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Add SQL function CASEFOLD().

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Useful for caseless matching. Similar to LOWER(), but avoids edge-case
problems with using LOWER() for caseless matching.

For collations that support it, CASEFOLD() handles characters with
more than two case variations or multi-character case variations. Some
characters may fold to uppercase. The results of case folding are also
more stable across Unicode versions than LOWER() or UPPER().

Discussion: https://postgr.es/m/a1886ddfcd8f60cb3e905c93009b646b4cfb74c5.camel%40j-davis.com
Reviewed-by: Ian Lawrence Barwick
This commit is contained in:
Jeff Davis
2025-01-24 14:56:22 -08:00
parent f15538cd27
commit bfc5992069
14 changed files with 278 additions and 3 deletions
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46
doc/src/sgml/func.sgml
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@ -2596,7 +2596,7 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<indexterm id="function-lower">
<primary>lower</primary>
</indexterm>
<function>lower</function> ( <type>text</type> )
@ -2657,7 +2657,7 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<indexterm id="function-normalize">
<primary>normalize</primary>
</indexterm>
<indexterm>
@ -3109,6 +3109,48 @@ SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>casefold</primary>
</indexterm>
<function>casefold</function> ( <type>text</type> )
<returnvalue>text</returnvalue>
</para>
<para>
Performs case folding of the input string according to the collation.
Case folding is similar to case conversion, but the purpose of case
folding is to facilitate case-insensitive comparison of strings,
whereas the purpose of case conversion is to convert to a particular
cased form. This function can only be used when the server encoding
is <literal>UTF8</literal>.
</para>
<para>
Ordinarily, case folding simply converts to lowercase, but there are a
few notable exceptions depending on the collation. For instance, the
character <literal>Σ</literal> (U+03A3) has two lowercase forms:
<literal>σ</literal> (U+03C3) and <literal>ς</literal> (U+03C2); case
folding in the <literal>PG_C_UTF8</literal> collation maps all three
forms to <literal>σ</literal>. Additionally, the result is not
necessarily lowercase; some characters may be folded to uppercase.
</para>
<para>
Case folding may change the length of the string. For instance, in
the <literal>PG_UNICODE_FAST</literal> collation, <literal>ß</literal>
(U+00DF) folds to <literal>ss</literal>.
</para>
<para>
<function>casefold</function> can be used for Unicode Default Caseless
Matching. It does not always preserve the normalized form of the
input string (see <xref linkend="function-normalize"/>).
</para>
<para>
The <literal>libc</literal> provider doesn't support case folding, so
<function>casefold</function> is identical to <xref
linkend="function-lower"/>.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>