Why this article?
First of all, because I had fun digging in the code.
Then, I was reading a lot about the improvements we will have in MySQL 5.6 and about some already present in 5.5. Most of them are well covered by people that certainly know more than me, so I read and read, but after a while became curious. I began reading the code and performing tests. I started to compare versions, like 5.1 – 5.5. – 5.6. One of the things I was looking for was how the new Purge thread mechanism works and what were its implications. I have to say that it seems to work better than the previous versions, and the Dimitry blog (see reference) seems to confirm that.
So again, why the article? Because I think there are some traps here and there, and I feel the need to write about them. The worse behavior is with MySQL 5.5. This is because in 5.5 we have an intermediate situation, where the purge is not fully …[Read more]