# F.16. hstore 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 semi-structured data. Keys and values are simply text strings. This module is considered “trusted”, that is, it can be installed by non-superusers who have `CREATE` privilege on the current database. ## F.16.1. `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 "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 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 ---------- "a"=>"1" ``` A value (but not a key) can be an SQL `NULL`. For example: ``` key => NULL ``` The `NULL` keyword is case-insensitive. Double-quote the `NULL` to treat it as the ordinary string “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 single-quote 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](https://www.postgresql.org/docs/14/sql-syntax-lexical.html#SQL-SYNTAX-STRINGS) 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 the `hstore` module are shown in [Table F.7](https://www.postgresql.org/docs/14/hstore.html#HSTORE-OP-TABLE), the functions in [Table F.8](https://www.postgresql.org/docs/14/hstore.html#HSTORE-FUNC-TABLE). #### **Table F.7. `hstore` Operators** |

Operator

Description

Example(s)

hstore -> text → text

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

'a=>x, b=>y'::hstore -> 'a' → x

| |

hstore -> text\[] → text\[]

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

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

| | `hstore` | |

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` `?` | |

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 - text → hstore

Deletes key from left operand.

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

| |

hstore - text\[] → hstore

Deletes keys from left operand.

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

| |

hstore - hstore → hstore

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

'a=>1, b=>2, c=>3'::hstore - '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 two-dimensional key/value array.

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

| #### **Table F.8. `hstore` Functions** |

Function

Description

Example(s)

| | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |

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 two-dimensional 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 single-item hstore.

hstore('a', 'b') → "a"=>"b"

| |

akeys ( hstore ) → text\[]

Extracts an hstore's keys as an array.

akeys('a=>1,b=>2') → {a,b}

| |

skeys ( hstore ) → setof text

Extracts an hstore's keys as a set.

skeys('a=>1,b=>2') →

| |

avals ( hstore ) → text\[]

Extracts an hstore's values as an array.

avals('a=>1,b=>2') → {1,2}

| |

svals ( hstore ) → setof text

Extracts an hstore's values as a set.

svals('a=>1,b=>2') →

| |

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}

| |

hstore\_to\_matrix ( hstore ) → text\[]

Extracts an hstore's keys and values as a two-dimensional array.

hstore\_to\_matrix('a=>1,b=>2') → {{a,1},{b,2}}

| |

hstore\_to\_json ( hstore ) → json

Converts an hstore to a json value, converting all non-null 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"}

| |

hstore\_to\_jsonb ( hstore ) → jsonb

Converts an hstore to a jsonb value, converting all non-null 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"}

| |

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}

| |

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}

| |

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"

| |

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') →

| |

exist ( hstore, text ) → boolean

Does hstore contain key?

exist('a=>1', 'a') → t

| |

defined ( hstore, text ) → boolean

Does hstore contain a non-NULL value for key?

defined('a=>NULL', 'a') → f

| |

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"

| |

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 --- b (1 row) UPDATE mytable SET h['c'] = 'new'; SELECT h FROM mytable; h ---------------------- "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 `->` 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.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); ``` `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.16.4. 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 --------------------------------------------- "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 ------+------+------ 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 ------+------+------ 123 | foo | baz (1 row) ``` ## F.16.5. 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 -----------+------- 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 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.16.7. 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 extensions for PL/Python are called `hstore_plpythonu`, `hstore_plpython2u`, and `hstore_plpython3u` (see [Section 46.1](https://www.postgresql.org/docs/14/plpython-python23.html) for the PL/Python naming convention). If you use them, `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 installation-time security hazards if a transform extension's schema contains objects defined by a hostile user. ## F.16.8. Authors Oleg Bartunov `<`[`oleg@sai.msu.su`](mailto:oleg@sai.msu.su)`>`, Moscow, Moscow University, Russia Teodor Sigaev `<`[`teodor@sigaev.ru`](mailto:teodor@sigaev.ru)`>`, Moscow, Delta-Soft Ltd., Russia Additional enhancements by Andrew Gierth `<`[`andrew@tao11.riddles.org.uk`](mailto:andrew@tao11.riddles.org.uk)`>`, United Kingdom --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.postgresql.tw/appendixes/additional-supplied-modules/hstore.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. 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