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299 lines
6.3 KiB
Plaintext
299 lines
6.3 KiB
Plaintext
<sect1 id="hstore">
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<title>hstore</title>
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<indexterm zone="hstore">
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<primary>hstore</primary>
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</indexterm>
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<para>
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The <literal>hstore</literal> module is usefull for storing (key,value) pairs.
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This module can be useful in different scenarios: case with many attributes
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rarely searched, semistructural data or a lazy DBA.
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</para>
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<sect2>
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<title>Operations</title>
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<itemizedlist>
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<listitem>
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<para>
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<literal>hstore -> text</literal> - get value , perl analogy $h{key}
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</para>
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<programlisting>
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select 'a=>q, b=>g'->'a';
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?
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------
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q
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</programlisting>
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<para>
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Note the use of parenthesis in the select below, because priority of 'is' is
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higher than that of '->':
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</para>
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<programlisting>
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SELECT id FROM entrants WHERE (info->'education_period') IS NOT NULL;
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>hstore || hstore</literal> - concatenation, perl analogy %a=( %b, %c );
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</para>
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<programlisting>
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regression=# select 'a=>b'::hstore || 'c=>d'::hstore;
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?column?
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--------------------
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"a"=>"b", "c"=>"d"
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(1 row)
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</programlisting>
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<para>
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but, notice
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</para>
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<programlisting>
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regression=# select 'a=>b'::hstore || 'a=>d'::hstore;
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?column?
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----------
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"a"=>"d"
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(1 row)
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>text => text</literal> - creates hstore type from two text strings
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</para>
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<programlisting>
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select 'a'=>'b';
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?column?
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----------
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"a"=>"b"
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>hstore @> hstore</literal> - contains operation, check if left operand contains right.
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</para>
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<programlisting>
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regression=# select 'a=>b, b=>1, c=>NULL'::hstore @> 'a=>c';
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?column?
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----------
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f
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(1 row)
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regression=# select 'a=>b, b=>1, c=>NULL'::hstore @> 'b=>1';
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?column?
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----------
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t
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(1 row)
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>hstore <@ hstore</literal> - contained operation, check if
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left operand is contained in right
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</para>
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<para>
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(Before PostgreSQL 8.2, the containment operators @> and <@ were
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respectively called @ and ~. These names are still available, but are
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deprecated and will eventually be retired. Notice that the old names
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are reversed from the convention formerly followed by the core geometric
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datatypes!)
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</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2>
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<title>Functions</title>
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<itemizedlist>
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<listitem>
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<para>
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<literal>akeys(hstore)</literal> - returns all keys from hstore as array
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</para>
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<programlisting>
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regression=# select akeys('a=>1,b=>2');
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akeys
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-------
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{a,b}
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>skeys(hstore)</literal> - returns all keys from hstore as strings
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</para>
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<programlisting>
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regression=# select skeys('a=>1,b=>2');
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skeys
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-------
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a
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b
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>avals(hstore)</literal> - returns all values from hstore as array
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</para>
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<programlisting>
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regression=# select avals('a=>1,b=>2');
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avals
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-------
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{1,2}
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>svals(hstore)</literal> - returns all values from hstore as
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strings
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</para>
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<programlisting>
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regression=# select svals('a=>1,b=>2');
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svals
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-------
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1
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2
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>delete (hstore,text)</literal> - delete (key,value) from hstore if
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key matches argument.
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</para>
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<programlisting>
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regression=# select delete('a=>1,b=>2','b');
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delete
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----------
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"a"=>"1"
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>each(hstore)</literal> - return (key, value) pairs
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</para>
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<programlisting>
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regression=# select * from each('a=>1,b=>2');
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key | value
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-----+-------
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a | 1
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b | 2
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>exist (hstore,text)</literal>
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</para>
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<para>
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<literal>hstore ? text</literal> - returns 'true if key is exists in hstore
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and false otherwise.
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</para>
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<programlisting>
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regression=# select exist('a=>1','a'), 'a=>1' ? 'a';
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exist | ?column?
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-------+----------
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t | t
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</programlisting>
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</listitem>
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<listitem>
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<para>
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<literal>defined (hstore,text)</literal> - returns true if key is exists in
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hstore and its value is not NULL.
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</para>
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<programlisting>
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regression=# select defined('a=>NULL','a');
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defined
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---------
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f
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</programlisting>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2>
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<title>Indices</title>
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<para>
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Module provides index support for '@>' and '?' operations.
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</para>
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<programlisting>
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CREATE INDEX hidx ON testhstore USING GIST(h);
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CREATE INDEX hidx ON testhstore USING GIN(h);
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</programlisting>
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</sect2>
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<sect2>
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<title>Examples</title>
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<para>
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Add a key:
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</para>
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<programlisting>
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UPDATE tt SET h=h||'c=>3';
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</programlisting>
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<para>
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Delete a key:
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</para>
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<programlisting>
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UPDATE tt SET h=delete(h,'k1');
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</programlisting>
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</sect2>
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<sect2>
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<title>Statistics</title>
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<para>
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hstore type, because of its intrinsic liberality, could contain a lot of
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different keys. Checking for valid keys is the task of application.
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Examples below demonstrate several techniques how to check keys statistics.
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</para>
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<para>
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Simple example
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</para>
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<programlisting>
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SELECT * FROM each('aaa=>bq, b=>NULL, ""=>1 ');
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</programlisting>
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<para>
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Using table
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</para>
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<programlisting>
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SELECT (each(h)).key, (each(h)).value INTO stat FROM testhstore ;
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</programlisting>
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<para>Online stat</para>
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<programlisting>
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SELECT key, count(*) FROM (SELECT (each(h)).key FROM testhstore) AS stat GROUP BY key ORDER BY count DESC, key;
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key | count
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-----------+-------
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line | 883
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query | 207
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pos | 203
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node | 202
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space | 197
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status | 195
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public | 194
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title | 190
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org | 189
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...................
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</programlisting>
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</sect2>
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<sect2>
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<title>Authors</title>
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<para>
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Oleg Bartunov <email>oleg@sai.msu.su</email>, Moscow, Moscow University, Russia
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</para>
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<para>
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Teodor Sigaev <email>teodor@sigaev.ru</email>, Moscow, Delta-Soft Ltd.,Russia
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</para>
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</sect2>
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</sect1>
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