutes, akonadi updated the $HOME/.local/share/akonadi/db_data/*
files, kmail using 100% CPU most of the time,
- Finally, 13 minutes after akonadi/parttable.ibd was last modified, also
akonadi/pimitemtable.ibd was last modified. The view updated to empty, as it
should.

Reproducible: Didn't try

Steps to Reproduce:
In a mixedmaildir resource with at least 2 mbox folders:
Select a folder.
Press Ctrl-. to view all mails.
Select one of the mails.
Press Ctrl-A to select all mails.
Drag-and-drop the selection to the other folder.


Actual Results:  
Run time that is quadratic in the number of mails.

Expected Results:  
Run time that is linear in the number of mails.
In previous releases of kmail, operations like this one used to take
minutes, not hours. Cf. https://bugs.kde.org/show_bug.cgi?id=104721

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