F.16.1.hstore External Representation

The text representation of anhstore, used for input and output, includes zero or morekey=>value pairs separated by commas. Some examples:

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

The order of the pairs is not significant (and may not be reproduced on output). Whitespace between pairs or around the=> sign is ignored. Double-quote keys and values that include whitespace, commas,=s or>s. To include a double quote or a backslash in a key or value, escape it with a backslash.

Each key in anhstore is unique. If you declare anhstore with duplicate keys, only one will be stored in thehstore and there is no guarantee as to which will be kept:

SELECT 'a=>1,a=>2'::hstore;  hstore---------- "a"=>"1"

A value (but not a key) can be an SQLNULL. For example:

key => NULL

TheNULL keyword is case-insensitive. Double-quote theNULL to treat it as the ordinary string"NULL".

On output, double quotes always surround keys and values, even when it's not strictly necessary.

F.16.2.hstore Operators and Functions

The operators provided by thehstore module are shown inTable F-6, the functions inTable F-7.

ab

12

 key | value-----+------- a   | 1 b   | 2

F.16.3. Indexes

hstore has GiST and GIN index support for the@>,?,?& and?| operators. For example:

CREATE INDEX hidx ON testhstore USING GIST (h);CREATE INDEX hidx ON testhstore USING GIN (h);

hstore also supportsbtree orhash indexes for the= operator. This allowshstore columns to be declaredUNIQUE, or to be used inGROUP BY,ORDER BY orDISTINCT expressions. The sort ordering forhstore values is not particularly useful, but these indexes may be useful for equivalence lookups. Create indexes for= comparisons as follows:

CREATE INDEX hidx ON testhstore USING BTREE (h);CREATE INDEX hidx ON testhstore USING HASH (h);

F.16.4. Examples

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

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

Delete a key:

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

Convert arecord to anhstore:

CREATE TABLE test (col1 integer, col2 text, col3 text);INSERT INTO test VALUES (123, 'foo', 'bar');SELECT hstore(t) FROM test AS t;                   hstore                    --------------------------------------------- "col1"=>"123", "col2"=>"foo", "col3"=>"bar"(1 row)

Convert anhstore to a predefinedrecord type:

CREATE TABLE test (col1 integer, col2 text, col3 text);SELECT * FROM populate_record(null::test,                              '"col1"=>"456", "col2"=>"zzz"'); col1 | col2 | col3 ------+------+------  456 | zzz  | (1 row)

Modify an existing record using the values from anhstore:

CREATE TABLE test (col1 integer, col2 text, col3 text);INSERT INTO test VALUES (123, 'foo', 'bar');SELECT (r).* FROM (SELECT t #= '"col3"=>"baz"' AS r FROM test t) s; col1 | col2 | col3 ------+------+------  123 | foo  | baz(1 row)

F.16.5. Statistics

Thehstore type, because of its intrinsic liberality, could contain a lot of different keys. Checking for valid keys is the task of the application. The following examples 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.16.6. Compatibility

As of PostgreSQL 9.0,hstore uses a different internal representation than previous versions. This presents no obstacle for dump/restore upgrades since the text representation (used in the dump) is unchanged.

In the event of a binary upgrade, upward compatibility is maintained by having the new code recognize old-format data. This will entail a slight performance penalty when processing data that has not yet been modified by the new code. It is possible to force an upgrade of all values in a table column by doing anUPDATE statement as follows:

UPDATE tablename SET hstorecol = hstorecol || '';

Another way to do it is:

ALTER TABLE tablename ALTER hstorecol TYPE hstore USING hstorecol || '';

TheALTER TABLE method requires an exclusive lock on the table, but does not result in bloating the table with old row versions.

F.16.7. Authors

Oleg Bartunov<oleg@sai.msu.su>, Moscow, Moscow University, Russia

Teodor Sigaev<teodor@sigaev.ru>, Moscow, Delta-Soft Ltd., Russia

Additional enhancements by Andrew Gierth<andrew@tao11.riddles.org.uk>, United Kingdom