<!-- doc/src/sgml/pgprewarm.sgml -->

<sect1 id="pgprewarm" xreflabel="pg_prewarm">
 <title>pg_prewarm</title>

 <indexterm zone="pgprewarm">
  <primary>pg_prewarm</primary>
 </indexterm>

 <para>
  The <filename>pg_prewarm</filename> module provides a convenient way
  to load relation data into either the operating system buffer cache
  or the <productname>PostgreSQL</productname> buffer cache.
 </para>

 <sect2>
  <title>Functions</title>

<synopsis>
pg_prewarm(regclass, mode text default 'buffer', fork text default 'main',
           first_block int8 default null,
           last_block int8 default null) RETURNS int8
</synopsis>

  <para>
   The first argument is the relation to be prewarmed.  The second argument
   is the prewarming method to be used, as further discussed below; the third
   is the relation fork to be prewarmed, usually <literal>main</literal>.
   The fourth argument is the first block number to prewarm
   (<literal>NULL</literal> is accepted as a synonym for zero).  The fifth
   argument is the last block number to prewarm (<literal>NULL</literal>
   means prewarm through the last block in the relation).  The return value
   is the number of blocks prewarmed.
  </para>

  <para>
   There are three available prewarming methods.  <literal>prefetch</literal>
   issues asynchronous prefetch requests to the operating system, if this is
   supported, or throws an error otherwise.  <literal>read</literal> reads
   the requested range of blocks; unlike <literal>prefetch</literal>, this is
   synchronous and supported on all platforms and builds, but may be slower.
   <literal>buffer</literal> reads the requested range of blocks into the
   database buffer cache.
  </para>

  <para>
   Note that with any of these methods, attempting to prewarm more blocks than
   can be cached &mdash; by the OS when using <literal>prefetch</literal> or
   <literal>read</literal>, or by <productname>PostgreSQL</productname> when
   using <literal>buffer</literal> &mdash; will likely result in lower-numbered
   blocks being evicted as higher numbered blocks are read in.  Prewarmed data
   also enjoys no special protection from cache evictions, so it is possible
   that other system activity may evict the newly prewarmed blocks shortly
   after they are read; conversely, prewarming may also evict other data from
   cache. For these reasons, prewarming is typically most useful at startup,
   when caches are largely empty.
  </para>
 </sect2>

 <sect2>
  <title>Author</title>

  <para>
   Robert Haas <email>rhaas@postgresql.org</email>
  </para>
 </sect2>

</sect1>
