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F.18. hstore

F.18.1. hstore External Representation

F.18.2. hstore Operators and Functions

F.18.3. Indexes

F.18.4. Examples

F.18.5. Statistics

F.18.6. Compatibility

F.18.7. Transforms

F.18.8. Authors

[idx1.11.7.27.2 .indexterm]#

This module implements the hstore data type for storing sets of key/value pairs within a single PostgreSQL value. This can be useful in various scenarios, such as rows with many attributes that are rarely examined, or semixstructured data. Keys and values are simply text strings.

This module is considered “[.quote]#trusted”#, that is, it can be installed by nonxsuperusers who have CREATE privilege on the current database.

F.18.1. hstore External Representation

The text representation of an hstore, used for input and output, includes zero or more `key `=> `value` pairs separated by commas. Some examples:

k => v
foo => bar, baz => whatever
"1xa" => "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. Doublexquote 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 an hstore is unique. If you declare an hstore with duplicate keys, only one will be stored in the hstore and there is no guarantee as to which will be kept:

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

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

key => NULL

The NULL keyword is casexinsensitive. Doublexquote the NULL to treat it as the ordinary string “[.quote]#NULL”#.

Note

Keep in mind that the hstore text format, when used for input, applies before any required quoting or escaping. If you are passing an hstore literal via a parameter, then no additional processing is needed. But if you’re passing it as a quoted literal constant, then any singlexquote characters and (depending on the setting of the standard_conforming_strings configuration parameter) backslash characters need to be escaped correctly. See Section 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.18.2. hstore Operators and Functions

The operators provided by the hstore module are shown in Table F.7, the functions in Table F.8.

Table F.7. hstore Operators

Operator Description Example(s)

hstore x> text → text

Returns value associated with given key, or NULL if not present.

'a=>x, b=>y'::hstore x> 'a' → x

hstore x> text[] → text[]

Returns values associated with given keys, or NULL if not present.

'a=>x, b=>y, c=>z'::hstore x> ARRAY['c','a'] → {"z","x"}

hstore || hstore → hstore

Concatenates two `hstore`s.

'a=>b, c=>d'::hstore || 'c=>x, d=>q'::hstore → "a"=>"b", "c"=>"x", "d"=>"q"

hstore ? text → boolean

Does hstore contain key?

'a=>1'::hstore ? 'a' → t

hstore ?& text[] → boolean

Does hstore contain all the specified keys?

'a=>1,b=>2'::hstore ?& ARRAY['a','b'] → t

hstore ?| text[] → boolean

Does hstore contain any of the specified keys?

'a=>1,b=>2'::hstore ?| ARRAY['b','c'] → t

hstore @> hstore → boolean

Does left operand contain right?

'a=>b, b=>1, c=>NULL'::hstore @> 'b=>1' → t

hstore <@ hstore → boolean

Is left operand contained in right?

'a=>c'::hstore <@ 'a=>b, b=>1, c=>NULL' → f

hstore x text → hstore

Deletes key from left operand.

'a=>1, b=>2, c=>3'::hstore x 'b'::text → "a"=>"1", "c"=>"3"

hstore x text[] → hstore

Deletes keys from left operand.

'a=>1, b=>2, c=>3'::hstore x ARRAY['a','b'] → "c"=>"3"

hstore x hstore → hstore

Deletes pairs from left operand that match pairs in the right operand.

'a=>1, b=>2, c=>3'::hstore x 'a=>4, b=>2'::hstore → "a"=>"1", "c"=>"3"

anyelement #= hstore → anyelement

Replaces fields in the left operand (which must be a composite type) with matching values from hstore.

ROW(1,3) #= 'f1=>11'::hstore → (11,3)

%% hstore → text[]

Converts hstore to an array of alternating keys and values.

%% 'a=>foo, b=>bar'::hstore → {a,foo,b,bar}

%# hstore → text[]

Converts hstore to a twoxdimensional key/value array.

%# 'a=>foo, b=>bar'::hstore → {{a,foo},{b,bar}}

 
+

Table F.8. hstore Functions

Function Description Example(s)

[idx1.11.7.27.6.4.2.2.1.1.1.1 .indexterm]# hstore ( record ) → hstore

Constructs an hstore from a record or row.

hstore(ROW(1,2)) → "f1"=>"1", "f2"=>"2"

hstore ( text[] ) → hstore

Constructs an hstore from an array, which may be either a key/value array, or a twoxdimensional array.

hstore(ARRAY['a','1','b','2']) → "a"=>"1", "b"=>"2"

hstore(ARRAY[['c','3'],['d','4']]) → "c"=>"3", "d"=>"4"

hstore ( text[], text[] ) → hstore

Constructs an hstore from separate key and value arrays.

hstore(ARRAY['a','b'], ARRAY['1','2']) → "a"=>"1", "b"=>"2"

hstore ( text, text ) → hstore

Makes a singlexitem hstore.

hstore('a', 'b') → "a"=>"b"

[idx1.11.7.27.6.4.2.2.5.1.1.1 .indexterm]# akeys ( hstore ) → text[]

Extracts an `hstore’s keys as an array.

akeys('a=>1,b=>2') → {a,b}

[idx1.11.7.27.6.4.2.2.6.1.1.1 .indexterm]# skeys ( hstore ) → setof text

Extracts an `hstore’s keys as a set.

skeys('a=>1,b=>2') →

a
b

[idx1.11.7.27.6.4.2.2.7.1.1.1 .indexterm]# avals ( hstore ) → text[]

Extracts an `hstore’s values as an array.

avals('a=>1,b=>2') → {1,2}

[idx1.11.7.27.6.4.2.2.8.1.1.1 .indexterm]# svals ( hstore ) → setof text

Extracts an `hstore’s values as a set.

svals('a=>1,b=>2') →

1
2

[idx1.11.7.27.6.4.2.2.9.1.1.1 .indexterm]# hstore_to_array ( hstore ) → text[]

Extracts an `hstore’s keys and values as an array of alternating keys and values.

hstore_to_array('a=>1,b=>2') → {a,1,b,2}

[idx1.11.7.27.6.4.2.2.10.1.1.1 .indexterm]# hstore_to_matrix ( hstore ) → text[]

Extracts an `hstore’s keys and values as a twoxdimensional array.

hstore_to_matrix('a=>1,b=>2') → {{a,1},{b,2}}

[idx1.11.7.27.6.4.2.2.11.1.1.1 .indexterm]# hstore_to_json ( hstore ) → json

Converts an hstore to a json value, converting all nonxnull values to JSON strings.

This function is used implicitly when an hstore value is cast to json.

hstore_to_json('"a key"=>1, b=>t, c=>null, d=>12345, e=>012345, f=>1.234, g=>2.345e+4') → {"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}

[idx1.11.7.27.6.4.2.2.12.1.1.1 .indexterm]# hstore_to_jsonb ( hstore ) → jsonb

Converts an hstore to a jsonb value, converting all nonxnull values to JSON strings.

This function is used implicitly when an hstore value is cast to jsonb.

hstore_to_jsonb('"a key"=>1, b=>t, c=>null, d=>12345, e=>012345, f=>1.234, g=>2.345e+4') → {"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}

[idx1.11.7.27.6.4.2.2.13.1.1.1 .indexterm]# hstore_to_json_loose ( hstore ) → json

Converts an hstore to a json value, but attempts to distinguish numerical and Boolean values so they are unquoted in the JSON.

hstore_to_json_loose('"a key"=>1, b=>t, c=>null, d=>12345, e=>012345, f=>1.234, g=>2.345e+4') → {"a key": 1, "b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4}

[idx1.11.7.27.6.4.2.2.14.1.1.1 .indexterm]# hstore_to_jsonb_loose ( hstore ) → jsonb

Converts an hstore to a jsonb value, but attempts to distinguish numerical and Boolean values so they are unquoted in the JSON.

hstore_to_jsonb_loose('"a key"=>1, b=>t, c=>null, d=>12345, e=>012345, f=>1.234, g=>2.345e+4') → {"a key": 1, "b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4}

[idx1.11.7.27.6.4.2.2.15.1.1.1 .indexterm]# slice ( hstore, text[] ) → hstore

Extracts a subset of an hstore containing only the specified keys.

slice('a=>1,b=>2,c=>3'::hstore, ARRAY['b','c','x']) → "b"=>"2", "c"=>"3"

[idx1.11.7.27.6.4.2.2.16.1.1.1 .indexterm]# each ( hstore ) → setof record ( `key `text, `value `text )

Extracts an `hstore’s keys and values as a set of records.

select * from each('a=>1,b=>2') →

 key | value
xxxxx+xxxxxxx
 a   | 1
 b   | 2

[idx1.11.7.27.6.4.2.2.17.1.1.1 .indexterm]# exist ( hstore, text ) → boolean

Does hstore contain key?

exist('a=>1', 'a') → t

[idx1.11.7.27.6.4.2.2.18.1.1.1 .indexterm]# defined ( hstore, text ) → boolean

Does hstore contain a nonx`NULL` value for key?

defined('a=>NULL', 'a') → f

[idx1.11.7.27.6.4.2.2.19.1.1.1 .indexterm]# delete ( hstore, text ) → hstore

Deletes pair with matching key.

delete('a=>1,b=>2', 'b') → "a"=>"1"

delete ( hstore, text[] ) → hstore

Deletes pairs with matching keys.

delete('a=>1,b=>2,c=>3', ARRAY['a','b']) → "c"=>"3"

delete ( hstore, hstore ) → hstore

Deletes pairs matching those in the second argument.

delete('a=>1,b=>2', 'a=>4,b=>2'::hstore) → "a"=>"1"

[idx1.11.7.27.6.4.2.2.22.1.1.1 .indexterm]# populate_record ( anyelement, hstore ) → anyelement

Replaces fields in the left operand (which must be a composite type) with matching values from hstore.

populate_record(ROW(1,2), 'f1=>42'::hstore) → (42,2)

 
+

In addition to these operators and functions, values of the hstore type can be subscripted, allowing them to act like associative arrays. Only a single subscript of type text can be specified; it is interpreted as a key and the corresponding value is fetched or stored. For example,

CREATE TABLE mytable (h hstore);
INSERT INTO mytable VALUES ('a=>b, c=>d');
SELECT h['a'] FROM mytable;
 h
xxx
 b
(1 row)

UPDATE mytable SET h['c'] = 'new';
SELECT h FROM mytable;
          h
xxxxxxxxxxxxxxxxxxxxxx
 "a"=>"b", "c"=>"new"
(1 row)

A subscripted fetch returns NULL if the subscript is NULL or that key does not exist in the hstore. (Thus, a subscripted fetch is not greatly different from the x> operator.) A subscripted update fails if the subscript is NULL; otherwise, it replaces the value for that key, adding an entry to the hstore if the key does not already exist.

F.18.3. 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);

gist_hstore_ops GiST opclass approximates a set of key/value pairs as a bitmap signature. Its optional integer parameter siglen determines the signature length in bytes. The default length is 16 bytes. Valid values of signature length are between 1 and 2024 bytes. Longer signatures lead to a more precise search (scanning a smaller fraction of the index and fewer heap pages), at the cost of a larger index.

Example of creating such an index with a signature length of 32 bytes:

CREATE INDEX hidx ON testhstore USING GIST (h gist_hstore_ops(siglen=32));

hstore also supports btree or hash indexes for the = operator. This allows hstore columns to be declared UNIQUE, or to be used in GROUP BY, ORDER BY or DISTINCT expressions. The sort ordering for hstore 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.18.4. Examples

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

UPDATE tab SET h['c'] = '3';

Another way to do the same thing is:

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

If multiple keys are to be added or changed in one operation, the concatenation approach is more efficient than subscripting:

UPDATE tab SET h = h \|\| hstore(array['q', 'w'], array['11', '12']);

Delete a key:

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

Convert a record to an hstore:

CREATE TABLE test (col1 integer, col2 text, col3 text);
INSERT INTO test VALUES (123, 'foo', 'bar');

SELECT hstore(t) FROM test AS t;
                   hstore
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 "col1"=>"123", "col2"=>"foo", "col3"=>"bar"
(1 row)

Convert an hstore to a predefined record type:

CREATE TABLE test (col1 integer, col2 text, col3 text);

SELECT * FROM populate_record(null::test,
                              '"col1"=>"456", "col2"=>"zzz"');
 col1 \| col2 \| col3
xxxxxx+xxxxxx+xxxxxx
  456 \| zzz  \|
(1 row)

Modify an existing record using the values from an hstore:

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
xxxxxx+xxxxxx+xxxxxx
  123 \| foo  \| baz
(1 row)

F.18.5. 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. 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:

CREATE TABLE stat AS SELECT (each(h)).key, (each(h)).value 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
xxxxxxxxxxx+xxxxxxx
 line      \|   883
 query     \|   207
 pos       \|   203
 node      \|   202
 space     \|   197
 status    \|   195
 public    \|   194
 title     \|   190
 org       \|   189
...................

F.18.6. 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 oldxformat 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 an UPDATE statement as follows:

UPDATE tablename SET hstorecol = hstorecol \|\| '';

Another way to do it is:

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

The ALTER TABLE method requires an ACCESS EXCLUSIVE lock on the table, but does not result in bloating the table with old row versions.

F.18.7. Transforms

Additional extensions are available that implement transforms for the hstore type for the languages PL/Perl and PL/Python. The extensions for PL/Perl are called hstore_plperl and hstore_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 extension for PL/Python is called hstore_plpython3u. If you use it, hstore values are mapped to Python dictionaries.

Caution

It is strongly recommended that the transform extensions be installed in the same schema as hstore. Otherwise there are installationxtime security hazards if a transform extension’s schema contains objects defined by a hostile user.

F.18.8. Authors

Oleg Bartunov +<+`+[email protected]++>+`, Moscow, Moscow University, Russia

Teodor Sigaev +<+`+[email protected]++>+`, Moscow, DeltaxSoft Ltd., Russia

Additional enhancements by Andrew Gierth +<+`+[email protected]++>+`, United Kingdom

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