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F.13. hstore

This module implements a data type hstore for storing sets of (key,value) pairs within a single PostgreSQL data field. This can be useful in various scenarios, such as rows with many attributes that are rarely examined, or semi-structured data.

In the current implementation, neither the key nor the value string can exceed 65535 bytes in length; an error will be thrown if this limit is exceeded. These maximum lengths may change in future releases.

F.13.1. hstore External Representation

The text representation of an hstore value includes zero or more key => value items, separated by commas. For example:

    k => v
    foo => bar, baz => whatever
    "1-a" => "anything at all"

The order of the items is not considered significant (and may not be reproduced on output). Whitespace between items or around the => sign is ignored. Use double quotes if a key or value includes whitespace, comma, = or >. To include a double quote or a backslash in a key or value, precede it with another backslash. (Keep in mind that depending on the setting of standard_conforming_strings, you may need to double backslashes in SQL literal strings.)

A value (but not a key) can be a SQL NULL. This is represented as

    key => NULL

The NULL keyword is not case-sensitive. Again, use double quotes if you want the string null to be treated as an ordinary data value.

Currently, double quotes are always used to surround key and value strings on output, even when this is not strictly necessary.

F.13.2. hstore Operators and Functions

Table F-5. hstore Operators

Operator Description Example Result
hstore -> text get value for key (null if not present) 'a=>x, b=>y'::hstore -> 'a' x
text => text make single-item hstore 'a' => 'b' "a"=>"b"
hstore || hstore concatenation 'a=>b, c=>d'::hstore || 'c=>x, d=>q'::hstore "a"=>"b", "c"=>"x", "d"=>"q"
hstore ? text does hstore contain key? 'a=>1'::hstore ? 'a' t
hstore @> hstore does left operand contain right? 'a=>b, b=>1, c=>NULL'::hstore @> 'b=>1' t
hstore <@ hstore is left operand contained in right? 'a=>c'::hstore <@ 'a=>b, b=>1, c=>NULL' f

(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!)

Note: The => operator is deprecated and may be removed in a future release. Use the hstore(text, text) function instead.

Table F-6. hstore Functions

Function Return Type Description Example Result
hstore(text, text) hstore make single-item hstore hstore('a', 'b') "a"=>"b"
akeys(hstore) text[] get hstore's keys as array akeys('a=>1,b=>2') {a,b}
skeys(hstore) setof text get hstore's keys as set skeys('a=>1,b=>2')
avals(hstore) text[] get hstore's values as array avals('a=>1,b=>2') {1,2}
svals(hstore) setof text get hstore's values as set svals('a=>1,b=>2')
each(hstore) setof (key text, value text) get hstore's keys and values as set select * from each('a=>1,b=>2')
 key | value
 a   | 1
 b   | 2
exist(hstore,text) boolean does hstore contain key? exist('a=>1','a') t
defined(hstore,text) boolean does hstore contain non-null value for key? defined('a=>NULL','a') f
delete(hstore,text) hstore delete any item matching key delete('a=>1,b=>2','b') "a"=>"1"

F.13.3. Indexes

hstore has index support for @> and ? operators. You can use either GiST or GIN index types. For example:

CREATE INDEX hidx ON testhstore USING GIST(h);

CREATE INDEX hidx ON testhstore USING GIN(h);

F.13.4. Examples

Add a key, or update an existing key with a new value:

UPDATE tab SET h = h || ('c' => '3');

Delete a key:

UPDATE tab SET h = delete(h, 'k1');

F.13.5. Statistics

The hstore type, because of its intrinsic liberality, could contain a lot of different keys. Checking for valid keys is the task of the application. Examples below demonstrate several techniques for checking keys and obtaining statistics.

Simple example:

SELECT * FROM each('aaa=>bq, b=>NULL, ""=>1');

Using a table:

SELECT (each(h)).key, (each(h)).value INTO stat FROM testhstore;

Online statistics:

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

F.13.6. Authors

Oleg Bartunov , Moscow, Moscow University, Russia

Teodor Sigaev , Moscow, Delta-Soft Ltd., Russia