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Re: need help from the list admin




----- Am 1. Apr 2016 um 17:52 schrieb shawn l.green shawn.l.green@xxxxxxxxxx:

>> 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)
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85764 Neuherberg
www.helmholtz-muenchen.de
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