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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 , Moscow, Moscow University, Russia
* Teodor Sigaev , 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
...................