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23.2. 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.

23.2.1. 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:

  1. 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.

  2. 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.

  3. 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";

23.2.2. 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 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 Section 23.3). 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.

23.2.2.1. Standard Collations #

On all platforms, the following collations are supported:

unicode

This SQL standard 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, and behavior may change if Postgres is built with a different version of ICU. (This collation has the same behavior as the ICU root locale; see und-x-icu (for undefined).)

ucs_basic

This SQL standard collation sorts using the Unicode code point values rather than natural language order, and only the ASCII letters A through Z are treated as letters. The behavior is efficient and stable across all versions. Only available for encoding UTF8. (This collation has the same behavior as the libc locale specification C in UTF8 encoding.)

pg_c_utf8

This collation sorts by Unicode code point values rather than natural language order. For the functions lower, initcap, and upper, it uses Unicode simple case mapping. For pattern matching (including regular expressions), it uses the POSIX Compatible variant of Unicode Compatibility Properties. Behavior is efficient and stable within a Postgres major version. This collation is only available for encoding UTF8.

C (equivalent to POSIX)

The C and POSIX collations are based on traditional C behavior. They sort by byte values rather than natural language order, and only the ASCII letters A through Z are treated as letters. The behavior is efficient and stable across all versions for a given database encoding, but behavior may vary between different database encodings.

default

The default collation selects the locale specified at database creation time.

Additional collations may be available depending on operating system support. The efficiency and stability of these additional collations depend on the collation provider, the provider version, and the locale.

23.2.2.2. 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, 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.

23.2.2.2.1. libc Collations #

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 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.

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.

23.2.2.2.2. 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. 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.

23.2.2.3. 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 pg_dump.

23.2.2.3.1. 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 23.2.2.3.3) or if the operating system has been upgraded to provide new locale definitions (in which case see also pg_import_system_collations()).

23.2.2.3.2. 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 23.2.3 for details and examples.

23.2.2.3.3. 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";

23.2.2.4. 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.

23.2.3. 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 23.2.3.2, or see Section 23.2.3.5 for more details.

23.2.3.1. 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 23.1 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 23.1. 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.

23.2.3.1.1. 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

23.2.3.2. Collation Settings for an ICU Locale #

Table 23.2 shows the available collation settings, which can be used as part of a language tag to customize a collation.

Table 23.2. ICU Collation Settings

Key Values Default Description
co emoji, phonebk, standard, ... standard Collation type. See Section 23.2.3.5 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 23.1 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 23.2.3.5 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 23.2.2.4). Additionally, some settings only take effect when the key ka is set to shifted (see Table 23.2).

23.2.3.3. 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.

23.2.3.4. 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

23.2.3.5. External References for ICU #

This section (Section 23.2.3) is only a brief overview of ICU behavior and language tags. Refer to the following documents for technical details, additional options, and new behavior: