postgresql/contrib/hstore/README.hstore

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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
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
* 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
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