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postgresql9.6-docs-9.6.22-1.mga7.noarch.rpm

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>24.2. Routine Reindexing</A
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>   In some situations it is worthwhile to rebuild indexes periodically
   with the <A
HREF="sql-reindex.html"
>REINDEX</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
HREF="sql-reindex.html"
>REINDEX</A
> can be used safely and easily in all cases.
   But since the command requires an exclusive table lock, it is
   often preferable to execute an index rebuild with a sequence of
   creation and replacement steps.  Index types that support
   <A
HREF="sql-createindex.html"
>CREATE INDEX</A
> with the <TT
CLASS="LITERAL"
>CONCURRENTLY</TT
>
   option can instead be recreated that way. If that is successful and the
   resulting index is valid, the original index can then be replaced by
   the newly built one using a combination of <A
HREF="sql-alterindex.html"
>ALTER INDEX</A
>
   and <A
HREF="sql-dropindex.html"
>DROP INDEX</A
>. When an index is used to enforce
   uniqueness or other constraints, <A
HREF="sql-altertable.html"
>ALTER TABLE</A
> might
   be necessary to swap the existing constraint with one enforced by
   the new index. Review this alternate multistep rebuild approach
   carefully before using it as there are limitations on which
   indexes can be reindexed this way, and errors must be handled.
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