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<h2 class="title" style="clear: both"><a id="mvcc"></a>Using Multiversion Concurrency Control</h2>
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<p>
Multiversion Concurrency Control (MVCC) enables snapshot
isolation. Snapshot isolation means that whenever a
transaction would take a read lock on a page, it
makes a copy of the page instead, and then performs its
operations on that copied page. This frees other writers
from blocking due to a read locks held by other
transactions.
</p>
<p>
You should use snapshot isolation whenever you have a lot
of read-only transactions operating at the same time that
read-write transactions. In this case, snapshot isolation
will improve transaction throughput, albeit at the cost of
greater resource usage.
</p>
<p>
MVCC is described in more detail in
<a href="../programmer_reference/transapp_read.html#snapshot_isolation" class="olink">Snapshot Isolation.</a>
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<p>
To use MVCC, you must enable it before you access any
database tables. Once MVCC is enabled, you can turn
snapshot isolation on and off at anytime during the life of
your application.
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<p>
To turn MVCC on, use:
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<p>
<span class="bold"><strong>PRAGMA multiversion</strong></span> = on | off;
</p>
<p>
This PRAGMA must be enabled before you access any database
tables during your application runtime, or an error is
returned. Turning MVCC on automatically enables snapshot
isolation. By default MVCC is turned off.
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<p>
Once MVCC is enabled, you can turn snapshot isolation on
and off using:
</p>
<p>
<span class="bold"><strong>PRAGMA snapshot_isolation</strong></span> = on | off;
</p>
<p>
This PRAGMA can be used at any time during the life of your
application <span class="emphasis"><em>after</em></span> MVCC has been turned
on. If you attempt to enable or disable snapshot isolation
before MVCC is enabled, an error is returned.
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