On 4/1/2016 10:08 AM, Lentes, Bernd wrote:
----- On Apr 1, 2016, at 3:12 PM, Bernd Lentes
bernd.len...@helmholtz-muenchen.de wrote:
Btw:
i read about isolation levels. REPEATABLE READ is the default for InnoDB.
http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_repeatable_read says:
"...so that all queries within a transaction see data from the same snapshot,
that is, the data as it was at the time the transaction started.".
http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_consistent_read says:
"With the repeatable read isolation level, the snapshot is based on the time
when the first read operation is performed".
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.
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.
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
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