July 2010
Intermediate to advanced
840 pages
16h 33m
English
Content preview from Joe Celko's SQL for Smarties, 3rd Edition
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32.5. SNAPSHOT Isolation: Optimistic Concurrency
Optimistic concurrency control is based on the idea that transactions are not very likely to conflict with each other, so we need to design a system to handle the problems as exceptions after they actually occur.
In Snapshot Isolation, each transaction reads data from a snapshot of the (committed) data as of the time the transaction started, called its Start_timestamp. This time may be any time before the transaction’s first read. A transaction running in Snapshot Isolation is never blocked attempting a read because it is working on its private copy of the data. But this means that at any time, each data item might have multiple versions, created by active and committed transactions.
When the ...
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