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Displaying posts with tag: optimizer_switch (reset)
How to Find Values of Session Variables With gdb

Usually I make notes while working on customer issues, reading the Slack channels or even scrolling across my Facebook news feed. Some of them are of "ToDo" kind, and this week while watching how my colleagues work on a very complicated crash/bug I noted that one of the problems they were discussing was how to find out from the core dump if the session behind the crashing thread had mrr=ON in the optimizer_switch.

I had already written more than once how to check MySQL threads one by one in gdb, depending on version and your real goal. For this post I decided to concentrate on checking the values of session variables and, specifically, individual switchable optimizations. This is actually …

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"I'm Winston Wolf, I solve problems."

My (few) readers are probably somewhat tired of boring topics of metadata locks and gdb breakpoints that I discuss a lot this year, so for this weekend I decided to concentrate on something less technical but still important to me - the way I prefer to follow while providing support for MySQL.

Before I continue, it's time to add the explicit disclaimer: the views on how support engineer should work expressed below are mine alone and not those of my current (or any previous) employer. Specific case I describe may be entirely fictional and has nothing to do with any real life customer. I love thy customers in reality... …

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Back on disk temporary tables

We already blog a lot on temporary tables on disk, how this could be bad for your workload and should be avoid.  Each temporary table will open a file descriptor,  external kernel call and by nature a well know file system slow operation.

We can point this benchmark simulating a working mail server


Monitoring of such queries can be trace via the status of

created_tmp_disk_tables 
Evidence of what is happening watching many Aria or MyISAM tables created on disk :

/usr/sbin/lsof  | grep "mysql" | grep "#"
mysqld     1855     mysql 1658u      REG                8,1       8192    1505932 …

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Showing entries 1 to 3