postgresql/contrib/hstore/README.hstore

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

[Online version] (http://www.sai.msu.su/~megera/oddmuse/index.cgi?Hstore)

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

LEGAL NOTICES: This module is released under BSD license (as PostgreSQL
itself)

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

(Before PostgreSQL 8.2, the containment operators @> and <@ were
respectively called @ and ~.  These names are still available, but are
deprecated and will eventually be retired.  Notice that the old names
are reversed from the convention formerly followed by the core geometric
datatypes!)

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

regression=# select exist('a=>1','a'), 'a=>1' ? 'a';
 exist | ?column? 
-------+----------
 t     | 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);
create index hidx on testhstore using gin(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.

          o simple example 

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

          o using table 

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

          o 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
...................