Re: Why does MySQL's performance decrease when queries are executed in parallel?

[Rick James]

I have often noticed InnoDB doing a table scan when the key is non-existent. I think it is a bug.

What version are you using?

XtraDB _may_ do a much better job of concurrency. It is known to do better when concurrency calls for 8 cores.

The high concurrency found in typical benchmarks is, in my opinion, very rarely realistic. I deal with a lot of high-traffic, online, servers; I rarely see more than one core's worth of CPU in use.

Note that your test is doing a full scan of one table, then reaching into the other table many times. These lead to a lot of locks, hence difficulty in getting much concurrency.