'"col3"=>"baz"' AS r FROM test t) s;
 col1 | col2 | col3
------+------+------
  123 | foo  | baz
(1 row)
</programlisting>
  </para>
 </sect2>

 <sect2 id="hstore-statistics">
  <title>Statistics</title>

  <para>
   The <type>hstore</type> type, because of its intrinsic liberality, could
   contain a lot of different keys. Checking for valid keys is the task of the
   application. The following examples demonstrate several techniques for
   checking keys and obtaining statistics.
  </para>

  <para>
   Simple example:
<programlisting>
SELECT * FROM each('aaa=&gt;bq, b=&gt;NULL, ""=&gt;1');
</programlisting>
  </para>

  <para>
   Using a table:
<programlisting>
CREATE TABLE stat AS SELECT (each(h)).key, (each(h)).value FROM testhstore;
</programlisting>
  </para>

  <para>
   Online statistics:
<programlisting>
SELECT key, count(*) FROM
  (SELECT (each(h)).key FROM testhstore) AS stat
  GROUP BY key
  ORDER BY count DESC, key;
    key    | count
-----------+-------
 line      |   883
 query     |   207
 pos       |   203
 node      |   202
 space     |   197
 status    |   195
 public    |   194
 title     |   190
 org       |   189
...................
</programlisting>
  </para>
 </sect2>

 <sect2 id="hstore-compatibility">
  <title>Compatibility</title>

  <para>
   As of PostgreSQL 9.0, <type>hstore</type> uses a different internal
   representation than previous versions. This presents no obstacle for
   dump/restore upgrades since the text representation (used in the dump) is
   unchanged.
  </para>

  <para>
   In the event of a binary upgrade, upward compatibility is maintained by
   having the new code recognize old-format data. This will entail a slight
   performance penalty when processing data that has not yet been modified by
   the new code. It is possible to force an upgrade of all values in a table
   column by doing an <literal>UPDATE</literal> statement as follows:
<programlisting>
UPDATE tablename SET hstorecol = hstorecol || '';
</programlisting>
  </para>

  <para>
   Another way to do it is:
<programlisting>
ALTER TABLE tablename ALTER hstorecol TYPE hstore USING hstorecol || '';
</programlisting>
   The <command>ALTER TABLE</command> method requires an
   <literal>ACCESS EXCLUSIVE</literal> lock on the table,
   but does not result in bloating the table with old row versions.
  </para>

 </sect2>

 <sect2 id="hstore-transforms">
  <title>Transforms</title>

  <para>
   Additional extensions are available that implement transforms for
   the <type>hstore</type> type for the languages PL/Perl and PL/Python.  The
   extensions for PL/Perl are called <literal>hstore_plperl</literal>
   and <literal>hstore_plperlu</literal>, for trusted and untrusted PL/Perl.
   If you install these transforms and specify them when creating a
   function, <type>hstore</type> values are mapped to Perl hashes.  The
   extension for PL/Python is called <literal>hstore_plpython3u</literal>.
   If you use it, <type>hstore</type> values are mapped to Python dictionaries.
  </para>
 </sect2>

 <sect2 id="hstore-authors">
  <title>Authors</title>

  <para>
   Oleg Bartunov <email>oleg@sai.msu.su</email>, Moscow, Moscow University, Russia
  </para>

  <para>
   Teodor Sigaev <email>teodor@sigaev.ru</email>, Moscow, Delta-Soft Ltd., Russia
  </para>

  <para>
   Additional enhancements by Andrew Gierth <email>andrew@tao11.riddles.org.uk</email>,
   United Kingdom
  </para>
 </sect2>

</sect1>