Hi, Lets see If I can help.
Overly long queries (transactions > in general) are bad for performance as a lot of unpurged versions > accumulate. > > In this sentence, I don't know the meaning about 'unpureged version > accumulate' > When rows are updated new versions are created. They are later removed by purge thread - only then no active transactions may need them. This is why long open transactions are expensive. > > > And I don't how to understanding 'open transaction'? > This is transaction which is started (opened) but not yet ended by commit or rollback. > > > Required for logical level replication to work properly. > > What's the meaning about logical level replication? > MySQL has statement level, also caused logical as well as row based replication. statement level replication requires updates to be serializable to work. > > > Can give problems for portable applications if you port from MySQL-4.0 to > later > > What's the meaning about this sentence? > > > This means you can run into the problems if you upgrade from MySQL 4.0 to later version. Probably is not much relevant any more. -- Peter Zaitsev, CEO, Percona Inc. Tel: +1 888 401 3401 ext 501 Skype: peter_zaitsev 24/7 Emergency Line +1 888 401 3401 ext 911 Percona Training Workshops http://www.percona.com/training/
