F.16. hstore
This module implements thehstore data type for storing sets of key/value pairs within a singlePostgres Pro value. This can be useful in various scenarios, such as rows with many attributes that are rarely examined, or semi-structured data. Keys and values are simply text strings.
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”.
Note
Keep in mind that thehstore text format, when used for input, appliesbefore any required quoting or escaping. If you are passing anhstore literal via a parameter, then no additional processing is needed. But if you're passing it as a quoted literal constant, then any single-quote characters and (depending on the setting of thestandard_conforming_strings configuration parameter) backslash characters need to be escaped correctly. SeeSection 4.1.2.1 for more on the handling of string constants.
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.8, the functions inTable F.9.
Table F.8. hstore Operators
| Operator | Description | Example | Result |
|---|---|---|---|
hstore->text | get value for key (NULL if not present) | 'a=>x, b=>y'::hstore -> 'a' | x |
hstore->text[] | get values for keys (NULL if not present) | 'a=>x, b=>y, c=>z'::hstore -> ARRAY['c','a'] | {"z","x"} |
hstore||hstore | concatenatehstores | 'a=>b, c=>d'::hstore || 'c=>x, d=>q'::hstore | "a"=>"b", "c"=>"x", "d"=>"q" |
hstore?text | doeshstore contain key? | 'a=>1'::hstore ? 'a' | t |
hstore?&text[] | doeshstore contain all specified keys? | 'a=>1,b=>2'::hstore ?& ARRAY['a','b'] | t |
hstore?|text[] | doeshstore contain any of the specified keys? | 'a=>1,b=>2'::hstore ?| ARRAY['b','c'] | 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 |
hstore-text | delete key from left operand | 'a=>1, b=>2, c=>3'::hstore - 'b'::text | "a"=>"1", "c"=>"3" |
hstore-text[] | delete keys from left operand | 'a=>1, b=>2, c=>3'::hstore - ARRAY['a','b'] | "c"=>"3" |
hstore-hstore | delete matching pairs from left operand | 'a=>1, b=>2, c=>3'::hstore - 'a=>4, b=>2'::hstore | "a"=>"1", "c"=>"3" |
record#=hstore | replace fields inrecord with matching values fromhstore | see Examples section | |
%%hstore | converthstore to array of alternating keys and values | %% 'a=>foo, b=>bar'::hstore | {a,foo,b,bar} |
%#hstore | converthstore to two-dimensional key/value array | %# 'a=>foo, b=>bar'::hstore | {{a,foo},{b,bar}} |
Note
Prior to PostgreSQL 8.2, the containment operators@> and<@ were called@ and~, respectively. These names are still available, but are deprecated and will eventually be removed. Notice that the old names are reversed from the convention formerly followed by the core geometric data types!
Table F.9. hstore Functions
Note
The functionhstore_to_json is used when anhstore value is cast tojson. Likewise,hstore_to_jsonb is used when anhstore value is cast tojsonb.
Note
The functionpopulate_record is actually declared withanyelement, notrecord, as its first argument, but it will reject non-record types with a run-time error.
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. Transforms
Additional extensions are available that implement transforms for thehstore type for the languages PL/Perl and PL/Python. The extensions for PL/Perl are calledhstore_plperl andhstore_plperlu, for trusted and untrusted PL/Perl. If you install these transforms and specify them when creating a function,hstore values are mapped to Perl hashes. The extensions for PL/Python are calledhstore_plpythonu,hstore_plpython2u, andhstore_plpython3u (seeSection 43.1 for the PL/Python naming convention). If you use them,hstore values are mapped to Python dictionaries.
F.16.8. 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