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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="COLLATION"
>22.2. Collation Support</A
></H1
><P
> 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
<TT
CLASS="SYMBOL"
>LC_COLLATE</TT
> and <TT
CLASS="SYMBOL"
>LC_CTYPE</TT
> settings
of a database cannot be changed after its creation.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33024"
>22.2.1. Concepts</A
></H2
><P
> Conceptually, every expression of a collatable data type has a
collation. (The built-in collatable data types are
<TT
CLASS="TYPE"
>text</TT
>, <TT
CLASS="TYPE"
>varchar</TT
>, and <TT
CLASS="TYPE"
>char</TT
>.
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.
</P
><P
> The collation of an expression can be the <SPAN
CLASS="QUOTE"
>"default"</SPAN
>
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.
</P
><P
> 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 <TT
CLASS="LITERAL"
>ORDER BY</TT
> clauses
and function or operator calls such as <TT
CLASS="LITERAL"
><</TT
>.
The collation to apply for an <TT
CLASS="LITERAL"
>ORDER BY</TT
> 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 <CODE
CLASS="FUNCTION"
>lower</CODE
>, <CODE
CLASS="FUNCTION"
>upper</CODE
>, and
<CODE
CLASS="FUNCTION"
>initcap</CODE
>; by pattern matching operators; and by
<CODE
CLASS="FUNCTION"
>to_char</CODE
> and related functions.
</P
><P
> 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.
</P
><P
> The <I
CLASS="FIRSTTERM"
>collation derivation</I
> 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
<TT
CLASS="LITERAL"
>COLLATE</TT
> 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:
<P
></P
></P><OL
TYPE="1"
><LI
><P
> 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.
</P
></LI
><LI
><P
> 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.
</P
></LI
><LI
><P
> 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.
</P
></LI
></OL
><P>
For example, consider this table definition:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE test1 (
a text COLLATE "de_DE",
b text COLLATE "es_ES",
...
);</PRE
><P>
Then in
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a < 'foo' FROM test1;</PRE
><P>
the <TT
CLASS="LITERAL"
><</TT
> comparison is performed according to
<TT
CLASS="LITERAL"
>de_DE</TT
> rules, because the expression combines an
implicitly derived collation with the default collation. But in
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a < ('foo' COLLATE "fr_FR") FROM test1;</PRE
><P>
the comparison is performed using <TT
CLASS="LITERAL"
>fr_FR</TT
> rules,
because the explicit collation derivation overrides the implicit one.
Furthermore, given
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a < b FROM test1;</PRE
><P>
the parser cannot determine which collation to apply, since the
<TT
CLASS="STRUCTFIELD"
>a</TT
> and <TT
CLASS="STRUCTFIELD"
>b</TT
> columns have conflicting
implicit collations. Since the <TT
CLASS="LITERAL"
><</TT
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a < b COLLATE "de_DE" FROM test1;</PRE
><P>
or equivalently
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a COLLATE "de_DE" < b FROM test1;</PRE
><P>
On the other hand, the structurally similar case
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a || b FROM test1;</PRE
><P>
does not result in an error, because the <TT
CLASS="LITERAL"
>||</TT
> operator
does not care about collations: its result is the same regardless
of the collation.
</P
><P
> 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
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM test1 ORDER BY a || 'foo';</PRE
><P>
the ordering will be done according to <TT
CLASS="LITERAL"
>de_DE</TT
> rules.
But this query:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM test1 ORDER BY a || b;</PRE
><P>
results in an error, because even though the <TT
CLASS="LITERAL"
>||</TT
> operator
doesn't need to know a collation, the <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause does.
As before, the conflict can be resolved with an explicit collation
specifier:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN33072"
>22.2.2. Managing Collations</A
></H2
><P
> A collation is an SQL schema object that maps an SQL name to
operating system locales. In particular, it maps to a combination
of <TT
CLASS="SYMBOL"
>LC_COLLATE</TT
> and <TT
CLASS="SYMBOL"
>LC_CTYPE</TT
>. (As
the name would suggest, the main purpose of a collation is to set
<TT
CLASS="SYMBOL"
>LC_COLLATE</TT
>, which controls the sort order. But
it is rarely necessary in practice to have an
<TT
CLASS="SYMBOL"
>LC_CTYPE</TT
> setting that is different from
<TT
CLASS="SYMBOL"
>LC_COLLATE</TT
>, so it is more convenient to collect
these under one concept than to create another infrastructure for
setting <TT
CLASS="SYMBOL"
>LC_CTYPE</TT
> per expression.) Also, a collation
is tied to a character set encoding (see <A
HREF="multibyte.html"
>Section 22.3</A
>).
The same collation name may exist for different encodings.
</P
><P
> On all platforms, the collations named <TT
CLASS="LITERAL"
>default</TT
>,
<TT
CLASS="LITERAL"
>C</TT
>, and <TT
CLASS="LITERAL"
>POSIX</TT
> are available. Additional
collations may be available depending on operating system support.
The <TT
CLASS="LITERAL"
>default</TT
> collation selects the <TT
CLASS="SYMBOL"
>LC_COLLATE</TT
>
and <TT
CLASS="SYMBOL"
>LC_CTYPE</TT
> values specified at database creation time.
The <TT
CLASS="LITERAL"
>C</TT
> and <TT
CLASS="LITERAL"
>POSIX</TT
> collations both specify
<SPAN
CLASS="QUOTE"
>"traditional C"</SPAN
> behavior, in which only the ASCII letters
<SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>A</TT
>"</SPAN
> through <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>Z</TT
>"</SPAN
>
are treated as letters, and sorting is done strictly by character
code byte values.
</P
><P
> If the operating system provides support for using multiple locales
within a single program (<CODE
CLASS="FUNCTION"
>newlocale</CODE
> and related functions),
then when a database cluster is initialized, <TT
CLASS="COMMAND"
>initdb</TT
>
populates the system catalog <TT
CLASS="LITERAL"
>pg_collation</TT
> with
collations based on all the locales it finds on the operating
system at the time. For example, the operating system might
provide a locale named <TT
CLASS="LITERAL"
>de_DE.utf8</TT
>.
<TT
CLASS="COMMAND"
>initdb</TT
> would then create a collation named
<TT
CLASS="LITERAL"
>de_DE.utf8</TT
> for encoding <TT
CLASS="LITERAL"
>UTF8</TT
>
that has both <TT
CLASS="SYMBOL"
>LC_COLLATE</TT
> and
<TT
CLASS="SYMBOL"
>LC_CTYPE</TT
> set to <TT
CLASS="LITERAL"
>de_DE.utf8</TT
>.
It will also create a collation with the <TT
CLASS="LITERAL"
>.utf8</TT
>
tag stripped off the name. So you could also use the collation
under the name <TT
CLASS="LITERAL"
>de_DE</TT
>, 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.
</P
><P
> In case a collation is needed that has different values for
<TT
CLASS="SYMBOL"
>LC_COLLATE</TT
> and <TT
CLASS="SYMBOL"
>LC_CTYPE</TT
>, a new
collation may be created using
the <A
HREF="sql-createcollation.html"
>CREATE COLLATION</A
> command. That 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.
</P
><P
> Within any particular database, only collations that use that
database's encoding are of interest. Other entries in
<TT
CLASS="LITERAL"
>pg_collation</TT
> are ignored. Thus, a stripped collation
name such as <TT
CLASS="LITERAL"
>de_DE</TT
> 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 <TT
CLASS="LITERAL"
>default</TT
>,
<TT
CLASS="LITERAL"
>C</TT
>, and <TT
CLASS="LITERAL"
>POSIX</TT
> collations can be used
regardless of the database encoding.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> considers distinct collation
objects to be incompatible even when they have identical properties.
Thus for example,
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a COLLATE "C" < b COLLATE "POSIX" FROM test1;</PRE
><P>
will draw an error even though the <TT
CLASS="LITERAL"
>C</TT
> and <TT
CLASS="LITERAL"
>POSIX</TT
>
collations have identical behaviors. Mixing stripped and non-stripped
collation names is therefore not recommended.
</P
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