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<title>12.5.Locking and Indexes</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="locking-indexes"></a>12.5.Locking and Indexes</h2></div></div></div>
<a name="id631879"></a><p> Though <span class="productname">PostgreSQL</span>
provides nonblocking read/write access to table
data, nonblocking read/write access is not currently offered for every
index access method implemented
in <span class="productname">PostgreSQL</span>.
The various index types are handled as follows:
</p>
<div class="variablelist"><dl>
<dt><span class="term"> B-tree and <acronym class="acronym">GiST</acronym> indexes
</span></dt>
<dd><p> Short-term share/exclusive page-level locks are used for
read/write access. Locks are released immediately after each
index row is fetched or inserted. These index types provide
the highest concurrency without deadlock conditions.
</p></dd>
<dt><span class="term"> Hash indexes
</span></dt>
<dd><p> Share/exclusive hash-bucket-level locks are used for read/write
access. Locks are released after the whole bucket is processed.
Bucket-level locks provide better concurrency than index-level
ones, but deadlock is possible since the locks are held longer
than one index operation.
</p></dd>
<dt><span class="term"> R-tree indexes
</span></dt>
<dd><p> Share/exclusive index-level locks are used for read/write access.
Locks are released after the entire command is done.
</p></dd>
</dl></div>
<p>
</p>
<p> Currently, B-tree indexes offer the best performance for concurrent
applications; since they also have more features than hash
indexes, they are the recommended index type for concurrent
applications that need to index scalar data. When dealing with
non-scalar data, B-trees are not useful, and GiST indexes should
be used instead. R-tree indexes are deprecated and are likely
to disappear entirely in a future release.
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