This chapter describes the available localization features from the point of view of the administrator. PostgreSQL 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 and . 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 . locale 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 .) 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 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 ORDER BYand locales The upper, lower, and initcap functions upperand locales lowerand locales 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 LIKEand locales regular expressionsand locales The to_char family of functions to_charand locales 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 for more information. Another approach is to create indexes using the C collation, as discussed in . 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, then PostgreSQL will by default 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 . 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. PostgreSQL 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 ICUICU library. ICU locales can only be used if support for ICU was configured when PostgreSQL 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: 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. 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. 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 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 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 or write to the developers' mailing list. collation 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. 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"; 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 ICUICU library. ICU locales can only be used if support for ICU was configured when PostgreSQL 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 ). 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. 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. 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 .) 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.) 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, initdb 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. For example, the operating system might provide a locale named de_DE.utf8. initdb 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 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. PostgreSQL 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. 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. initdb 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. If the standard and predefined collations are not sufficient, users can create their own collation objects using the SQL command . 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 pg_dump. 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 ) or if the operating system has been upgraded to provide new locale definitions (in which case see also pg_import_system_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 for details and examples. The command 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"; 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. 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 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 , or see for more details. 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. 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. 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. 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 shows the available collation settings, which can be used as part of a language tag to customize a collation. Key Values Default Description co emoji, phonebk, standard, ... standard Collation type. See 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 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 for additional options and details. For many collation settings, you must create the collation with deterministic set to false for the setting to have the desired effect (see ). Additionally, some settings only take effect when the key ka is set to shifted (see ). 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. 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 . This small example creates a collation based on the root locale with a tailoring rule: 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. ' < '?' < '`' < ':' < '#' < '@' < \' < '=' < '"' <*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 This section () is only a brief overview of ICU behavior and language tags. Refer to the following documents for technical details, additional options, and new behavior: Unicode Technical Standard #35 BCP 47 CLDR repository character set The character set support in PostgreSQL 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 PostgreSQL database cluster using 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. (On Windows, however, UTF-8 encoding can be used with any locale.) If you have ICU support configured, ICU-provided locales can be used with most but not all server-side encodings. shows the character sets available for use in PostgreSQL. Name Description Language Server? ICU? Bytes/&zwsp;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 No 1–3 EUC_KR Extended UNIX Code-KR Korean Yes Yes 1–3 EUC_TW Extended UNIX Code-TW Traditional Chinese, Taiwanese Yes Yes 1–3 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 KOI8-R Cyrillic (Russian) Yes Yes 1 KOI8 KOI8U KOI8-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 No 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 PostgreSQL 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 PostgreSQL will be unable to help you by converting or validating non-ASCII characters. initdb defines the default character set (encoding) for a PostgreSQL cluster. For example, initdb -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, initdb 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 . 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 | postgres | 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) On most modern operating systems, PostgreSQL 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. PostgreSQL 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. PostgreSQL supports automatic character set conversion between server and client for many combinations of character sets ( shows which ones). To enable automatic character set conversion, you have to tell PostgreSQL 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 () 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 . 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. PostgreSQL allows conversion between any two character sets for which a conversion function is listed in the pg_conversion system catalog. PostgreSQL comes with some predefined conversions, as summarized in and shown in more detail in . You can create a new conversion using the SQL command . (To be used for automatic client/server conversions, a conversion must be marked as default for its character set pair.) 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 to 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 Conversion Name 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 . 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_cn UTF8 EUC_CN utf8_to_euc_jp UTF8 EUC_JP 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 These are good sources to start learning about various kinds of encoding systems. CJKV Information Processing: Chinese, Japanese, Korean & Vietnamese Computing Contains detailed explanations of EUC_JP, EUC_CN, EUC_KR, EUC_TW. The web site of the Unicode Consortium. RFC 3629 UTF-8 (8-bit UCS/Unicode Transformation Format) is defined here.