[Kde-pim] [PATCH] Akonadi database optimization

[Tobias König]

Hej,

as a proud user of KMail 4.5 I faced some performance problems lately which 
happened when entering a folder with many (in words 200) emails.
In this case KMail was waiting nearly 30 seconds until it continued to show
the emails in the message list.

After some debugging, blaming nearly every component of the PIM stack (sorry 
Kevin, it was not the maildirresource ;)) and adding debug statements all over 
the places, mysqld was identified as the culprit. The problematic call was the 
fetch statement for all header payloads of all mails in the current folder.
Just for fun I changed the fetch scope to FullPayload and to my suprise it was 
_faster_ then only fetching the headers.
So the SQL query, which varies for both calls must be the issue.
After some discussion with our MySQL-optimizer-Guru (aka milian) we came to 
the conclusion that the 'AND PimItemTable.id >= 1' condition in the statement 
leads to a performance bottleneck. The reason is that the >= operator will
imply a so called range-join which has a bad performance.
Fortunately this special condition is not really needed, because the 'id' 
column will always contain values >= 1 per definition. So we can just optimize 
this condition away and KMail 4.5 works as fast as it is supposed to do :)

Attached is a patch that fixes QueryHelper to not add unnecessary >= 
conditions. Does somebody see any problems with it?

Ciao,
Tobias

P.S.: Isotopp pointed out another performance problem that we could solve by 
handling the PLD parts in a special way. I'll discuss this with Volker on 
monday.
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