Adding upstream version 15.5.upstream/15.5

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 293 insertions, 0 deletions

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+<!-- doc/src/sgml/citext.sgml -->
+
+<sect1 id="citext" xreflabel="citext">
+ <title>citext</title>
+
+ <indexterm zone="citext">
+  <primary>citext</primary>
+ </indexterm>
+
+ <para>
+  The <filename>citext</filename> module provides a case-insensitive
+  character string type, <type>citext</type>. Essentially, it internally calls
+  <function>lower</function> when comparing values. Otherwise, it behaves almost
+  exactly like <type>text</type>.
+ </para>
+
+ <tip>
+  <para>
+   Consider using <firstterm>nondeterministic collations</firstterm> (see
+   <xref linkend="collation-nondeterministic"/>) instead of this module.  They
+   can be used for case-insensitive comparisons, accent-insensitive
+   comparisons, and other combinations, and they handle more Unicode special
+   cases correctly.
+  </para>
+ </tip>
+
+ <para>
+  This module is considered <quote>trusted</quote>, that is, it can be
+  installed by non-superusers who have <literal>CREATE</literal> privilege
+  on the current database.
+ </para>
+
+ <sect2>
+  <title>Rationale</title>
+
+  <para>
+   The standard approach to doing case-insensitive matches
+   in <productname>PostgreSQL</productname> has been to use the <function>lower</function>
+   function when comparing values, for example
+
+<programlisting>
+SELECT * FROM tab WHERE lower(col) = LOWER(?);
+</programlisting>
+  </para>
+
+  <para>
+   This works reasonably well, but has a number of drawbacks:
+  </para>
+
+   <itemizedlist>
+    <listitem>
+     <para>
+      It makes your SQL statements verbose, and you always have to remember to
+      use <function>lower</function> on both the column and the query value.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      It won't use an index, unless you create a functional index using
+      <function>lower</function>.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      If you declare a column as <literal>UNIQUE</literal> or <literal>PRIMARY
+      KEY</literal>, the implicitly generated index is case-sensitive.  So it's
+      useless for case-insensitive searches, and it won't enforce
+      uniqueness case-insensitively.
+     </para>
+    </listitem>
+   </itemizedlist>
+
+   <para>
+    The <type>citext</type> data type allows you to eliminate calls
+    to <function>lower</function> in SQL queries, and allows a primary key to
+    be case-insensitive. <type>citext</type> is locale-aware, just
+    like <type>text</type>, which means that the matching of upper case and
+    lower case characters is dependent on the rules of
+    the database's <literal>LC_CTYPE</literal> setting. Again, this behavior is
+    identical to the use of <function>lower</function> in queries. But because it's
+    done transparently by the data type, you don't have to remember to do
+    anything special in your queries.
+   </para>
+
+ </sect2>
+
+ <sect2>
+  <title>How to Use It</title>
+
+  <para>
+   Here's a simple example of usage:
+
+<programlisting>
+CREATE TABLE users (
+    nick CITEXT PRIMARY KEY,
+    pass TEXT   NOT NULL
+);
+
+INSERT INTO users VALUES ( 'larry',  sha256(random()::text::bytea) );
+INSERT INTO users VALUES ( 'Tom',    sha256(random()::text::bytea) );
+INSERT INTO users VALUES ( 'Damian', sha256(random()::text::bytea) );
+INSERT INTO users VALUES ( 'NEAL',   sha256(random()::text::bytea) );
+INSERT INTO users VALUES ( 'Bj&oslash;rn',  sha256(random()::text::bytea) );
+
+SELECT * FROM users WHERE nick = 'Larry';
+</programlisting>
+
+   The <command>SELECT</command> statement will return one tuple, even though
+   the <structfield>nick</structfield> column was set to <literal>larry</literal> and the query
+   was for <literal>Larry</literal>.
+  </para>
+ </sect2>
+
+ <sect2>
+  <title>String Comparison Behavior</title>
+
+  <para>
+   <type>citext</type> performs comparisons by converting each string to lower
+   case (as though <function>lower</function> were called) and then comparing the
+   results normally.  Thus, for example, two strings are considered equal
+   if <function>lower</function> would produce identical results for them.
+  </para>
+
+  <para>
+   In order to emulate a case-insensitive collation as closely as possible,
+   there are <type>citext</type>-specific versions of a number of string-processing
+   operators and functions.  So, for example, the regular expression
+   operators <literal>~</literal> and <literal>~*</literal> exhibit the same behavior when
+   applied to <type>citext</type>: they both match case-insensitively.
+   The same is true
+   for <literal>!~</literal> and <literal>!~*</literal>, as well as for the
+   <literal>LIKE</literal> operators <literal>~~</literal> and <literal>~~*</literal>, and
+   <literal>!~~</literal> and <literal>!~~*</literal>. If you'd like to match
+   case-sensitively, you can cast the operator's arguments to <type>text</type>.
+  </para>
+
+  <para>
+   Similarly, all of the following functions perform matching
+   case-insensitively if their arguments are <type>citext</type>:
+  </para>
+
+  <itemizedlist>
+   <listitem>
+    <para>
+      <function>regexp_match()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>regexp_matches()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>regexp_replace()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>regexp_split_to_array()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>regexp_split_to_table()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>replace()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>split_part()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>strpos()</function>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+      <function>translate()</function>
+    </para>
+   </listitem>
+  </itemizedlist>
+
+  <para>
+   For the regexp functions, if you want to match case-sensitively, you can
+   specify the <quote>c</quote> flag to force a case-sensitive match.  Otherwise,
+   you must cast to <type>text</type> before using one of these functions if
+   you want case-sensitive behavior.
+  </para>
+
+ </sect2>
+
+ <sect2>
+  <title>Limitations</title>
+
+   <itemizedlist>
+    <listitem>
+     <para>
+      <type>citext</type>'s case-folding behavior depends on
+      the <literal>LC_CTYPE</literal> setting of your database. How it compares
+      values is therefore determined when the database is created.
+      It is not truly
+      case-insensitive in the terms defined by the Unicode standard.
+      Effectively, what this means is that, as long as you're happy with your
+      collation, you should be happy with <type>citext</type>'s comparisons. But
+      if you have data in different languages stored in your database, users
+      of one language may find their query results are not as expected if the
+      collation is for another language.
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      As of <productname>PostgreSQL</productname> 9.1, you can attach a
+      <literal>COLLATE</literal> specification to <type>citext</type> columns or data
+      values.  Currently, <type>citext</type> operators will honor a non-default
+      <literal>COLLATE</literal> specification while comparing case-folded strings,
+      but the initial folding to lower case is always done according to the
+      database's <literal>LC_CTYPE</literal> setting (that is, as though
+      <literal>COLLATE "default"</literal> were given).  This may be changed in a
+      future release so that both steps follow the input <literal>COLLATE</literal>
+      specification.
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+       <type>citext</type> is not as efficient as <type>text</type> because the
+       operator functions and the B-tree comparison functions must make copies
+       of the data and convert it to lower case for comparisons.  Also, only
+       <type>text</type> can support B-Tree deduplication.  However,
+       <type>citext</type> is slightly more efficient than using
+       <function>lower</function> to get case-insensitive matching.
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      <type>citext</type> doesn't help much if you need data to compare
+      case-sensitively in some contexts and case-insensitively in other
+      contexts.  The standard answer is to use the <type>text</type> type and
+      manually use the <function>lower</function> function when you need to compare
+      case-insensitively; this works all right if case-insensitive comparison
+      is needed only infrequently.  If you need case-insensitive behavior most
+      of the time and case-sensitive infrequently, consider storing the data
+      as <type>citext</type> and explicitly casting the column to <type>text</type>
+      when you want case-sensitive comparison.  In either situation, you will
+      need two indexes if you want both types of searches to be fast.
+    </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      The schema containing the <type>citext</type> operators must be
+      in the current <varname>search_path</varname> (typically <literal>public</literal>);
+      if it is not, the normal case-sensitive <type>text</type> operators
+      will be invoked instead.
+    </para>
+    </listitem>
+
+    <listitem>
+     <para>
+      The approach of lower-casing strings for comparison does not handle some
+      Unicode special cases correctly, for example when one upper-case letter
+      has two lower-case letter equivalents.  Unicode distinguishes between
+      <firstterm>case mapping</firstterm> and <firstterm>case
+      folding</firstterm> for this reason.  Use nondeterministic collations
+      instead of <type>citext</type> to handle that correctly.
+     </para>
+    </listitem>
+   </itemizedlist>
+ </sect2>
+
+ <sect2>
+  <title>Author</title>
+
+  <para>
+   David E. Wheeler <email>david@kineticode.com</email>
+  </para>
+
+  <para>
+    Inspired by the original <type>citext</type> module by Donald Fraser.
+  </para>
+
+ </sect2>
+
+</sect1>