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
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
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
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
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", ... );
SELECT a < 'foo' FROM test1;
< 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
because the explicit collation derivation overrides the implicit one.
SELECT a < b FROM test1;
the parser cannot determine which collation to apply, since the
b columns have conflicting
implicit collations. Since the
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;
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
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
But this query:
SELECT * FROM test1 ORDER BY a || b;
results in an error, because even though the
doesn't need to know a collation, the
ORDER BY clause does.
As before, the conflict can be resolved with an explicit collation
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
libc, which uses the locales provided by the
operating system C library. These are the locales that most tools
provided by the operating system use. Another provider
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
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
LC_CTYPE per expression.) Also,
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
184.108.40.206. Standard Collations
On all platforms, the collations named
POSIX are available. Additional
collations may be available depending on operating system support.
default collation selects the
LC_CTYPE values specified at database creation time.
POSIX collations both specify
“traditional C” behavior, in which only the ASCII letters
A” through “
are treated as letters, and sorting is done strictly by character
code byte values.
Additionally, the SQL standard collation name
is available for encoding
UTF8. It is equivalent
C and sorts by Unicode code point.
220.127.116.11. 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,
populates the system catalog
collations based on all the locales it finds in the operating
system at the time.
To inspect the currently available locales, use the query
* FROM pg_collation, or the command
18.104.22.168.1. libc Collations
For example, the operating system might
provide a locale named
initdb would then create a collation named
de_DE.utf8 for encoding
that has both
LC_CTYPE set to
It will also create a collation with the
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
The default set of collations provided by
directly to the locales installed in the operating system, which can be
listed using the command
locale -a. In case
libc collation is needed that has different values
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
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 less thing you need to change if you decide to change to
another database encoding. Note however that the
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
collations have identical behaviors. Mixing stripped and non-stripped
collation names is therefore not recommended.
22.214.171.124.2. 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”
-x-icu appended, to distinguish them from
Here are some example collations that might be created:
German collation, default variant
German collation for Austria, default variant
(There are also, say,
de-CH-x-icu, but as of this writing, they are equivalent to
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
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”.
126.96.36.199. 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
pg_catalog, like all predefined objects.
User-defined collations should be created in user schemas. This also
ensures that they are saved by
188.8.131.52.1. 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
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 184.108.40.206.3) or if the operating system has
been upgraded to provide new locale definitions (in which case see
220.127.116.11.2. ICU Collations
ICU allows collations to be customized beyond the basic language+country
set that is preloaded by
initdb. Users are encouraged
to define their own collation objects that make use of these facilities to
suit the sorting behavior to their requirements.
and http://userguide.icu-project.org/collation/api for
information on ICU locale naming. The set of acceptable names and
attributes depends on the particular ICU version.
Here are some examples:
CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');
CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de@collation=phonebook');
German collation with phone book collation type
The first example selects the ICU locale using a “language tag” per BCP 47. The second example uses the traditional ICU-specific locale syntax. The first style is preferred going forward, but it is not supported by older ICU versions.
Note that you can name the collation objects in the SQL environment anything you want. In this example, we follow the naming style that the predefined collations use, which in turn also follow BCP 47, but that is not required for user-defined collations.
CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = 'und-u-co-emoji');
CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = '@collation=emoji');
Root collation with Emoji collation type, per Unicode Technical Standard #51
Observe how in the traditional ICU locale naming system, the root locale is selected by an empty string.
CREATE COLLATION digitslast (provider = icu, locale = 'en-u-kr-latn-digit');
CREATE COLLATION digitslast (provider = icu, locale = 'en@colReorder=latn-digit');
Sort digits after Latin letters. (The default is digits before letters.)
CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');
CREATE COLLATION upperfirst (provider = icu, locale = 'en@colCaseFirst=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-latn-digit');
CREATE COLLATION special (provider = icu, locale = 'en@colCaseFirst=upper;colReorder=latn-digit');
Combines both of the above options.
CREATE COLLATION numeric (provider = icu, locale = 'en-u-kn-true');
CREATE COLLATION numeric (provider = icu, locale = 'en@colNumeric=yes');
Numeric ordering, sorts sequences of digits by their numeric value, for example:
A-123(also known as natural sort).
Technical Standard #35
and BCP 47 for
details. The list of possible collation types (
subtag) can be found in
The ICU Locale
Explorer can be used to check the details of a particular locale
definition. The examples using the
k* subtags require
at least ICU version 54.
Note that while this system allows creating collations that “ignore
case” or “ignore accents” or similar (using the
ks key), in order for such collations to act in a
truly case- or accent-insensitive manner, they also need to be declared as not
see Section 18.104.22.168.
Otherwise, any strings that compare equal according to the collation but
are not byte-wise equal will be sorted according to their byte values.
By design, ICU will accept almost any string as a locale name and match it to the closest locale it can provide, using the fallback procedure described in its documentation. Thus, there will be no direct feedback if a collation specification is composed using features that the given ICU installation does not actually support. It is therefore recommended to create application-level test cases to check that the collation definitions satisfy one's requirements.
22.214.171.124.3. 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";
126.96.36.199. 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
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. 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.