I'm trying to generate a few graphs using the sysbench benchmark (default configuration) trying to show the relationship between deadlocks and isolation level in MySQL.
But I get some strage results: I was under the impression that repeatable read would have more deadlocks than read committed (which is the case), but significantly more than read uncommitted. In fact it turns out that read uncommitted has more deadlocks than either.
Is this normal? And if so, why?
Deadlock can happen in any isolation level. It's hard to tell without the actual tests, but I guess that in case of read commited / repeatable read, if you have to read a value of a row being updated, the value is being read from the rollback log, but in case of read uncommited rollback log is not used, so it the row is locked for update, the read has to wait for the actual value to be written. But it's a wild guess, having more deadlocks in read uncommited is a strange behaviour and most likely - implementation dependent. I would be interested if you could provide the actual tests, and if the test can be repeated in different versions of MySQL.
