----- Am 1. Apr 2016 um 17:52 schrieb shawn l.green shawn.l.gr...@oracle.com:
>> What is true ? when the transaction started or when the first read is >> performed ? > Until you need to establish a snapshot of the data, then you don't need > a snapshot position. > > The transaction physically begins (rows begin to be protected against > changes by other transactions) with the first read. OK. But only when the first read is in a transaction and isolation level is REPEATABLE READ the query is "snapshotted" for further queries, so seeing the same result ? When several SELECT are issued not inside a transaction they always get the current data ? > Consider the alternative: If we started protecting data with the START > TRANSACTION command we would need to protect every row in every table in > every database. That is simply not efficient. YES. > We protect the pages that contain the rows that are physically required > by the individual transaction. This is a much smaller locking footprint > and is much easier to manage. Bernd Helmholtz Zentrum Muenchen Deutsches Forschungszentrum fuer Gesundheit und Umwelt (GmbH) Ingolstaedter Landstr. 1 85764 Neuherberg www.helmholtz-muenchen.de Aufsichtsratsvorsitzende: MinDir'in Baerbel Brumme-Bothe Geschaeftsfuehrer: Prof. Dr. Guenther Wess, Dr. Alfons Enhsen, Renate Schlusen (komm.) Registergericht: Amtsgericht Muenchen HRB 6466 USt-IdNr: DE 129521671 -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql
