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191 lines
4.1 KiB
Plaintext
191 lines
4.1 KiB
Plaintext
Hstore - contrib module for storing (key,value) pairs
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[Online version] (http://www.sai.msu.su/~megera/oddmuse/index.cgi?Hstore)
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Motivation
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Many attributes rarely searched, semistructural data, lazy DBA
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Authors
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* Oleg Bartunov <oleg@sai.msu.su>, Moscow, Moscow University, Russia
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* Teodor Sigaev <teodor@sigaev.ru>, Moscow, Delta-Soft Ltd.,Russia
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LEGAL NOTICES: This module is released under BSD license (as PostgreSQL
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itself)
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Operations
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* hstore -> text - get value , perl analogy $h{key}
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select 'a=>q, b=>g'->'a';
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?
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------
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q
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* hstore || hstore - concatenation, perl analogy %a=( %b, %c );
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regression=# select 'a=>b'::hstore || 'c=>d'::hstore;
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?column?
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--------------------
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"a"=>"b", "c"=>"d"
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(1 row)
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but, notice
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regression=# select 'a=>b'::hstore || 'a=>d'::hstore;
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?column?
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----------
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"a"=>"d"
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(1 row)
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* text => text - creates hstore type from two text strings
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select 'a'=>'b';
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?column?
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----------
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"a"=>"b"
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* hstore @> hstore - contains operation, check if left operand contains right.
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regression=# select 'a=>b, b=>1, c=>NULL'::hstore @> 'a=>c';
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?column?
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----------
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f
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(1 row)
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regression=# select 'a=>b, b=>1, c=>NULL'::hstore @> 'b=>1';
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?column?
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----------
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t
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(1 row)
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* hstore <@ hstore - contained operation, check if left operand is contained
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in right
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(Before PostgreSQL 8.2, the containment operators @> and <@ were
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respectively called @ and ~. These names are still available, but are
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deprecated and will eventually be retired. Notice that the old names
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are reversed from the convention formerly followed by the core geometric
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datatypes!)
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Functions
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* akeys(hstore) - returns all keys from hstore as array
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regression=# select akeys('a=>1,b=>2');
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akeys
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-------
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{a,b}
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* skeys(hstore) - returns all keys from hstore as strings
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regression=# select skeys('a=>1,b=>2');
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skeys
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-------
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a
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b
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* avals(hstore) - returns all values from hstore as array
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regression=# select avals('a=>1,b=>2');
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avals
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-------
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{1,2}
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* svals(hstore) - returns all values from hstore as strings
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regression=# select svals('a=>1,b=>2');
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svals
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-------
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1
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2
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* delete (hstore,text) - delete (key,value) from hstore if key matches
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argument.
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regression=# select delete('a=>1,b=>2','b');
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delete
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----------
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"a"=>"1"
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* each(hstore) return (key, value) pairs
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regression=# select * from each('a=>1,b=>2');
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key | value
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-----+-------
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a | 1
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b | 2
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* exist (hstore,text) - returns 'true if key is exists in hstore and
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false otherwise.
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regression=# select exist('a=>1','a');
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exist
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----------
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t
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* defined (hstore,text) - returns true if key is exists in hstore and
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its value is not NULL.
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regression=# select defined('a=>NULL','a');
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defined
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---------
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f
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Indices
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Module provides index support for '@>' and '<@' operations.
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create index hidx on testhstore using gist(h);
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Note
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Use parenthesis in select below, because priority of 'is' is higher than that of '->'
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select id from entrants where (info->'education_period') is not null;
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Examples
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* add key
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update tt set h=h||'c=>3';
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* delete key
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update tt set h=delete(h,'k1');
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* Statistics
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hstore type, because of its intrinsic liberality, could contain a lot of
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different keys. Checking for valid keys is the task of application.
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Examples below demonstrate several techniques how to check keys statistics.
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o simple example
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select * from each('aaa=>bq, b=>NULL, ""=>1 ');
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o using table
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select (each(h)).key, (each(h)).value into stat from testhstore ;
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o online stat
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select key, count(*) from (select (each(h)).key from testhstore) as stat group by key order by count desc, key;
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key | count
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-----------+-------
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line | 883
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query | 207
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pos | 203
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node | 202
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space | 197
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status | 195
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public | 194
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title | 190
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org | 189
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...................
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In the current implementation, neither the key nor the value
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string can exceed 65535 bytes in length; an error will be thrown if this
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limit is exceeded. These maximum lengths may change in future releases.
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