Localization

This chapter describes the available localization features from the point of view of the administrator. QHB supports two localization facilities:

Using the locale features of the operating system to provide locale-specific collation order, number formatting, translated messages, and other aspects. This is covered in Section Locale Support and Section Collation Support.
Providing a number of different character sets to support storing text in all kinds of languages, and providing character set translation between client and server. This is covered in Section Character Set Support.

Locale Support

Locale support refers to an application respecting cultural preferences regarding alphabets, sorting, number formatting, etc. QHB 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.

Overview

Locale support is automatically initialized when a database cluster is created using qhb_bootstrap (or initdb). qhb_bootstrap 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 qhb_bootstrap exactly which locale to use by specifying the --locale option. For example:

qhb_bootstrap --locale=ru_RU

This example for Unix systems sets the locale to Russian (ru) as spoken in Russia (RU). 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, ru_RU.utf8 represents the Russian language (ru) as spoken in Russia (RU), 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.

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:

Category	Controlled Area
LC_COLLATE	String sort order строк
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 qhb_bootstrap options to override the locale choice for a specific category. For instance, to set the locale to Russian, but use U.S. rules for formatting currency, use qhb_bootstrap --locale=ru_RU --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 Collation Support). The default values for these categories are determined when qhb_bootstrap 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 Locale and Formatting for details). The values that are chosen by qhb_bootstrap are actually only written into the configuration file qhb.conf to serve as defaults when the server is started. If you remove these assignments from qhb.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.

Note
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.

Behavior

The locale settings influence the following SQL features:

Sort order in queries using ORDER BY or the standard comparison operators on textual data
The upper, lower, and initcap functions
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 to_char family of functions
The ability to use indexes with LIKE clauses

The drawback of using locales other than C or POSIX in QHB 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 QHB 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 Operator Classes and Operator Families for more information. Another approach is to create indexes using the C collation, as discussed in Section Collation Support.

Selecting Locales

Locales can be selected in different scopes depending on requirements. The above overview showed how locales are specified using qhb_bootstrap 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, then QHB will by default also behave according to that locale.
As shown above, command-line options for qhb_bootstrap 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 Collation Support. 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.

Locale Providers

QHB supports multiple locale providers. This specifies which library supplies the locale data. One standard provider name is libc, which uses the locales provided by the operating system C library. These are the locales used by most tools provided by the operating system. Another provider is icu, which uses the external ICU library. ICU locales can only be used if support for ICU was configured when QHB was built.

The commands and tools that select the locale settings, as described above, each have an option to select the locale provider. The examples shown earlier all use the libc provider, which is the default. Here is an example to initialize a database cluster using the ICU provider:

qhb_bootstrap --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.

Which locale provider to use depends on individual requirements. For most basic uses, either provider will give adequate results. For the libc provider, it depends on what the operating system offers; some operating systems are better than others. For advanced uses, ICU offers more locale variants and customization options.

ICU Locales

ICU Locale Names

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');

Locale Canonicalization and Validation

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.

Language Tag

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 ICU Custom Collations for details and additional examples of using language tags with custom collation information for the locale.

Problems

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 QHB 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.

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.

Collation Support

The collation feature allows specifying the sort order and character classification behavior of data per-column, or even per-operation. This alleviates the restriction that the LC_COLLATE and LC_CTYPE settings of a database cannot be changed after its creation.

Concepts

Conceptually, every expression of a collatable data type has a collation. (The built-in collatable data types are text, varchar, and char. User-defined base types can also be marked collatable, and of course a domain over a collatable data type is collatable.) If the expression is a column reference, the collation of the expression is the defined collation of the column. If the expression is a constant, the collation is the default collation of the data type of the constant. The collation of a more complex expression is derived from the collations of its inputs, as described below.

The collation of an expression can be the “default” collation, which means the locale settings defined for the database. It is also possible for an expression's collation to be indeterminate. In such cases, ordering operations and other operations that need to know the collation will fail.

When the database system has to perform an ordering or a character classification, it uses the collation of the input expression. This happens, for example, with ORDER BY clauses and function or operator calls such as <. The collation to apply for an ORDER BY clause is simply the collation of the sort key. The collation to apply for a function or operator call is derived from the arguments, as described below. In addition to comparison operators, collations are taken into account by functions that convert between lower and upper case letters, such as lower, upper, and initcap; by pattern matching operators; and by to_char and related functions.

For a function or operator call, the collation that is derived by examining the argument collations is used at run time for performing the specified operation. If the result of the function or operator call is of a collatable data type, the collation is also used at parse time as the defined collation of the function or operator expression, in case there is a surrounding expression that requires knowledge of its collation.

The collation derivation of an expression can be implicit or explicit. This distinction affects how collations are combined when multiple different collations appear in an expression. An explicit collation derivation occurs when a COLLATE clause is used; all other collation derivations are implicit. When multiple collations need to be combined, for example in a function call, the following rules are used:

If any input expression has an explicit collation derivation, then all explicitly derived collations among the input expressions must be the same, otherwise an error is raised. If any explicitly derived collation is present, that is the result of the collation combination.
Otherwise, all input expressions must have the same implicit collation derivation or the default collation. If any non-default collation is present, that is the result of the collation combination. Otherwise, the result is the default collation.
If there are conflicting non-default implicit collations among the input expressions, then the combination is deemed to have indeterminate collation. This is not an error condition unless the particular function being invoked requires knowledge of the collation it should apply. If it does, an error will be raised at run-time.

For example, consider this table definition:

CREATE TABLE test1 (
a text COLLATE "de_DE",
b text COLLATE "es_ES",
...
);

Then in

SELECT a < 'foo' FROM test1;

the < comparison is performed according to de_DE rules, because the expression combines an implicitly derived collation with the default collation. But in

SELECT a < ('foo' COLLATE "fr_FR") FROM test1;

the comparison is performed using fr_FR rules, because the explicit collation derivation overrides the implicit one. Furthermore, given

SELECT a < b FROM test1;

the parser cannot determine which collation to apply, since the a and b columns have conflicting implicit collations. Since the < operator does need to know which collation to use, this will result in an error. The error can be resolved by attaching an explicit collation specifier to either input expression, thus:

SELECT a < b COLLATE "de_DE" FROM test1;

or equivalently

SELECT a COLLATE "de_DE" < b FROM test1;

On the other hand, the structurally similar case

SELECT a || b FROM test1;

does not result in an error, because the || operator does not care about collations: its result is the same regardless of the collation.

The collation assigned to a function or operator's combined input expressions is also considered to apply to the function or operator's result, if the function or operator delivers a result of a collatable data type. So, in

SELECT * FROM test1 ORDER BY a || 'foo';

the ordering will be done according to de_DE rules. But this query:

SELECT * FROM test1 ORDER BY a || b;

results in an error, because even though the || operator doesn't need to know a collation, the ORDER BY clause does. As before, the conflict can be resolved with an explicit collation specifier:

SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";

Managing Collations

A collation is an SQL schema object that maps an SQL name to locales provided by libraries installed in the operating system. A collation definition has a provider that specifies which library supplies the locale data. One standard provider name is libc, which uses the locales provided by the operating system C library. These are the locales used by most tools provided by the operating system. Another provider is icu, which uses the external ICU library. ICU locales can only be used if support for ICU was configured when QHB was built.

A collation object provided by libc maps to a combination of LC_COLLATE and LC_CTYPE settings, as accepted by the setlocale() system library call. (As the name would suggest, the main purpose of a collation is to set LC_COLLATE, which controls the sort order. But it is rarely necessary in practice to have an LC_CTYPE setting that is different from LC_COLLATE, so it is more convenient to collect these under one concept than to create another infrastructure for setting LC_CTYPE per expression.) Also, a libc collation is tied to a character set encoding (see Section Character Set Support). The same collation name may exist for different encodings.

A collation object provided by icu maps to a named collator provided by the ICU library. ICU does not support separate “collate” and “ctype” settings, so they are always the same. Also, ICU collations are independent of the encoding, so there is always only one ICU collation of a given name in a database.

Standard Collations

On all platforms, the collations named default, C, and POSIX are available. Additional collations may be available depending on operating system support. The default collation selects the LC_COLLATE and LC_CTYPE values specified at database creation time. The C and POSIX collations both specify “traditional C” behavior, in which only the ASCII letters “A” through “Z” are treated as letters, and sorting is done strictly by character code byte values.

Note
The C and POSIX locales may behave differently depending on the database encoding.

Additionally, two SQL standard collation names are available:

unicode

This collation sorts using the Unicode Collation Algorithm with the Default Unicode Collation Element Table. It is available in all encodings. ICU support is required to use this collation. (This collation has the same behavior as the ICU root locale; see und-x-icu (for “undefined”).)

ucs_basic

This collation sorts by Unicode code point. It is only available for encoding UTF8. (This collation has the same behavior as the libc locale specification C in UTF8 encoding.)

Predefined Collations

If the operating system provides support for using multiple locales within a single program (newlocale and related functions), or if support for ICU is configured, then when a database cluster is initialized, qhb_bootstrap populates the system catalog pg_collation with collations based on all the locales it finds in the operating system at the time.

To inspect the currently available locales, use the query SELECT * FROM pg_collation, or the command \dOS+ in psql.

libc Collations

For example, the operating system might provide a locale named de_DE.utf8. qhb_bootstrap would then create a collation named de_DE.utf8 for encoding UTF8 that has both LC_COLLATE and LC_CTYPE set to de_DE.utf8. It will also create a collation with the .utf8 tag stripped off the name. So you could also use the collation under the name de_DE, which is less cumbersome to write and makes the name less encoding-dependent. Note that, nevertheless, the initial set of collation names is platform-dependent.

The default set of collations provided by libc map directly to the locales installed in the operating system, which can be listed using the command locale -a. In case a libc collation is needed that has different values for LC_COLLATE and LC_CTYPE, or if new locales are installed in the operating system after the database system was initialized, then a new collation may be created using the CREATE COLLATION command. New operating system locales can also be imported en masse using the pg_import_system_collations() function.

Within any particular database, only collations that use that database's encoding are of interest. Other entries in pg_collation are ignored. Thus, a stripped collation name such as de_DE can be considered unique within a given database even though it would not be unique globally. Use of the stripped collation names is recommended, since it will make one fewer thing you need to change if you decide to change to another database encoding. Note however that the default, C, and POSIX collations can be used regardless of the database encoding.

QHB considers distinct collation objects to be incompatible even when they have identical properties. Thus for example,

SELECT a COLLATE "C" < b COLLATE "POSIX" FROM test1;

will draw an error even though the C and POSIX collations have identical behaviors. Mixing stripped and non-stripped collation names is therefore not recommended.

ICU Collations

With ICU, it is not sensible to enumerate all possible locale names. ICU uses a particular naming system for locales, but there are many more ways to name a locale than there are actually distinct locales. qhb_bootstrap uses the ICU APIs to extract a set of distinct locales to populate the initial set of collations. Collations provided by ICU are created in the SQL environment with names in BCP 47 language tag format, with a “private use” extension -x-icu appended, to distinguish them from libc locales.

Here are some example collations that might be created:

de-x-icu
German collation, default variant

de-AT-x-icu
German collation for Austria, default variant
(There are also, say, de-DE-x-icu or de-CH-x-icu, but as of this writing, they are equivalent to de-x-icu).

und-x-icu (for “undefined”)
ICU “root” collation. Use this to get a reasonable language-agnostic sort order.

Some (less frequently used) encodings are not supported by ICU. When the database encoding is one of these, ICU collation entries in pg_collation are ignored. Attempting to use one will draw an error along the lines of “collation "de-x-icu" for encoding "WIN874" does not exist”.

Creating New Collation Objects

If the standard and predefined collations are not sufficient, users can create their own collation objects using the SQL command CREATE COLLATION.

The standard and predefined collations are in the schema pg_catalog, like all predefined objects. User-defined collations should be created in user schemas. This also ensures that they are saved by qhb_dump.

Creating libc Collations

New libc collations can be created like this:

CREATE COLLATION german (provider = libc, locale = 'de_DE');

The exact values that are acceptable for the locale clause in this command depend on the operating system. On Unix-like systems, the command locale -a will show a list.

Since the predefined libc collations already include all collations defined in the operating system when the database instance is initialized, it is not often necessary to manually create new ones. Reasons might be if a different naming system is desired (in which case see also Section Copying Collations ) or if the operating system has been upgraded to provide new locale definitions (in which case see also pg_import_system_collations()).

Creating ICU Collations

ICU collations can be created like:

CREATE COLLATION german (provider = icu, locale = 'de-DE');

ICU locales are specified as a BCP 47 Language Tag, but can also accept most libc-style locale names. If possible, libc-style locale names are transformed into language tags.

New ICU collations can customize collation behavior extensively by including collation attributes in the language tag. See Section ICU Custom Collations for details and examples.

Copying Collations

The command CREATE COLLATION can also be used to create a new collation from an existing collation, which can be useful to be able to use operating-system- independent collation names in applications, create compatibility names, or use an ICU-provided collation under a more readable name. For example:

CREATE COLLATION german FROM "de_DE";

CREATE COLLATION french FROM "fr-x-icu";

Nondeterministic Collations

A collation is either deterministic or nondeterministic. A deterministic collation uses deterministic comparisons, which means that it considers strings to be equal only if they consist of the same byte sequence. Nondeterministic comparison may determine strings to be equal even if they consist of different bytes. Typical situations include case-insensitive comparison, accent-insensitive comparison, as well as comparison of strings in different Unicode normal forms. It is up to the collation provider to actually implement such insensitive comparisons; the deterministic flag only determines whether ties are to be broken using bytewise comparison. See also Unicode Technical Standard 10 for more information on the terminology.

To create a nondeterministic collation, specify the property deterministic = false to CREATE COLLATION, for example:

CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false);

This example would use the standard Unicode collation in a nondeterministic way. In particular, this would allow strings in different normal forms to be compared correctly. More interesting examples make use of the ICU customization facilities explained above. For example:

CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-true', deterministic = false);

All standard and predefined collations are deterministic, all user-defined collations are deterministic by default. While nondeterministic collations give a more “correct” behavior, especially when considering the full power of Unicode and its many special cases, they also have some drawbacks. Foremost, their use leads to a performance penalty. Note, in particular, that B-tree cannot use deduplication with indexes that use a nondeterministic collation. Also, certain operations are not possible with nondeterministic collations, such as pattern matching operations. Therefore, they should be used only in cases where they are specifically wanted.

Tip
To deal with text in different Unicode normalization forms, it is also an option to use the functions/expressions normalize and is normalized to preprocess or check the strings, instead of using nondeterministic collations. There are different trade-offs for each approach.

ICU Custom Collations

ICU allows extensive control over collation behavior by defining new collations with collation settings as a part of the language tag. These settings can modify the collation order to suit a variety of needs. For instance:

-- ignore differences in accents and case
CREATE COLLATION ignore_accent_case (provider = icu, deterministic = false, locale = 'und-u-ks-level1');
SELECT 'Å' = 'A' COLLATE ignore_accent_case; -- true
SELECT 'z' = 'Z' COLLATE ignore_accent_case; -- true

-- upper case letters sort before lower case.
CREATE COLLATION upper_first (provider = icu, locale = 'und-u-kf-upper');
SELECT 'B' < 'b' COLLATE upper_first; -- true

-- treat digits numerically and ignore punctuation
CREATE COLLATION num_ignore_punct (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-kn');
SELECT 'id-45' < 'id-123' COLLATE num_ignore_punct; -- true
SELECT 'w;x*y-z' = 'wxyz' COLLATE num_ignore_punct; -- true

Many of the available options are described in Section Collation Settings for an ICU Locale, or see Section External References for ICU for more details.

ICU Comparison Levels

Comparison of two strings (collation) in ICU is determined by a multi-level process, where textual features are grouped into "levels". Treatment of each level is controlled by the collation settings. Higher levels correspond to finer textual features.

Table 1 shows which textual feature differences are considered significant when determining equality at the given level. The Unicode character U+2063 is an invisible separator, and as seen in the table, is ignored for at all levels of comparison less than identic.

Table 1. ICU Collation Levels

Level	Description	'f' = 'f'	'ab' = U&'a\2063b'	'x-y' = 'x_y'	'g' = 'G'	'n' = 'ñ'	'y' = 'z'
level1	Base Character	true	true	true	true	true	false
level2	Accents	true	true	true	true	false	false
level3	Case/Variants	true	true	true	false	false	false
level4	Punctuation	true	true	false	false	false	false
identic	All	true	false	false	false	false	false

At every level, even with full normalization off, basic normalization is performed. For example, 'á' may be composed of the code points U&'\0061\0301' or the single code point U&'\00E1', and those sequences will be considered equal even at the identic level. To treat any difference in code point representation as distinct, use a collation created with deterministic set to true.

Collation Level Examples

CREATE COLLATION level3 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level3');
CREATE COLLATION level4 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level4');
CREATE COLLATION identic (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-identic');

-- invisible separator ignored at all levels except identic
SELECT 'ab' = U&'a\2063b' COLLATE level4; -- true
SELECT 'ab' = U&'a\2063b' COLLATE identic; -- false

-- punctuation ignored at level3 but not at level 4
SELECT 'x-y' = 'x_y' COLLATE level3; -- true
SELECT 'x-y' = 'x_y' COLLATE level4; -- false

Collation Settings for an ICU Locale

Table 2 shows the available collation settings, which can be used as part of a language tag to customize a collation.

Table 2. ICU Collation Settings

Key	Values	Default	Description
co	emoji, phonebk, standard, ...	standard	Collation type. See Section External References for ICU for additional options and details.
ka	noignore, shifted	noignore	If set to shifted, causes some characters (e.g. punctuation or space) to be ignored in comparison. Key ks must be set to level3 or lower to take effect. Set key kv to control which character classes are ignored.
kb	true, false	false	Backwards comparison for the level 2 differences. For example, locale und-u-kb sorts 'àe' before 'aé'.
kc	true, false	false	Separates case into a "level 2.5" that falls between accents and other level 3 features. If set to true and ks is set to level1, will ignore accents but take case into account.
kf	upper, lower, false	false	If set to upper, upper case sorts before lower case. If set to lower, lower case sorts before upper case. If set to false, the sort depends on the rules of the locale.
kn	true, false	false	If set to true, numbers within a string are treated as a single numeric value rather than a sequence of digits. For example, 'id-45' sorts before 'id-123'.
kk	true, false	false	Enable full normalization; may affect performance. Basic normalization is performed even when set to false. Locales for languages that require full normalization typically enable it by default. Full normalization is important in some cases, such as when multiple accents are applied to a single character. For example, the code point sequences U&'\0065\0323\0302' and U&'\0065\0302\0323' represent an e with circumflex and dot-below accents applied in different orders. With full normalization on, these code point sequences are treated as equal; otherwise they are unequal.
kr	space, punct, symbol, currency, digit, script-id		Set to one or more of the valid values, or any BCP 47 script-id, e.g. latn ("Latin") or grek ("Greek"). Multiple values are separated by "-". Redefines the ordering of classes of characters; those characters belonging to a class earlier in the list sort before characters belonging to a class later in the list. For instance, the value digit-currency-space (as part of a language tag like und-u-kr-digit-currency-space) sorts punctuation before digits and spaces.
ks	level1, level2, level3, level4, identic	level3	Sensitivity (or "strength") when determining equality, with level1 the least sensitive to differences and identic the most sensitive to differences. See Table 1 for details.
kv	space, punct, symbol, currency	punct	Classes of characters ignored during comparison at level 3. Setting to a later value includes earlier values; e.g. symbol also includes punct and space in the characters to be ignored. Key ka must be set to shifted and key ks must be set to level3 or lower to take effect.

Defaults may depend on locale. The above table is not meant to be complete. See Section External References for ICU for additional options and details.

Note
For many collation settings, you must create the collation with deterministic set to false for the setting to have the desired effect (see Section Nondeterministic Collations). Additionally, some settings only take effect when the key ka is set to shifted (see Table 2).

Collation Settings Examples

CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');

German collation with phone book collation type

CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = 'und-u-co-emoji');

Root collation with Emoji collation type, per Unicode Technical Standard #51

CREATE COLLATION latinlast (provider = icu, locale = 'en-u-kr-grek-latn');

Sort Greek letters before Latin ones. (The default is Latin before Greek.)

CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');

Sort upper-case letters before lower-case letters. (The default is lower-case letters first.)

CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-grek-latn');

Combines both of the above options.

ICU Tailoring Rules

If the options provided by the collation settings shown above are not sufficient the order of collation elements can be changed with tailoring rules, whose syntax is detailed at https://unicode-org.github.io/icu/userguide/collation/customization/.

This small example creates a collation based on the root locale with a tailoring rule:

CREATE COLLATION custom (provider = icu, locale = 'und', rules = '&V << w <<< W');

With this rule, the letter “W” is sorted after “V”, but is treated as a secondary difference similar to an accent. Rules like this are contained in the locale definitions of some languages. (Of course, if a locale definition already contains the desired rules, then they don't need to be specified again explicitly.)

Here is a more complex example. The following statement sets up a collation named ebcdic with rules to sort US-ASCII characters in the order of the EBCDIC encoding.

CREATE COLLATION ebcdic (provider = icu, locale = 'und',
rules = $$
& ' ' < '.' < '<' < '(' < '+' < \|
< '&' < '!' < '$' < '*' < ')' < ';'
< '-' < '/' < ',' < '%' < '_' < '>' < '?'
< '`' < ':' < '#' < '@' < \' < '=' < '"'
<*a-r < '~' <*s-z < '^' < '[' < ']'
< '{' <*A-I < '}' <*J-R < '\' <*S-Z <*0-9
$$);

SELECT c
FROM (VALUES ('a'), ('b'), ('A'), ('B'), ('1'), ('2'), ('!'), ('^')) AS x(c)
ORDER BY c COLLATE ebcdic;
 c
---
 !
 a
 b
 ^
 A
 B
 1
 2

External References for ICU

Section ICU Custom Collations is only a brief overview of ICU behavior and language tags. Refer to the following documents for technical details, additional options, and new behavior:

Character Set Support

The character set support in QHB allows you to store text in a variety of character sets (also called encodings), including single-byte character sets such as the ISO 8859 series and multiple-byte character sets such as EUC (Extended Unix Code), UTF-8, and Mule internal code. All supported character sets can be used transparently by clients, but a few are not supported for use within the server (that is, as a server-side encoding). The default character set is selected while initializing your QHB database cluster using qhb_bootstrap (or initdb). It can be overridden when you create a database, so you can have multiple databases each with a different character set.

An important restriction, however, is that each database's character set must be compatible with the database's LC_CTYPE (character classification) and LC_COLLATE (string sort order) locale settings. For C or POSIX locale, any character set is allowed, but for other libc-provided locales there is only one character set that will work correctly. If you have ICU support configured, ICU-provided locales can be used with most but not all server-side encodings.

Supported Character Sets

Table 3 shows the character sets available for use in QHB.

Table 3. QHB Character Sets

Name	Description	Language	Server?	ICU?	Bytes/Char	Aliases
BIG5	Big Five	Traditional Chinese	No	No	1-2	WIN950, Windows950
EUC_CN	Extended UNIX Code-CN	Simplified Chinese	Yes	Yes	1-3
EUC_JP	Extended UNIX Code-JP	Japanese	Yes	Yes	1-3
EUC_JIS_2004	Extended UNIX Code-JP, JIS X 0213	Japanese	Yes	Нет	1-3
EUC_KR	Extended UNIX Code-KR	Korean	Yes	Yes	1-3
EUC_TW	Extended UNIX Code-TW	Traditional Chinese, Taiwanese	Yes	Yes	1-4
GB18030	National Standard	Chinese	No	No	1-4
GBK	Extended National Standard	Simplified Chinese	No	No	1-2	WIN936, Windows936
ISO_8859_5	ISO 8859-5, ECMA 113	Latin/Cyrillic	Yes	Yes	1
ISO_8859_6	ISO 8859-6, ECMA 114	Latin/Arabic	Yes	Yes	1
ISO_8859_7	ISO 8859-7, ECMA 118	Latin/Greek	Yes	Yes	1
ISO_8859_8	ISO 8859-8, ECMA 121	Latin/Hebrew	Yes	Yes	1
JOHAB	JOHAB	Korean (Hangul)	No	No	1-3
KOI8R	KOI 8-R	Cyrillic (Russian)	Yes	Yes	1	KOI8
KOI8U	KOI 8-U	Cyrillic (Ukrainian)	Yes	Yes	1
LATIN1	ISO 8859-1, ECMA 94	Western European	Yes	Yes	1	ISO88591
LATIN2	ISO 8859-2, ECMA 94	Central European	Yes	Yes	1	ISO88592
LATIN3	ISO 8859-3, ECMA 94	South European	Yes	Yes	1	ISO88593
LATIN4	ISO 8859-4, ECMA 94	North European	Yes	Yes	1	ISO88594
LATIN5	ISO 8859-9, ECMA 128	Turkish	Yes	Yes	1	ISO88599
LATIN6	ISO 8859-10, ECMA 144	Nordic	Yes	Yes	1	ISO885910
LATIN7	ISO 8859-13	Baltic	Yes	Yes	1	ISO885913
LATIN8	ISO 8859-14	Celtic	Yes	Yes	1	ISO885914
LATIN9	ISO 8859-15	LATIN1 with Euro and accents	Yes	Yes	1	ISO885915
LATIN10	ISO 8859-16, ASRO SR 14111	Romanian	Yes	No	1	ISO885916
MULE_INTERNAL	Mule internal code	Multilingual Emacs	Yes	No	1-4
SJIS	Shift JIS	Japanese	No	No	1-2	Mskanji, ShiftJIS, WIN932, Windows932
SHIFT_JIS_2004	Shift JIS, JIS X 0213	Japanese	No	No	1-2
SQL_ASCII	unspecified (see text)	any	Yes	No	1
UHC	Unified Hangul Code	Korean	Нет	No	1-2	WIN949, Windows949
UTF8	Unicode, 8-bit	all	Yes	Yes	1-4	Unicode
WIN866	Windows CP866	Cyrillic	Yes	Yes	1	ALT
WIN874	Windows CP874	Thai	Yes	No	1
WIN1250	Windows CP1250	Central European	Yes	Yes	1
WIN1251	Windows CP1251	Cyrillic	Yes	Yes	1	WIN
WIN1252	Windows CP1252	Western European	Yes	Yes	1
WIN1253	Windows CP1253	Greek	Yes	Yes	1
WIN1254	Windows CP1254	Turkish	Yes	Yes	1
WIN1255	Windows CP1255	Hebrew	Yes	Yes	1
WIN1256	Windows CP1256	Arabic	Yes	Yes	1
WIN1257	Windows CP1257	Baltic	Yes	Yes	1
WIN1258	Windows CP1258	Vietnamese	Yes	Yes	1	ABC, TCVN, TCVN5712, VSCII

Not all client APIs support all the listed character sets. For example, the QHB JDBC driver does not support MULE_INTERNAL, LATIN6, LATIN8, and LATIN10.

The SQL_ASCII setting behaves considerably differently from the other settings. When the server character set is SQL_ASCII, the server interprets byte values 0–127 according to the ASCII standard, while byte values 128–255 are taken as uninterpreted characters. No encoding conversion will be done when the setting is SQL_ASCII. Thus, this setting is not so much a declaration that a specific encoding is in use, as a declaration of ignorance about the encoding. In most cases, if you are working with any non-ASCII data, it is unwise to use the SQL_ASCII setting because QHB will be unable to help you by converting or validating non-ASCII characters.

Setting the Character Set

qhb_bootstrap defines the default character set (encoding) for a QHB cluster. For example,

qhb_bootstrap -E EUC_JP

sets the default character set to EUC_JP (Extended Unix Code for Japanese). You can use --encoding instead of -E if you prefer longer option strings. If no -E or --encoding option is given, qhb_bootstrap attempts to determine the appropriate encoding to use based on the specified or default locale.

You can specify a non-default encoding at database creation time, provided that the encoding is compatible with the selected locale:

createdb -E EUC_KR -T template0 --lc-collate=ko_KR.euckr --lc-ctype=ko_KR.euckr korean

This will create a database named korean that uses the character set EUC_KR, and locale ko_KR. Another way to accomplish this is to use this SQL command:

CREATE DATABASE korean WITH ENCODING 'EUC_KR' `LC_COLLATE`='ko_KR.euckr' `LC_CTYPE`='ko_KR.euckr' TEMPLATE=template0;

Notice that the above commands specify copying the template0 database. When copying any other database, the encoding and locale settings cannot be changed from those of the source database, because that might result in corrupt data. For more information see Section Template Databases.

The encoding for a database is stored in the system catalog pg_database. You can see it by using the psql -l option or the \l command.

$ psql -l
                                         List of databases
   Name    |  Owner   | Encoding  |  Collation  |    Ctype    |          Access Privileges          
-----------+----------+-----------+-------------+-------------+-------------------------------------
 clocaledb | hlinnaka | SQL_ASCII | C           | C           |
 englishdb | hlinnaka | UTF8      | en_GB.UTF8  | en_GB.UTF8  |
 japanese  | hlinnaka | UTF8      | ja_JP.UTF8  | ja_JP.UTF8  |
 korean    | hlinnaka | EUC_KR    | ko_KR.euckr | ko_KR.euckr |
 qhb       | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  |
 template0 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
 template1 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
(7 rows)

IMPORTANT
On most modern operating systems, QHB can determine which character set is implied by the LC_CTYPE setting, and it will enforce that only the matching database encoding is used. On older systems it is your responsibility to ensure that you use the encoding expected by the locale you have selected. A mistake in this area is likely to lead to strange behavior of locale-dependent operations such as sorting.

QHB will allow superusers to create databases with SQL_ASCII encoding even when LC_CTYPE is not C or POSIX. As noted above, SQL_ASCII does not enforce that the data stored in the database has any particular encoding, and so this choice poses risks of locale-dependent misbehavior. Using this combination of settings is deprecated and may someday be forbidden altogether.

Automatic Character Set Conversion Between Server and Client

QHB supports automatic character set conversion between server and client for many combinations of character sets (Section Available Character Set Conversions) shows which ones).

To enable automatic character set conversion, you have to tell QHB the character set (encoding) you would like to use in the client. There are several ways to accomplish this:

Using the \encoding command in psql. \encoding allows you to change client encoding on the fly. For example, to change the encoding to SJIS, type:

\encoding SJIS

libpq (Section Control Functions) has functions to control the client encoding.
Using SET client_encoding TO. Setting the client encoding can be done with this SQL command:

SET CLIENT_ENCODING TO 'value';

Also you can use the standard SQL syntax SET NAMES for this purpose:

SET NAMES 'value';

To query the current client encoding:

SHOW client_encoding;

To return to the default encoding:

RESET client_encoding;

Using PGCLIENTENCODING. If the environment variable PGCLIENTENCODING is defined in the client's environment, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)
Using the configuration variable client_encoding. If the client_encoding variable is set, that client encoding is automatically selected when a connection to the server is made. (This can subsequently be overridden using any of the other methods mentioned above.)

If the conversion of a particular character is not possible — suppose you chose EUC_JP for the server and LATIN1 for the client, and some Japanese characters are returned that do not have a representation in LATIN1 — an error is reported.

If the client character set is defined as SQL_ASCII, encoding conversion is disabled, regardless of the server's character set. (However, if the server's character set is not SQL_ASCII, the server will still check that incoming data is valid for that encoding; so the net effect is as though the client character set were the same as the server's.) Just as for the server, use of SQL_ASCII is unwise unless you are working with all-ASCII data.

Available Character Set Conversions

QHB allows conversion between any two character sets for which a conversion function is listed in the pg_conversion system catalog. QHB comes with some predefined conversions, as summarized in Table 4 and shown in more detail in Table 5. You can create a new conversion using the SQL command CREATE CONVERSION. (To be used for automatic client/server conversions, a conversion must be marked as “default” for its character set pair.)

Table 4. Built-in Client/Server Character Set Conversions

Server Character Set	Available Client Character Sets
BIG5	not supported as a server encoding
EUC_CN	EUC_CN, MULE_INTERNAL, UTF8
EUC_JP	EUC_JP, MULE_INTERNAL, SJIS, UTF8
EUC_JIS_2004	EUC_JIS_2004, SHIFT_JIS_2004, UTF8
EUC_KR	EUC_KR, MULE_INTERNAL, UTF8
EUC_TW	EUC_TW, BIG5, MULE_INTERNAL, UTF8
GB18030	not supported as a server encoding
GBK	not supported as a server encoding
ISO_8859_5	ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN866, WIN1251
ISO_8859_6	ISO_8859_6, UTF8
ISO_8859_7	ISO_8859_7, UTF8
ISO_8859_8	ISO_8859_8, UTF8
JOHAB	not supported as a server encoding
KOI8R	KOI8R, ISO_8859_5, MULE_INTERNAL, UTF8, WIN866, WIN1251
KOI8U	KOI8U, UTF8
LATIN1	LATIN1, MULE_INTERNAL, UTF8
LATIN2	LATIN2, MULE_INTERNAL, UTF8, WIN1250
LATIN3	LATIN3, MULE_INTERNAL, UTF8
LATIN4	LATIN4, MULE_INTERNAL, UTF8
LATIN5	LATIN5, UTF8
LATIN6	LATIN6, UTF8
LATIN7	LATIN7, UTF8
LATIN8	LATIN8, UTF8
LATIN9	LATIN9, UTF8
LATIN10	LATIN10, UTF8
MULE_INTERNAL	MULE_INTERNAL, BIG5, EUC_CN, EUC_JP, EUC_KR, EUC_TW, ISO_8859_5, KOI8R, LATIN1 - LATIN4, SJIS, WIN866, WIN1250, WIN1251
SJIS	not supported as a server encoding
SHIFT_JIS_2004	not supported as a server encoding
SQL_ASCII	any (no conversion will be performed)
UHC	not supported as a server encoding
UTF8	all supported encodings
WIN866	WIN866, ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN1251
WIN874	WIN874, UTF8
WIN1250	WIN1250, LATIN2, MULE_INTERNAL, UTF8
WIN1251	WIN1251, ISO_8859_5, KOI8R, MULE_INTERNAL, UTF8, WIN866
WIN1252	WIN1252, UTF8
WIN1253	WIN1253, UTF8
WIN1254	WIN1254, UTF8
WIN1255	WIN1255, UTF8
WIN1256	WIN1256, UTF8
WIN1257	WIN1257, UTF8
WIN1258	WIN1258, UTF8

Table 5. All Built-in Character Set Conversions

Conversion Name ¹	Source Encoding	Destination Encoding
big5_to_euc_tw	BIG5	EUC_TW
big5_to_mic	BIG5	MULE_INTERNAL
big5_to_utf8	BIG5	UTF8
euc_cn_to_mic	EUC_CN	MULE_INTERNAL
euc_cn_to_utf8	EUC_CN	UTF8
euc_jp_to_mic	EUC_JP	MULE_INTERNAL
euc_jp_to_sjis	EUC_JP	SJIS
euc_jp_to_utf8	EUC_JP	UTF8
euc_kr_to_mic	EUC_KR	MULE_INTERNAL
euc_kr_to_utf8	EUC_KR	UTF8
euc_tw_to_big5	EUC_TW	BIG5
euc_tw_to_mic	EUC_TW	MULE_INTERNAL
euc_tw_to_utf8	EUC_TW	UTF8
gb18030_to_utf8	GB18030	UTF8
gbk_to_utf8	GBK	UTF8
iso_8859_10_to_utf8	LATIN6	UTF8
iso_8859_13_to_utf8	LATIN7	UTF8
iso_8859_14_to_utf8	LATIN8	UTF8
iso_8859_15_to_utf8	LATIN9	UTF8
iso_8859_16_to_utf8	LATIN10	UTF8
iso_8859_1_to_mic	LATIN1	MULE_INTERNAL
iso_8859_1_to_utf8	LATIN1	UTF8
iso_8859_2_to_mic	LATIN2	MULE_INTERNAL
iso_8859_2_to_utf8	LATIN2	UTF8
iso_8859_2_to_windows_1250	LATIN2	WIN1250
iso_8859_3_to_mic	LATIN3	MULE_INTERNAL
iso_8859_3_to_utf8	LATIN3	UTF8
iso_8859_4_to_mic	LATIN4	MULE_INTERNAL
iso_8859_4_to_utf8	LATIN4	UTF8
iso_8859_5_to_koi8_r	ISO_8859_5	KOI8R
iso_8859_5_to_mic	ISO_8859_5	MULE_INTERNAL
iso_8859_5_to_utf8	ISO_8859_5	UTF8
iso_8859_5_to_windows_1251	ISO_8859_5	WIN1251
iso_8859_5_to_windows_866	ISO_8859_5	WIN866
iso_8859_6_to_utf8	ISO_8859_6	UTF8
iso_8859_7_to_utf8	ISO_8859_7	UTF8
iso_8859_8_to_utf8	ISO_8859_8	UTF8
iso_8859_9_to_utf8	LATIN5	UTF8
johab_to_utf8	JOHAB	UTF8
koi8_r_to_iso_8859_5	KOI8R	ISO_8859_5
koi8_r_to_mic	KOI8R	MULE_INTERNAL
koi8_r_to_utf8	KOI8R	UTF8
koi8_r_to_windows_1251	KOI8R	WIN1251
koi8_r_to_windows_866	KOI8R	WIN866
koi8_u_to_utf8	KOI8U	UTF8
mic_to_big5	MULE_INTERNAL	BIG5
mic_to_euc_cn	MULE_INTERNAL	EUC_CN
mic_to_euc_jp	MULE_INTERNAL	EUC_JP
mic_to_euc_kr	MULE_INTERNAL	EUC_KR
mic_to_euc_tw	MULE_INTERNAL	EUC_TW
mic_to_iso_8859_1	MULE_INTERNAL	LATIN1
mic_to_iso_8859_2	MULE_INTERNAL	LATIN2
mic_to_iso_8859_3	MULE_INTERNAL	LATIN3
mic_to_iso_8859_4	MULE_INTERNAL	LATIN4
mic_to_iso_8859_5	MULE_INTERNAL	ISO_8859_5
mic_to_koi8_r	MULE_INTERNAL	KOI8R
mic_to_sjis	MULE_INTERNAL	SJIS
mic_to_windows_1250	MULE_INTERNAL	WIN1250
mic_to_windows_1251	MULE_INTERNAL	WIN1251
mic_to_windows_866	MULE_INTERNAL	WIN866
sjis_to_euc_jp	SJIS	EUC_JP
sjis_to_mic	SJIS	MULE_INTERNAL
sjis_to_utf8	SJIS	UTF8
windows_1258_to_utf8	WIN1258	UTF8
uhc_to_utf8	UHC	UTF8
utf8_to_big5	UTF8	BIG5
utf8_to_euc_jp	UTF8	EUC_JP
utf8_to_euc_cn	UTF8	EUC_CN
utf8_to_euc_kr	UTF8	EUC_KR
utf8_to_euc_tw	UTF8	EUC_TW
utf8_to_gb18030	UTF8	GB18030
utf8_to_gbk	UTF8	GBK
utf8_to_iso_8859_1	UTF8	LATIN1
utf8_to_iso_8859_10	UTF8	LATIN6
utf8_to_iso_8859_13	UTF8	LATIN7
utf8_to_iso_8859_14	UTF8	LATIN8
utf8_to_iso_8859_15	UTF8	LATIN9
utf8_to_iso_8859_16	UTF8	LATIN10
utf8_to_iso_8859_2	UTF8	LATIN2
utf8_to_iso_8859_3	UTF8	LATIN3
utf8_to_iso_8859_4	UTF8	LATIN4
utf8_to_iso_8859_5	UTF8	ISO_8859_5
utf8_to_iso_8859_6	UTF8	ISO_8859_6
utf8_to_iso_8859_7	UTF8	ISO_8859_7
utf8_to_iso_8859_8	UTF8	ISO_8859_8
utf8_to_iso_8859_9	UTF8	LATIN5
utf8_to_johab	UTF8	JOHAB
utf8_to_koi8_r	UTF8	KOI8R
utf8_to_koi8_u	UTF8	KOI8U
utf8_to_sjis	UTF8	SJIS
utf8_to_windows_1258	UTF8	WIN1258
utf8_to_uhc	UTF8	UHC
utf8_to_windows_1250	UTF8	WIN1250
utf8_to_windows_1251	UTF8	WIN1251
utf8_to_windows_1252	UTF8	WIN1252
utf8_to_windows_1253	UTF8	WIN1253
utf8_to_windows_1254	UTF8	WIN1254
utf8_to_windows_1255	UTF8	WIN1255
utf8_to_windows_1256	UTF8	WIN1256
utf8_to_windows_1257	UTF8	WIN1257
utf8_to_windows_866	UTF8	WIN866
utf8_to_windows_874	UTF8	WIN874
windows_1250_to_iso_8859_2	WIN1250	LATIN2
windows_1250_to_mic	WIN1250	MULE_INTERNAL
windows_1250_to_utf8	WIN1250	UTF8
windows_1251_to_iso_8859_5	WIN1251	ISO_8859_5
windows_1251_to_koi8_r	WIN1251	KOI8R
windows_1251_to_mic	WIN1251	MULE_INTERNAL
windows_1251_to_utf8	WIN1251	UTF8
windows_1251_to_windows_866	WIN1251	WIN866
windows_1252_to_utf8	WIN1252	UTF8
windows_1256_to_utf8	WIN1256	UTF8
windows_866_to_iso_8859_5	WIN866	ISO_8859_5
windows_866_to_koi8_r	WIN866	KOI8R
windows_866_to_mic	WIN866	MULE_INTERNAL
windows_866_to_utf8	WIN866	UTF8
windows_866_to_windows_1251	WIN866	WIN
windows_874_to_utf8	WIN874	UTF8
euc_jis_2004_to_utf8	EUC_JIS_2004	UTF8
utf8_to_euc_jis_2004	UTF8	EUC_JIS_2004
shift_jis_2004_to_utf8	SHIFT_JIS_2004	UTF8
utf8_to_shift_jis_2004	UTF8	SHIFT_JIS_2004
euc_jis_2004_to_shift_jis_2004	EUC_JIS_2004	SHIFT_JIS_2004
shift_jis_2004_to_euc_jis_2004	SHIFT_JIS_2004	EUC_JIS_2004

The conversion names follow a standard naming scheme: The official name of the source encoding with all non-alphanumeric characters replaced by underscores, followed by _to_, followed by the similarly processed destination encoding name. Therefore, these names sometimes deviate from the customary encoding names shown in Table 3.