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

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

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

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

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

regression=# select isdefined('a=>NULL','a');
 isdefined 
-----------
 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.

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