Locale support refers to an application respecting cultural preferences regarding alphabets, sorting, number formatting, etc. PostgreSQL uses the standard ISO C and POSIX locale facilities provided by the server operating system. For additional information refer to the documentation of your system.
Locale support is automatically initialized when a database
cluster is created using initdb.
initdb will initialize the database cluster
with the locale setting of its execution environment by default,
so if your system is already set to use the locale that you want
in your database cluster then there is nothing else you need to
do. If you want to use a different locale (or you are not sure
which locale your system is set to), you can instruct
initdb exactly which locale to use by
specifying the --locale option. For example:
initdb --locale=sv_SE
This example for Unix systems sets the locale to Swedish
(sv) as spoken
in Sweden (SE). Other possibilities might include
en_US (U.S. English) and fr_CA (French
Canadian). If more than one character set can be used for a
locale then the specifications can take the form
language_territory.codeset. For example,
fr_BE.UTF-8 represents the French language (fr) as
spoken in Belgium (BE), with a UTF-8 character set
encoding.
What locales are available on your
system under what names depends on what was provided by the operating
system vendor and what was installed. On most Unix systems, the command
locale -a will provide a list of available locales.
Windows uses more verbose locale names, such as German_Germany
or Swedish_Sweden.1252, but the principles are the same.
Occasionally it is useful to mix rules from several locales, e.g., use English collation rules but Spanish messages. To support that, a set of locale subcategories exist that control only certain aspects of the localization rules:
LC_COLLATE | String sort order |
LC_CTYPE | Character classification (What is a letter? Its upper-case equivalent?) |
LC_MESSAGES | Language of messages |
LC_MONETARY | Formatting of currency amounts |
LC_NUMERIC | Formatting of numbers |
LC_TIME | Formatting of dates and times |
The category names translate into names of
initdb options to override the locale choice
for a specific category. For instance, to set the locale to
French Canadian, but use U.S. rules for formatting currency, use
initdb --locale=fr_CA --lc-monetary=en_US.
If you want the system to behave as if it had no locale support,
use the special locale name C, or equivalently
POSIX.
Some locale categories must have their values
fixed when the database is created. You can use different settings
for different databases, but once a database is created, you cannot
change them for that database anymore. LC_COLLATE
and LC_CTYPE are these categories. They affect
the sort order of indexes, so they must be kept fixed, or indexes on
text columns would become corrupt.
(But you can alleviate this restriction using collations, as discussed
in Section 23.2.)
The default values for these
categories are determined when initdb is run, and
those values are used when new databases are created, unless
specified otherwise in the CREATE DATABASE command.
The other locale categories can be changed whenever desired
by setting the server configuration parameters
that have the same name as the locale categories (see Section 19.11.2 for details). The values
that are chosen by initdb are actually only written
into the configuration file postgresql.conf to
serve as defaults when the server is started. If you remove these
assignments from postgresql.conf then the
server will inherit the settings from its execution environment.
Note that the locale behavior of the server is determined by the environment variables seen by the server, not by the environment of any client. Therefore, be careful to configure the correct locale settings before starting the server. A consequence of this is that if client and server are set up in different locales, messages might appear in different languages depending on where they originated.
When we speak of inheriting the locale from the execution
environment, this means the following on most operating systems:
For a given locale category, say the collation, the following
environment variables are consulted in this order until one is
found to be set: LC_ALL, LC_COLLATE
(or the variable corresponding to the respective category),
LANG. If none of these environment variables are
set then the locale defaults to C.
Some message localization libraries also look at the environment
variable LANGUAGE which overrides all other locale
settings for the purpose of setting the language of messages. If
in doubt, please refer to the documentation of your operating
system, in particular the documentation about
gettext.
To enable messages to be translated to the user's preferred language,
NLS must have been selected at build time
(configure --enable-nls). All other locale support is
built in automatically.
The locale settings influence the following SQL features:
Sort order in queries using ORDER BY or the standard
comparison operators on textual data
Pattern matching operators (LIKE, SIMILAR TO,
and POSIX-style regular expressions); locales affect both case
insensitive matching and the classification of characters by
character-class regular expressions
The ability to use indexes with LIKE clauses
The drawback of using locales other than C or
POSIX in PostgreSQL is its performance
impact. It slows character handling and prevents ordinary indexes
from being used by LIKE. For this reason use locales
only if you actually need them.
As a workaround to allow PostgreSQL to use indexes
with LIKE clauses under a non-C locale, several custom
operator classes exist. These allow the creation of an index that
performs a strict character-by-character comparison, ignoring
locale comparison rules. Refer to Section 11.10
for more information. Another approach is to create indexes using
the C collation, as discussed in
Section 23.2.
Locales can be selected in different scopes depending on requirements.
The above overview showed how locales are specified using
initdb to set the defaults for the entire cluster. The
following list shows where locales can be selected. Each item provides
the defaults for the subsequent items, and each lower item allows
overriding the defaults on a finer granularity.
As explained above, the environment of the operating system provides the defaults for the locales of a newly initialized database cluster. In many cases, this is enough: if the operating system is configured for the desired language/territory, by default PostgreSQL will also behave according to that locale.
As shown above, command-line options for initdb
specify the locale settings for a newly initialized database cluster.
Use this if the operating system does not have the locale configuration
you want for your database system.
A locale can be selected separately for each database. The SQL command
CREATE DATABASE and its command-line equivalent
createdb have options for that. Use this for example
if a database cluster houses databases for multiple tenants with
different requirements.
Locale settings can be made for individual table columns. This uses an SQL object called collation and is explained in Section 23.2. Use this for example to sort data in different languages or customize the sort order of a particular table.
Finally, locales can be selected for an individual query. Again, this uses SQL collation objects. This could be used to change the sort order based on run-time choices or for ad-hoc experimentation.
A locale provider specifies which library defines the locale behavior for collations and character classifications.
The commands and tools that select the locale settings, as described above, each have an option to select the locale provider. Here is an example to initialize a database cluster using the ICU provider:
initdb --locale-provider=icu --icu-locale=en
See the description of the respective commands and programs for
details. Note that you can mix locale providers at different
granularities, for example use libc by default for the
cluster but have one database that uses the icu
provider, and then have collation objects using either provider within
those databases.
Regardless of the locale provider, the operating system is still used to provide some locale-aware behavior, such as messages (see lc_messages).
The available locale providers are listed below:
builtin
The builtin provider uses built-in operations. Only
the C and C.UTF-8 locales are
supported for this provider.
The C locale behavior is identical to the
C locale in the libc provider. When using this
locale, the behavior may depend on the database encoding.
The C.UTF-8 locale is available only for when the
database encoding is UTF-8, and the behavior is
based on Unicode. The collation uses the code point values only. The
regular expression character classes are based on the "POSIX
Compatible" semantics, and the case mapping is the "simple" variant.
icu
The icu provider uses the external
ICU
library. PostgreSQL must have been
configured with support.
ICU provides collation and character classification behavior that is
independent of the operating system and database encoding, which is
preferable if you expect to transition to other platforms without any
change in results. LC_COLLATE and
LC_CTYPE can be set independently of the ICU
locale.
For the ICU provider, results may depend on the version of the ICU library used, as it is updated to reflect changes in natural language over time.
libc
The libc provider uses the operating system's C
library. The collation and character classification behavior is
controlled by the settings LC_COLLATE and
LC_CTYPE, so they cannot be set independently.
The same locale name may have different behavior on different platforms when using the libc provider.
The ICU format for the locale name is a Language Tag.
CREATE COLLATION mycollation1 (provider = icu, locale = 'ja-JP'); CREATE COLLATION mycollation2 (provider = icu, locale = 'fr');
When defining a new ICU collation object or database with ICU as the provider, the given locale name is transformed ("canonicalized") into a language tag if not already in that form. For instance,
CREATE COLLATION mycollation3 (provider = icu, locale = 'en-US-u-kn-true'); NOTICE: using standard form "en-US-u-kn" for locale "en-US-u-kn-true" CREATE COLLATION mycollation4 (provider = icu, locale = 'de_DE.utf8'); NOTICE: using standard form "de-DE" for locale "de_DE.utf8"
If you see this notice, ensure that the provider and
locale are the expected result. For consistent results
when using the ICU provider, specify the canonical language tag instead of relying on the
transformation.
A locale with no language name, or the special language name
root, is transformed to have the language
und ("undefined").
ICU can transform most libc locale names, as well as some other formats, into language tags for easier transition to ICU. If a libc locale name is used in ICU, it may not have precisely the same behavior as in libc.
If there is a problem interpreting the locale name, or if the locale name represents a language or region that ICU does not recognize, you will see the following warning:
CREATE COLLATION nonsense (provider = icu, locale = 'nonsense'); WARNING: ICU locale "nonsense" has unknown language "nonsense" HINT: To disable ICU locale validation, set parameter icu_validation_level to DISABLED. CREATE COLLATION
icu_validation_level controls how the message is
reported. Unless set to ERROR, the collation will
still be created, but the behavior may not be what the user intended.
A language tag, defined in BCP 47, is a standardized identifier used to identify languages, regions, and other information about a locale.
Basic language tags are simply
language-region;
or even just language. The
language is a language code
(e.g. fr for French), and
region is a region code
(e.g. CA for Canada). Examples:
ja-JP, de, or
fr-CA.
Collation settings may be included in the language tag to customize collation behavior. ICU allows extensive customization, such as sensitivity (or insensitivity) to accents, case, and punctuation; treatment of digits within text; and many other options to satisfy a variety of uses.
To include this additional collation information in a language tag,
append -u, which indicates there are additional
collation settings, followed by one or more
-key-value
pairs. The key is the key for a collation setting and
value is a valid value for that setting. For
boolean settings, the -key
may be specified without a corresponding
-value, which implies a
value of true.
For example, the language tag en-US-u-kn-ks-level2
means the locale with the English language in the US region, with
collation settings kn set to true
and ks set to level2. Those
settings mean the collation will be case-insensitive and treat a sequence
of digits as a single number:
CREATE COLLATION mycollation5 (provider = icu, deterministic = false, locale = 'en-US-u-kn-ks-level2'); SELECT 'aB' = 'Ab' COLLATE mycollation5 as result; result -------- t (1 row) SELECT 'N-45' < 'N-123' COLLATE mycollation5 as result; result -------- t (1 row)
See Section 23.2.3 for details and additional examples of using language tags with custom collation information for the locale.
If locale support doesn't work according to the explanation above,
check that the locale support in your operating system is
correctly configured. To check what locales are installed on your
system, you can use the command locale -a if
your operating system provides it.
Check that PostgreSQL is actually using the locale
that you think it is. The LC_COLLATE and LC_CTYPE
settings are determined when a database is created, and cannot be
changed except by creating a new database. Other locale
settings including LC_MESSAGES and LC_MONETARY
are initially determined by the environment the server is started
in, but can be changed on-the-fly. You can check the active locale
settings using the SHOW command.
The directory src/test/locale in the source
distribution contains a test suite for
PostgreSQL's locale support.
Client applications that handle server-side errors by parsing the text of the error message will obviously have problems when the server's messages are in a different language. Authors of such applications are advised to make use of the error code scheme instead.
Maintaining catalogs of message translations requires the on-going efforts of many volunteers that want to see PostgreSQL speak their preferred language well. If messages in your language are currently not available or not fully translated, your assistance would be appreciated. If you want to help, refer to Chapter 57 or write to the developers' mailing list.