Hstore - contrib module for storing (key,value) pairs

Online version

Hstore is a part of PostgreSQL distribution since 8.2 version !
Changes: functions ''isexists'' and ''isdefined'' were renamed to ''exist'' and
''defined''. Users of old versions encouraged to upgrade.

Motivation

Many attributes rarely searched, semistructural data, lazy DBA

Authors

  * Oleg Bartunov <oleg@sai.msu.su>, Moscow, Moscow University, Russia
  * Teodor Sigaev <teodor@sigaev.ru>, Moscow, Delta-Soft Ltd.,Russia

License

Stable version, included into PostgreSQL distribution, released under BSD
license. Development version, available from this site, released under the GNU
General Public License, version 2 (June 1991).

Operations

  * hstore ? text - get value , perl analogy $h{key}

select 'a=>q, b=>g'->'a';
  ?
------
  q

  * hstore || hstore - concatenation, perl analogy %a=( %b, %c );

regression=# select 'a=>b'::hstore || 'c=>d'::hstore;
      ?column?
--------------------
 "a"=>"b", "c"=>"d"
(1 row)

but, notice

regression=# select 'a=>b'::hstore || 'a=>d'::hstore;
 ?column?
----------
 "a"=>"d"
(1 row)

  * text => text - creates hstore type from two text strings

select 'a'=>'b';
  ?column?
----------
  "a"=>"b"

  * hstore @ hstore - contains operation, check if left operand contains right.

regression=# select 'a=>b, b=>1, c=>NULL'::hstore @ 'a=>c';
 ?column?
----------
 f
(1 row)

regression=# select 'a=>b, b=>1, c=>NULL'::hstore @ 'b=>1';
 ?column?
----------
 t
(1 row)

  * hstore ~ hstore - contained operation, check if left operand is contained
    in right

Functions

  * akeys(hstore) - returns all keys from hstore as array

regression=# select akeys('a=>1,b=>2');
 akeys
-------
 {a,b}

  * skeys(hstore) - returns all keys from hstore as strings

regression=# select skeys('a=>1,b=>2');
 skeys
-------
 a
 b

  * avals(hstore) - returns all values from hstore as array

regression=# select avals('a=>1,b=>2');
 avals
-------
 {1,2}

  * svals(hstore) - returns all values from hstore as strings

regression=# select svals('a=>1,b=>2');
 svals
-------
 1
 2

  * delete (hstore,text) - delete (key,value) from hstore if key matches
    argument.

regression=# select delete('a=>1,b=>2','b');
  delete
----------
 "a"=>"1"

  * each(hstore) return (key, value) pairs

regression=# select * from each('a=>1,b=>2');
 key | value
-----+-------
 a   | 1
 b   | 2

  * exist(hstore,text) - returns 'true if key is exists in hstore and false
    otherwise.

regression=# select exist('a=>1','a');
 exists
----------
 t

  * defined (hstore,text) - returns true if key is exists in hstore and its
    value is not NULL.

regression=# select defined('a=>NULL','a');
 defined
-----------
 f

Indices

Module provides index support for '@' and '~' operations.

create index hidx on testhstore using gist(h);

Note

Use parenthesis in select below, because priority of 'is' is higher than that
of '->'

select id from entrants where (info->'education_period') is not null;

Examples

  * add key

update tt set h=h||'c=>3';

  * delete key

update tt set h=delete(h,'k1');

  * Statistics

hstore type, because of its intrinsic liberality, could contain a lot of
different keys. Checking for valid keys is the task of application. Examples
below demonstrate several techniques how to check keys statistics.

      + simple example

select * from each('aaa=>bq, b=>NULL, ""=>1 ');

      + using table

select (each(h)).key, (each(h)).value into stat from testhstore ;

      + online stat

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
...................