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

<sect1 id="hstore" xreflabel="hstore">
 <title>hstore</title>

 <indexterm zone="hstore">
  <primary>hstore</primary>
 </indexterm>

 <para>
  This module implements the <type>hstore</type> data type for storing sets of
  key/value pairs within a single <productname>PostgreSQL</productname> value.
  This can be useful in various scenarios, such as rows with many attributes
  that are rarely examined, or semi-structured data.  Keys and values are
  simply text strings.
 </para>

 <para>
  This module is considered <quote>trusted</quote>, that is, it can be
  installed by non-superusers who have <literal>CREATE</literal> privilege
  on the current database.
 </para>

 <sect2>
  <title><type>hstore</type> External Representation</title>

  <para>

   The text representation of an <type>hstore</type>, used for input and output,
   includes zero or more <replaceable>key</replaceable> <literal>=&gt;</literal>
   <replaceable>value</replaceable> pairs separated by commas. Some examples:

<synopsis>
k =&gt; v
foo =&gt; bar, baz =&gt; whatever
"1-a" =&gt; "anything at all"
</synopsis>

   The order of the pairs is not significant (and may not be reproduced on
   output). Whitespace between pairs or around the <literal>=&gt;</literal> sign is
   ignored. Double-quote keys and values that include whitespace, commas,
   <literal>=</literal>s or <literal>&gt;</literal>s. To include a double quote or a
   backslash in a key or value, escape it with a backslash.
  </para>

  <para>
   Each key in an <type>hstore</type> is unique. If you declare an <type>hstore</type>
   with duplicate keys, only one will be stored in the <type>hstore</type> and
   there is no guarantee as to which will be kept:

<programlisting>
SELECT 'a=&gt;1,a=&gt;2'::hstore;
  hstore
----------
 "a"=&gt;"1"
</programlisting>
  </para>

  <para>
   A value (but not a key) can be an SQL <literal>NULL</literal>. For example:

<programlisting>
key =&gt; NULL
</programlisting>

   The <literal>NULL</literal> keyword is case-insensitive. Double-quote the
   <literal>NULL</literal> to treat it as the ordinary string <quote>NULL</quote>.
  </para>

  <note>
  <para>
   Keep in mind that the <type>hstore</type> text format, when used for input,
   applies <emphasis>before</emphasis> any required quoting or escaping. If you are
   passing an <type>hstore</type> literal via a parameter, then no additional
   processing is needed. But if you're passing it as a quoted literal
   constant, then any single-quote characters and (depending on the setting of
   the <varname>standard_conforming_strings</varname> configuration parameter)
   backslash characters need to be escaped correctly. See
   <xref linkend="sql-syntax-strings"/> for more on the handling of string
   constants.
  </para>
  </note>

  <para>
   On output, double quotes always surround keys and values, even when it's
   not strictly necessary.
  </para>

 </sect2>

 <sect2>
  <title><type>hstore</type> Operators and Functions</title>

  <para>
   The operators provided by the <literal>hstore</literal> module are
   shown in <xref linkend="hstore-op-table"/>, the functions
   in <xref linkend="hstore-func-table"/>.
  </para>

  <table id="hstore-op-table">
   <title><type>hstore</type> Operators</title>

   <tgroup cols="4">
    <thead>
     <row>
      <entry>Operator</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><type>hstore</type> <literal>-&gt;</literal> <type>text</type></entry>
      <entry>get value for key (<literal>NULL</literal> if not present)</entry>
      <entry><literal>'a=&gt;x, b=&gt;y'::hstore -&gt; 'a'</literal></entry>
      <entry><literal>x</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>-&gt;</literal> <type>text[]</type></entry>
      <entry>get values for keys (<literal>NULL</literal> if not present)</entry>
      <entry><literal>'a=&gt;x, b=&gt;y, c=&gt;z'::hstore -&gt; ARRAY['c','a']</literal></entry>
      <entry><literal>{"z","x"}</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>||</literal> <type>hstore</type></entry>
      <entry>concatenate <type>hstore</type>s</entry>
      <entry><literal>'a=&gt;b, c=&gt;d'::hstore || 'c=&gt;x, d=&gt;q'::hstore</literal></entry>
      <entry><literal>"a"=&gt;"b", "c"=&gt;"x", "d"=&gt;"q"</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>?</literal> <type>text</type></entry>
      <entry>does <type>hstore</type> contain key?</entry>
      <entry><literal>'a=&gt;1'::hstore ? 'a'</literal></entry>
      <entry><literal>t</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>?&amp;</literal> <type>text[]</type></entry>
      <entry>does <type>hstore</type> contain all specified keys?</entry>
      <entry><literal>'a=&gt;1,b=&gt;2'::hstore ?&amp; ARRAY['a','b']</literal></entry>
      <entry><literal>t</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>?|</literal> <type>text[]</type></entry>
      <entry>does <type>hstore</type> contain any of the specified keys?</entry>
      <entry><literal>'a=&gt;1,b=&gt;2'::hstore ?| ARRAY['b','c']</literal></entry>
      <entry><literal>t</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>@&gt;</literal> <type>hstore</type></entry>
      <entry>does left operand contain right?</entry>
      <entry><literal>'a=&gt;b, b=&gt;1, c=&gt;NULL'::hstore @&gt; 'b=&gt;1'</literal></entry>
      <entry><literal>t</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>&lt;@</literal> <type>hstore</type></entry>
      <entry>is left operand contained in right?</entry>
      <entry><literal>'a=&gt;c'::hstore &lt;@ 'a=&gt;b, b=&gt;1, c=&gt;NULL'</literal></entry>
      <entry><literal>f</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>-</literal> <type>text</type></entry>
      <entry>delete key from left operand</entry>
      <entry><literal>'a=&gt;1, b=&gt;2, c=&gt;3'::hstore - 'b'::text</literal></entry>
      <entry><literal>"a"=&gt;"1", "c"=&gt;"3"</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>-</literal> <type>text[]</type></entry>
      <entry>delete keys from left operand</entry>
      <entry><literal>'a=&gt;1, b=&gt;2, c=&gt;3'::hstore - ARRAY['a','b']</literal></entry>
      <entry><literal>"c"=&gt;"3"</literal></entry>
     </row>

     <row>
      <entry><type>hstore</type> <literal>-</literal> <type>hstore</type></entry>
      <entry>delete matching pairs from left operand</entry>
      <entry><literal>'a=&gt;1, b=&gt;2, c=&gt;3'::hstore - 'a=&gt;4, b=&gt;2'::hstore</literal></entry>
      <entry><literal>"a"=&gt;"1", "c"=&gt;"3"</literal></entry>
     </row>

     <row>
      <entry><type>record</type> <literal>#=</literal> <type>hstore</type></entry>
      <entry>replace fields in <type>record</type> with matching values from <type>hstore</type></entry>
      <entry>see Examples section</entry>
      <entry></entry>
     </row>

     <row>
      <entry><literal>%%</literal> <type>hstore</type></entry>
      <entry>convert <type>hstore</type> to array of alternating keys and values</entry>
      <entry><literal>%% 'a=&gt;foo, b=&gt;bar'::hstore</literal></entry>
      <entry><literal>{a,foo,b,bar}</literal></entry>
     </row>

     <row>
      <entry><literal>%#</literal> <type>hstore</type></entry>
      <entry>convert <type>hstore</type> to two-dimensional key/value array</entry>
      <entry><literal>%# 'a=&gt;foo, b=&gt;bar'::hstore</literal></entry>
      <entry><literal>{{a,foo},{b,bar}}</literal></entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <note>
  <para>
   Prior to PostgreSQL 8.2, the containment operators <literal>@&gt;</literal>
   and <literal>&lt;@</literal> were called <literal>@</literal> and <literal>~</literal>,
   respectively. These names are still available, but are deprecated and will
   eventually be removed. Notice that the old names are reversed from the
   convention formerly followed by the core geometric data types!
   </para>
  </note>

  <table id="hstore-func-table">
   <title><type>hstore</type> Functions</title>

   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry><function>hstore(record)</function><indexterm><primary>hstore</primary></indexterm></entry>
      <entry><type>hstore</type></entry>
      <entry>construct an <type>hstore</type> from a record or row</entry>
      <entry><literal>hstore(ROW(1,2))</literal></entry>
      <entry><literal>f1=&gt;1,f2=&gt;2</literal></entry>
     </row>

     <row>
      <entry><function>hstore(text[])</function></entry>
      <entry><type>hstore</type></entry>
      <entry>construct an <type>hstore</type> from an array, which may be either
       a key/value array, or a two-dimensional array</entry>
      <entry><literal>hstore(ARRAY['a','1','b','2']) || hstore(ARRAY[['c','3'],['d','4']])</literal></entry>
      <entry><literal>a=&gt;1, b=&gt;2, c=&gt;3, d=&gt;4</literal></entry>
     </row>

     <row>
      <entry><function>hstore(text[], text[])</function></entry>
      <entry><type>hstore</type></entry>
      <entry>construct an <type>hstore</type> from separate key and value arrays</entry>
      <entry><literal>hstore(ARRAY['a','b'], ARRAY['1','2'])</literal></entry>
      <entry><literal>"a"=&gt;"1","b"=&gt;"2"</literal></entry>
     </row>

     <row>
      <entry><function>hstore(text, text)</function></entry>
      <entry><type>hstore</type></entry>
      <entry>make single-item <type>hstore</type></entry>
      <entry><literal>hstore('a', 'b')</literal></entry>
      <entry><literal>"a"=&gt;"b"</literal></entry>
     </row>

     <row>
      <entry><function>akeys(hstore)</function><indexterm><primary>akeys</primary></indexterm></entry>
      <entry><type>text[]</type></entry>
      <entry>get <type>hstore</type>'s keys as an array</entry>
      <entry><literal>akeys('a=&gt;1,b=&gt;2')</literal></entry>
      <entry><literal>{a,b}</literal></entry>
     </row>

     <row>
      <entry><function>skeys(hstore)</function><indexterm><primary>skeys</primary></indexterm></entry>
      <entry><type>setof text</type></entry>
      <entry>get <type>hstore</type>'s keys as a set</entry>
      <entry><literal>skeys('a=&gt;1,b=&gt;2')</literal></entry>
      <entry>
<programlisting>
a
b
</programlisting></entry>
     </row>

     <row>
      <entry><function>avals(hstore)</function><indexterm><primary>avals</primary></indexterm></entry>
      <entry><type>text[]</type></entry>
      <entry>get <type>hstore</type>'s values as an array</entry>
      <entry><literal>avals('a=&gt;1,b=&gt;2')</literal></entry>
      <entry><literal>{1,2}</literal></entry>
     </row>

     <row>
      <entry><function>svals(hstore)</function><indexterm><primary>svals</primary></indexterm></entry>
      <entry><type>setof text</type></entry>
      <entry>get <type>hstore</type>'s values as a set</entry>
      <entry><literal>svals('a=&gt;1,b=&gt;2')</literal></entry>
      <entry>
<programlisting>
1
2
</programlisting></entry>
     </row>

     <row>
      <entry><function>hstore_to_array(hstore)</function><indexterm><primary>hstore_to_array</primary></indexterm></entry>
      <entry><type>text[]</type></entry>
      <entry>get <type>hstore</type>'s keys and values as an array of alternating
       keys and values</entry>
      <entry><literal>hstore_to_array('a=&gt;1,b=&gt;2')</literal></entry>
      <entry><literal>{a,1,b,2}</literal></entry>
     </row>

     <row>
      <entry><function>hstore_to_matrix(hstore)</function><indexterm><primary>hstore_to_matrix</primary></indexterm></entry>
      <entry><type>text[]</type></entry>
      <entry>get <type>hstore</type>'s keys and values as a two-dimensional array</entry>
      <entry><literal>hstore_to_matrix('a=&gt;1,b=&gt;2')</literal></entry>
      <entry><literal>{{a,1},{b,2}}</literal></entry>
     </row>

     <row>
      <entry><function>hstore_to_json(hstore)</function><indexterm><primary>hstore_to_json</primary></indexterm></entry>
      <entry><type>json</type></entry>
      <entry>get <type>hstore</type> as a <type>json</type> value, converting
       all non-null values to JSON strings</entry>
      <entry><literal>hstore_to_json('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
      <entry><literal>{"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}</literal></entry>
     </row>

     <row>
      <entry><function>hstore_to_jsonb(hstore)</function><indexterm><primary>hstore_to_jsonb</primary></indexterm></entry>
      <entry><type>jsonb</type></entry>
      <entry>get <type>hstore</type> as a <type>jsonb</type> value, converting
       all non-null values to JSON strings</entry>
      <entry><literal>hstore_to_jsonb('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
      <entry><literal>{"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}</literal></entry>
     </row>

     <row>
      <entry><function>hstore_to_json_loose(hstore)</function><indexterm><primary>hstore_to_json_loose</primary></indexterm></entry>
      <entry><type>json</type></entry>
      <entry>get <type>hstore</type> as a <type>json</type> value, but attempt to distinguish numerical and Boolean values so they are unquoted in the JSON</entry>
      <entry><literal>hstore_to_json_loose('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
      <entry><literal>{"a key": 1, "b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4}</literal></entry>
     </row>

     <row>
      <entry><function>hstore_to_jsonb_loose(hstore)</function><indexterm><primary>hstore_to_jsonb_loose</primary></indexterm></entry>
      <entry><type>jsonb</type></entry>
      <entry>get <type>hstore</type> as a <type>jsonb</type> value, but attempt to distinguish numerical and Boolean values so they are unquoted in the JSON</entry>
      <entry><literal>hstore_to_jsonb_loose('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
      <entry><literal>{"a key": 1, "b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4}</literal></entry>
     </row>

     <row>
      <entry><function>slice(hstore, text[])</function><indexterm><primary>slice</primary></indexterm></entry>
      <entry><type>hstore</type></entry>
      <entry>extract a subset of an <type>hstore</type></entry>
      <entry><literal>slice('a=&gt;1,b=&gt;2,c=&gt;3'::hstore, ARRAY['b','c','x'])</literal></entry>
      <entry><literal>"b"=&gt;"2", "c"=&gt;"3"</literal></entry>
     </row>

     <row>
      <entry><function>each(hstore)</function><indexterm><primary>each</primary></indexterm></entry>
      <entry><type>setof(key text, value text)</type></entry>
      <entry>get <type>hstore</type>'s keys and values as a set</entry>
      <entry><literal>select * from each('a=&gt;1,b=&gt;2')</literal></entry>
      <entry>
<programlisting>
 key | value
-----+-------
 a   | 1
 b   | 2
</programlisting></entry>
     </row>

     <row>
      <entry><function>exist(hstore,text)</function><indexterm><primary>exist</primary></indexterm></entry>
      <entry><type>boolean</type></entry>
      <entry>does <type>hstore</type> contain key?</entry>
      <entry><literal>exist('a=&gt;1','a')</literal></entry>
      <entry><literal>t</literal></entry>
     </row>

     <row>
      <entry><function>defined(hstore,text)</function><indexterm><primary>defined</primary></indexterm></entry>
      <entry><type>boolean</type></entry>
      <entry>does <type>hstore</type> contain non-<literal>NULL</literal> value for key?</entry>
      <entry><literal>defined('a=&gt;NULL','a')</literal></entry>
      <entry><literal>f</literal></entry>
     </row>

     <row>
      <entry><function>delete(hstore,text)</function><indexterm><primary>delete</primary></indexterm></entry>
      <entry><type>hstore</type></entry>
      <entry>delete pair with matching key</entry>
      <entry><literal>delete('a=&gt;1,b=&gt;2','b')</literal></entry>
      <entry><literal>"a"=&gt;"1"</literal></entry>
     </row>

     <row>
      <entry><function>delete(hstore,text[])</function></entry>
      <entry><type>hstore</type></entry>
      <entry>delete pairs with matching keys</entry>
      <entry><literal>delete('a=&gt;1,b=&gt;2,c=&gt;3',ARRAY['a','b'])</literal></entry>
      <entry><literal>"c"=&gt;"3"</literal></entry>
     </row>

     <row>
      <entry><function>delete(hstore,hstore)</function></entry>
      <entry><type>hstore</type></entry>
      <entry>delete pairs matching those in the second argument</entry>
      <entry><literal>delete('a=&gt;1,b=&gt;2','a=&gt;4,b=&gt;2'::hstore)</literal></entry>
      <entry><literal>"a"=&gt;"1"</literal></entry>
     </row>

     <row>
      <entry><function>populate_record(record,hstore)</function><indexterm><primary>populate_record</primary></indexterm></entry>
      <entry><type>record</type></entry>
      <entry>replace fields in <type>record</type> with matching values from <type>hstore</type></entry>
      <entry>see Examples section</entry>
      <entry></entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <note>
   <para>
     The function <function>hstore_to_json</function> is used when
     an <type>hstore</type> value is cast to <type>json</type>.
     Likewise, <function>hstore_to_jsonb</function> is used when
     an <type>hstore</type> value is cast to <type>jsonb</type>.
   </para>
  </note>

  <note>
   <para>
    The function <function>populate_record</function> is actually declared
    with <type>anyelement</type>, not <type>record</type>, as its first argument,
    but it will reject non-record types with a run-time error.
   </para>
  </note>
 </sect2>

 <sect2>
  <title>Indexes</title>

  <para>
   <type>hstore</type> has GiST and GIN index support for the <literal>@&gt;</literal>,
   <literal>?</literal>, <literal>?&amp;</literal> and <literal>?|</literal> operators. For example:
  </para>
<programlisting>
CREATE INDEX hidx ON testhstore USING GIST (h);

CREATE INDEX hidx ON testhstore USING GIN (h);
</programlisting>

  <para>
   <literal>gist_hstore_ops</literal> GiST opclass approximates a set of
   key/value pairs as a bitmap signature.  Its optional integer parameter
   <literal>siglen</literal> determines the
   signature length in bytes.  The default length is 16 bytes.
   Valid values of signature length are between 1 and 2024 bytes.  Longer
   signatures lead to a more precise search (scanning a smaller fraction of the index and
   fewer heap pages), at the cost of a larger index.
  </para>

  <para>
   Example of creating such an index with a signature length of 32 bytes:
  </para>
<programlisting>
  CREATE INDEX hidx ON testhstore USING GIST (h gist_hstore_ops(siglen=32));
</programlisting>

  <para>
   <type>hstore</type> also supports <type>btree</type> or <type>hash</type> indexes for
   the <literal>=</literal> operator. This allows <type>hstore</type> columns to be
   declared <literal>UNIQUE</literal>, or to be used in <literal>GROUP BY</literal>,
   <literal>ORDER BY</literal> or <literal>DISTINCT</literal> expressions. The sort ordering
   for <type>hstore</type> values is not particularly useful, but these indexes
   may be useful for equivalence lookups. Create indexes for <literal>=</literal>
   comparisons as follows:
  </para>
<programlisting>
CREATE INDEX hidx ON testhstore USING BTREE (h);

CREATE INDEX hidx ON testhstore USING HASH (h);
</programlisting>
 </sect2>

 <sect2>
  <title>Examples</title>

  <para>
   Add a key, or update an existing key with a new value:
<programlisting>
UPDATE tab SET h = h || hstore('c', '3');
</programlisting>
  </para>

  <para>
   Delete a key:
<programlisting>
UPDATE tab SET h = delete(h, 'k1');
</programlisting>
  </para>

  <para>
   Convert a <type>record</type> to an <type>hstore</type>:
<programlisting>
CREATE TABLE test (col1 integer, col2 text, col3 text);
INSERT INTO test VALUES (123, 'foo', 'bar');

SELECT hstore(t) FROM test AS t;
                   hstore                    
---------------------------------------------
 "col1"=&gt;"123", "col2"=&gt;"foo", "col3"=&gt;"bar"
(1 row)
</programlisting>
  </para>

  <para>
   Convert an <type>hstore</type> to a predefined <type>record</type> type:
<programlisting>
CREATE TABLE test (col1 integer, col2 text, col3 text);

SELECT * FROM populate_record(null::test,
                              '"col1"=&gt;"456", "col2"=&gt;"zzz"');
 col1 | col2 | col3 
------+------+------
  456 | zzz  | 
(1 row)
</programlisting>
  </para>

  <para>
   Modify an existing record using the values from an <type>hstore</type>:
<programlisting>
CREATE TABLE test (col1 integer, col2 text, col3 text);
INSERT INTO test VALUES (123, 'foo', 'bar');

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

 <sect2>
  <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>
SELECT (each(h)).key, (each(h)).value INTO stat 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>
  <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 exclusive lock on the table,
   but does not result in bloating the table with old row versions.
  </para>

 </sect2>

 <sect2>
  <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
   extensions for PL/Python are
   called <literal>hstore_plpythonu</literal>, <literal>hstore_plpython2u</literal>,
   and <literal>hstore_plpython3u</literal>
   (see <xref linkend="plpython-python23"/> for the PL/Python naming
   convention).  If you use them, <type>hstore</type> values are mapped to
   Python dictionaries.
  </para>

  <para>
   Of these additional extensions, <literal>hstore_plperl</literal> is
   considered trusted; the rest are not.
  </para>
 </sect2>

 <sect2>
  <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>
