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.
 
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”.
  
   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 for more on the handling of string
   constants.
  
On output, double quotes always surround keys and values, even when it's not strictly necessary.
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 | Result | 
|---|---|---|---|
| hstore->text | get value for key ( NULLif not present) | 'a=>x, b=>y'::hstore -> 'a' | x | 
| hstore->text[] | get values for keys ( NULLif not present) | 'a=>x, b=>y, c=>z'::hstore -> ARRAY['c','a'] | {"z","x"} | 
| hstore||hstore | concatenate hstores | 'a=>b, c=>d'::hstore || 'c=>x, d=>q'::hstore | "a"=>"b", "c"=>"x", "d"=>"q" | 
| hstore?text | does hstorecontain key? | 'a=>1'::hstore ? 'a' | t | 
| hstore?&text[] | does hstorecontain all specified keys? | 'a=>1,b=>2'::hstore ?& ARRAY['a','b'] | t | 
| hstore?|text[] | does hstorecontain 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 in recordwith matching values fromhstore | see Examples section | |
| %%hstore | convert hstoreto array of alternating keys and values | %% 'a=>foo, b=>bar'::hstore | {a,foo,b,bar} | 
| %#hstore | convert hstoreto two-dimensional key/value array | %# 'a=>foo, b=>bar'::hstore | {{a,foo},{b,bar}} | 
   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.8. hstore Functions
     The function hstore_to_json is used when
     an hstore value is cast to json.
     Likewise, hstore_to_jsonb is used when
     an hstore value is cast to jsonb.
   
    The function populate_record is actually declared
    with anyelement, not record, as its first argument,
    but it will reject non-record types with a run-time error.
   
   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 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);
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 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)
   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:
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
...................
   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 exclusive lock on the table,
   but does not result in bloating the table with old row versions.
  
   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 45.1 for the PL/Python naming
   convention).  If you use them, hstore values are mapped to
   Python dictionaries.
  
   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