<!--
doc/src/sgml/ref/reindex.sgml
PostgreSQL documentation
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<refentry id="sql-reindex">
 <indexterm zone="sql-reindex">
  <primary>REINDEX</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>REINDEX</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>REINDEX</refname>
  <refpurpose>rebuild indexes</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } [ CONCURRENTLY ] <replaceable class="parameter">name</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>REINDEX</command> rebuilds an index using the data
   stored in the index's table, replacing the old copy of the index. There are
   several scenarios in which to use <command>REINDEX</command>:

   <itemizedlist>
    <listitem>
     <para>
      An index has become corrupted, and no longer contains valid
      data. Although in theory this should never happen, in
      practice indexes can become corrupted due to software bugs or
      hardware failures.  <command>REINDEX</command> provides a
      recovery method.
     </para>
    </listitem>

    <listitem>
     <para>
      An index has become <quote>bloated</quote>, that is it contains many
      empty or nearly-empty pages.  This can occur with B-tree indexes in
      <productname>PostgreSQL</productname> under certain uncommon access
      patterns. <command>REINDEX</command> provides a way to reduce
      the space consumption of the index by writing a new version of
      the index without the dead pages. See <xref
      linkend="routine-reindex"/> for more information.
     </para>
    </listitem>

    <listitem>
     <para>
      You have altered a storage parameter (such as fillfactor)
      for an index, and wish to ensure that the change has taken full effect.
     </para>
    </listitem>

    <listitem>
     <para>
      If an index build fails with the <literal>CONCURRENTLY</literal> option,
      this index is left as <quote>invalid</quote>. Such indexes are useless
      but it can be convenient to use <command>REINDEX</command> to rebuild
      them. Note that only <command>REINDEX INDEX</command> is able
      to perform a concurrent build on an invalid index.
     </para>
    </listitem>

   </itemizedlist></para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>INDEX</literal></term>
    <listitem>
     <para>
      Recreate the specified index.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>TABLE</literal></term>
    <listitem>
     <para>
      Recreate all indexes of the specified table.  If the table has a
      secondary <quote>TOAST</quote> table, that is reindexed as well.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SCHEMA</literal></term>
    <listitem>
     <para>
      Recreate all indexes of the specified schema.  If a table of this
      schema has a secondary <quote>TOAST</quote> table, that is reindexed as
      well. Indexes on shared system catalogs are also processed.
      This form of <command>REINDEX</command> cannot be executed inside a
      transaction block.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DATABASE</literal></term>
    <listitem>
     <para>
      Recreate all indexes within the current database.
      Indexes on shared system catalogs are also processed.
      This form of <command>REINDEX</command> cannot be executed inside a
      transaction block.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SYSTEM</literal></term>
    <listitem>
     <para>
      Recreate all indexes on system catalogs within the current database.
      Indexes on shared system catalogs are included.
      Indexes on user tables are not processed.
      This form of <command>REINDEX</command> cannot be executed inside a
      transaction block.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name of the specific index, table, or database to be
      reindexed.  Index and table names can be schema-qualified.
      Presently, <command>REINDEX DATABASE</command> and <command>REINDEX SYSTEM</command>
      can only reindex the current database, so their parameter must match
      the current database's name.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CONCURRENTLY</literal></term>
    <listitem>
     <para>
      When this option is used, <productname>PostgreSQL</productname> will rebuild the
      index without taking any locks that prevent concurrent inserts,
      updates, or deletes on the table; whereas a standard index rebuild
      locks out writes (but not reads) on the table until it's done.
      There are several caveats to be aware of when using this option
      &mdash; see <xref linkend="sql-reindex-concurrently"
      endterm="sql-reindex-concurrently-title"/>.
     </para>
     <para>
      For temporary tables, <command>REINDEX</command> is always
      non-concurrent, as no other session can access them, and
      non-concurrent reindex is cheaper.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Prints a progress report as each index is reindexed.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   If you suspect corruption of an index on a user table, you can
   simply rebuild that index, or all indexes on the table, using
   <command>REINDEX INDEX</command> or <command>REINDEX TABLE</command>.
  </para>

  <para>
   Things are more difficult if you need to recover from corruption of
   an index on a system table.  In this case it's important for the
   system to not have used any of the suspect indexes itself.
   (Indeed, in this sort of scenario you might find that server
   processes are crashing immediately at start-up, due to reliance on
   the corrupted indexes.)  To recover safely, the server must be started
   with the <option>-P</option> option, which prevents it from using
   indexes for system catalog lookups.
  </para>

  <para>
   One way to do this is to shut down the server and start a single-user
   <productname>PostgreSQL</productname> server
   with the <option>-P</option> option included on its command line.
   Then, <command>REINDEX DATABASE</command>, <command>REINDEX SYSTEM</command>,
   <command>REINDEX TABLE</command>, or <command>REINDEX INDEX</command> can be
   issued, depending on how much you want to reconstruct.  If in
   doubt, use <command>REINDEX SYSTEM</command> to select
   reconstruction of all system indexes in the database.  Then quit
   the single-user server session and restart the regular server.
   See the <xref linkend="app-postgres"/> reference page for more
   information about how to interact with the single-user server
   interface.
  </para>

  <para>
   Alternatively, a regular server session can be started with
   <option>-P</option> included in its command line options.
   The method for doing this varies across clients, but in all
   <application>libpq</application>-based clients, it is possible to set
   the <envar>PGOPTIONS</envar> environment variable to <literal>-P</literal>
   before starting the client.  Note that while this method does not
   require locking out other clients, it might still be wise to prevent
   other users from connecting to the damaged database until repairs
   have been completed.
  </para>

  <para>
   <command>REINDEX</command> is similar to a drop and recreate of the index
   in that the index contents are rebuilt from scratch.  However, the locking
   considerations are rather different.  <command>REINDEX</command> locks out writes
   but not reads of the index's parent table.  It also takes an
   <literal>ACCESS EXCLUSIVE</literal> lock on the specific index being processed,
   which will block reads that attempt to use that index.  In contrast,
   <command>DROP INDEX</command> momentarily takes an
   <literal>ACCESS EXCLUSIVE</literal> lock on the parent table, blocking both
   writes and reads.  The subsequent <command>CREATE INDEX</command> locks out
   writes but not reads; since the index is not there, no read will attempt to
   use it, meaning that there will be no blocking but reads might be forced
   into expensive sequential scans.
  </para>

  <para>
   Reindexing a single index or table requires being the owner of that
   index or table.  Reindexing a schema or database requires being the
   owner of that schema or database.  Note that is therefore sometimes
   possible for non-superusers to rebuild indexes of tables owned by
   other users.  However, as a special exception, when
   <command>REINDEX DATABASE</command>, <command>REINDEX SCHEMA</command>
   or <command>REINDEX SYSTEM</command> is issued by a non-superuser,
   indexes on shared catalogs will be skipped unless the user owns the
   catalog (which typically won't be the case).  Of course, superusers
   can always reindex anything.
  </para>

  <para>
   Reindexing partitioned tables or partitioned indexes is not supported.
   Each individual partition can be reindexed separately instead.
  </para>

  <refsect2 id="sql-reindex-concurrently">
   <title id="sql-reindex-concurrently-title">Rebuilding Indexes Concurrently</title>

   <indexterm zone="sql-reindex-concurrently">
    <primary>index</primary>
    <secondary>rebuilding concurrently</secondary>
   </indexterm>

   <para>
    Rebuilding an index can interfere with regular operation of a database.
    Normally <productname>PostgreSQL</productname> locks the table whose index is rebuilt
    against writes and performs the entire index build with a single scan of the
    table. Other transactions can still read the table, but if they try to
    insert, update, or delete rows in the table they will block until the
    index rebuild is finished. This could have a severe effect if the system is
    a live production database. Very large tables can take many hours to be
    indexed, and even for smaller tables, an index rebuild can lock out writers
    for periods that are unacceptably long for a production system.
   </para>

   <para>
    <productname>PostgreSQL</productname> supports rebuilding indexes with minimum locking
    of writes.  This method is invoked by specifying the
    <literal>CONCURRENTLY</literal> option of <command>REINDEX</command>. When this option
    is used, <productname>PostgreSQL</productname> must perform two scans of the table
    for each index that needs to be rebuilt and wait for termination of
    all existing transactions that could potentially use the index.
    This method requires more total work than a standard index
    rebuild and takes significantly longer to complete as it needs to wait
    for unfinished transactions that might modify the index. However, since
    it allows normal operations to continue while the index is being rebuilt, this
    method is useful for rebuilding indexes in a production environment. Of
    course, the extra CPU, memory and I/O load imposed by the index rebuild
    may slow down other operations.
   </para>

   <para>
    The following steps occur in a concurrent reindex.  Each step is run in a
    separate transaction.  If there are multiple indexes to be rebuilt, then
    each step loops through all the indexes before moving to the next step.

    <orderedlist>
     <listitem>
      <para>
       A new transient index definition is added to the catalog
       <literal>pg_index</literal>.  This definition will be used to replace
       the old index.  A <literal>SHARE UPDATE EXCLUSIVE</literal> lock at
       session level is taken on the indexes being reindexed as well as their
       associated tables to prevent any schema modification while processing.
      </para>
     </listitem>

     <listitem>
      <para>
       A first pass to build the index is done for each new index.  Once the
       index is built, its flag <literal>pg_index.indisready</literal> is
       switched to <quote>true</quote> to make it ready for inserts, making it
       visible to other sessions once the transaction that performed the build
       is finished.  This step is done in a separate transaction for each
       index.
      </para>
     </listitem>

     <listitem>
      <para>
       Then a second pass is performed to add tuples that were added while the
       first pass was running.  This step is also done in a separate
       transaction for each index.
      </para>
     </listitem>

     <listitem>
      <para>
       All the constraints that refer to the index are changed to refer to the
       new index definition, and the names of the indexes are changed.  At
       this point, <literal>pg_index.indisvalid</literal> is switched to
       <quote>true</quote> for the new index and to <quote>false</quote> for
       the old, and a cache invalidation is done causing all sessions that
       referenced the old index to be invalidated.
      </para>
     </listitem>

     <listitem>
      <para>
       The old indexes have <literal>pg_index.indisready</literal> switched to
       <quote>false</quote> to prevent any new tuple insertions, after waiting
       for running queries that might reference the old index to complete.
      </para>
     </listitem>

     <listitem>
      <para>
       The old indexes are dropped.  The <literal>SHARE UPDATE
       EXCLUSIVE</literal> session locks for the indexes and the table are
       released.
      </para>
     </listitem>
    </orderedlist>
   </para>

   <para>
    If a problem arises while rebuilding the indexes, such as a
    uniqueness violation in a unique index, the <command>REINDEX</command>
    command will fail but leave behind an <quote>invalid</quote> new index in addition to
    the pre-existing one. This index will be ignored for querying purposes
    because it might be incomplete; however it will still consume update
    overhead. The <application>psql</application> <command>\d</command> command will report
    such an index as <literal>INVALID</literal>:

<programlisting>
postgres=# \d tab
       Table "public.tab"
 Column |  Type   | Modifiers
--------+---------+-----------
 col    | integer |
Indexes:
    "idx" btree (col)
    "idx_ccnew" btree (col) INVALID
</programlisting>

    If the index marked <literal>INVALID</literal> is suffixed
    <literal>ccnew</literal>, then it corresponds to the transient
    index created during the concurrent operation, and the recommended
    recovery method is to drop it using <literal>DROP INDEX</literal>,
    then attempt <command>REINDEX CONCURRENTLY</command> again.
    If the invalid index is instead suffixed <literal>ccold</literal>,
    it corresponds to the original index which could not be dropped;
    the recommended recovery method is to just drop said index, since the
    rebuild proper has been successful.
   </para>

   <para>
    Regular index builds permit other regular index builds on the same table
    to occur simultaneously, but only one concurrent index build can occur on a
    table at a time. In both cases, no other types of schema modification on
    the table are allowed meanwhile.  Another difference is that a regular
    <command>REINDEX TABLE</command> or <command>REINDEX INDEX</command>
    command can be performed within a transaction block, but <command>REINDEX
    CONCURRENTLY</command> cannot.
   </para>

   <para>
    Like any long-running transaction, <command>REINDEX</command> on a table
    can affect which tuples can be removed by concurrent
    <command>VACUUM</command> on any other table.
   </para>

   <para>
    <command>REINDEX SYSTEM</command> does not support
    <command>CONCURRENTLY</command> since system catalogs cannot be reindexed
    concurrently.
   </para>

   <para>
    Furthermore, indexes for exclusion constraints cannot be reindexed
    concurrently.  If such an index is named directly in this command, an
    error is raised.  If a table or database with exclusion constraint indexes
    is reindexed concurrently, those indexes will be skipped.  (It is possible
    to reindex such indexes without the <command>CONCURRENTLY</command> option.)
   </para>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Rebuild a single index:

<programlisting>
REINDEX INDEX my_index;
</programlisting>
  </para>

  <para>
   Rebuild all the indexes on the table <literal>my_table</literal>:

<programlisting>
REINDEX TABLE my_table;
</programlisting>
  </para>

  <para>
   Rebuild all indexes in a particular database, without trusting the
   system indexes to be valid already:

<programlisting>
$ <userinput>export PGOPTIONS="-P"</userinput>
$ <userinput>psql broken_db</userinput>
...
broken_db=&gt; REINDEX DATABASE broken_db;
broken_db=&gt; \q
</programlisting></para>

  <para>
   Rebuild indexes for a table, without blocking read and write operations
   on involved relations while reindexing is in progress:

<programlisting>
REINDEX TABLE CONCURRENTLY my_broken_table;
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>REINDEX</command> command in the SQL standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-createindex"/></member>
   <member><xref linkend="sql-dropindex"/></member>
   <member><xref linkend="app-reindexdb"/></member>
  </simplelist>
 </refsect1>
</refentry>
