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In some situations it is worthwhile to rebuild indexes periodically
with the <a class="xref" href="sql-reindex.html" title="REINDEX"><span class="refentrytitle">REINDEX</span></a> command or a series of individual
rebuilding steps.
</p><p>
B-tree index pages that have become completely empty are reclaimed for
re-use. However, there is still a possibility
of inefficient use of space: if all but a few index keys on a page have
been deleted, the page remains allocated. Therefore, a usage
pattern in which most, but not all, keys in each range are eventually
deleted will see poor use of space. For such usage patterns,
periodic reindexing is recommended.
</p><p>
The potential for bloat in non-B-tree indexes has not been well
researched. It is a good idea to periodically monitor the index's physical
size when using any non-B-tree index type.
</p><p>
Also, for B-tree indexes, a freshly-constructed index is slightly faster to
access than one that has been updated many times because logically
adjacent pages are usually also physically adjacent in a newly built index.
(This consideration does not apply to non-B-tree indexes.) It
might be worthwhile to reindex periodically just to improve access speed.
</p><p>
<a class="xref" href="sql-reindex.html" title="REINDEX"><span class="refentrytitle">REINDEX</span></a> can be used safely and easily in all cases.
This command requires an <code class="literal">ACCESS EXCLUSIVE</code> lock by
default, hence it is often preferable to execute it with its
<code class="literal">CONCURRENTLY</code> option, which requires only a
<code class="literal">SHARE UPDATE EXCLUSIVE</code> lock.
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