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>Appendix F. Additional Supplied Modules</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="CITEXT"
>F.7. citext</A
></H1
><P
> The <TT
CLASS="FILENAME"
>citext</TT
> module provides a case-insensitive
character string type, <TT
CLASS="TYPE"
>citext</TT
>. Essentially, it internally calls
<CODE
CLASS="FUNCTION"
>lower</CODE
> when comparing values. Otherwise, it behaves almost
exactly like <TT
CLASS="TYPE"
>text</TT
>.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146765"
>F.7.1. Rationale</A
></H2
><P
> The standard approach to doing case-insensitive matches
in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has been to use the <CODE
CLASS="FUNCTION"
>lower</CODE
>
function when comparing values, for example
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM tab WHERE lower(col) = LOWER(?);</PRE
><P>
</P
><P
> This works reasonably well, but has a number of drawbacks:
</P
><P
></P
><UL
><LI
><P
> It makes your SQL statements verbose, and you always have to remember to
use <CODE
CLASS="FUNCTION"
>lower</CODE
> on both the column and the query value.
</P
></LI
><LI
><P
> It won't use an index, unless you create a functional index using
<CODE
CLASS="FUNCTION"
>lower</CODE
>.
</P
></LI
><LI
><P
> If you declare a column as <TT
CLASS="LITERAL"
>UNIQUE</TT
> or <TT
CLASS="LITERAL"
>PRIMARY
KEY</TT
>, the implicitly generated index is case-sensitive. So it's
useless for case-insensitive searches, and it won't enforce
uniqueness case-insensitively.
</P
></LI
></UL
><P
> The <TT
CLASS="TYPE"
>citext</TT
> data type allows you to eliminate calls
to <CODE
CLASS="FUNCTION"
>lower</CODE
> in SQL queries, and allows a primary key to
be case-insensitive. <TT
CLASS="TYPE"
>citext</TT
> is locale-aware, just
like <TT
CLASS="TYPE"
>text</TT
>, which means that the matching of upper case and
lower case characters is dependent on the rules of
the database's <TT
CLASS="LITERAL"
>LC_CTYPE</TT
> setting. Again, this behavior is
identical to the use of <CODE
CLASS="FUNCTION"
>lower</CODE
> 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.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146790"
>F.7.2. How to Use It</A
></H2
><P
> Here's a simple example of usage:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE users (
nick CITEXT PRIMARY KEY,
pass TEXT NOT NULL
);
INSERT INTO users VALUES ( 'larry', md5(random()::text) );
INSERT INTO users VALUES ( 'Tom', md5(random()::text) );
INSERT INTO users VALUES ( 'Damian', md5(random()::text) );
INSERT INTO users VALUES ( 'NEAL', md5(random()::text) );
INSERT INTO users VALUES ( 'Bjørn', md5(random()::text) );
SELECT * FROM users WHERE nick = 'Larry';</PRE
><P>
The <TT
CLASS="COMMAND"
>SELECT</TT
> statement will return one tuple, even though
the <TT
CLASS="STRUCTFIELD"
>nick</TT
> column was set to <TT
CLASS="LITERAL"
>larry</TT
> and the query
was for <TT
CLASS="LITERAL"
>Larry</TT
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146798"
>F.7.3. String Comparison Behavior</A
></H2
><P
> <TT
CLASS="TYPE"
>citext</TT
> performs comparisons by converting each string to lower
case (as though <CODE
CLASS="FUNCTION"
>lower</CODE
> were called) and then comparing the
results normally. Thus, for example, two strings are considered equal
if <CODE
CLASS="FUNCTION"
>lower</CODE
> would produce identical results for them.
</P
><P
> In order to emulate a case-insensitive collation as closely as possible,
there are <TT
CLASS="TYPE"
>citext</TT
>-specific versions of a number of string-processing
operators and functions. So, for example, the regular expression
operators <TT
CLASS="LITERAL"
>~</TT
> and <TT
CLASS="LITERAL"
>~*</TT
> exhibit the same behavior when
applied to <TT
CLASS="TYPE"
>citext</TT
>: they both match case-insensitively.
The same is true
for <TT
CLASS="LITERAL"
>!~</TT
> and <TT
CLASS="LITERAL"
>!~*</TT
>, as well as for the
<TT
CLASS="LITERAL"
>LIKE</TT
> operators <TT
CLASS="LITERAL"
>~~</TT
> and <TT
CLASS="LITERAL"
>~~*</TT
>, and
<TT
CLASS="LITERAL"
>!~~</TT
> and <TT
CLASS="LITERAL"
>!~~*</TT
>. If you'd like to match
case-sensitively, you can cast the operator's arguments to <TT
CLASS="TYPE"
>text</TT
>.
</P
><P
> Similarly, all of the following functions perform matching
case-insensitively if their arguments are <TT
CLASS="TYPE"
>citext</TT
>:
</P
><P
></P
><UL
><LI
><P
> <CODE
CLASS="FUNCTION"
>regexp_matches()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>regexp_replace()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>regexp_split_to_array()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>regexp_split_to_table()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>replace()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>split_part()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>strpos()</CODE
>
</P
></LI
><LI
><P
> <CODE
CLASS="FUNCTION"
>translate()</CODE
>
</P
></LI
></UL
><P
> For the regexp functions, if you want to match case-sensitively, you can
specify the <SPAN
CLASS="QUOTE"
>"c"</SPAN
> flag to force a case-sensitive match. Otherwise,
you must cast to <TT
CLASS="TYPE"
>text</TT
> before using one of these functions if
you want case-sensitive behavior.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146847"
>F.7.4. Limitations</A
></H2
><P
></P
><UL
><LI
><P
> <TT
CLASS="TYPE"
>citext</TT
>'s case-folding behavior depends on
the <TT
CLASS="LITERAL"
>LC_CTYPE</TT
> 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 <TT
CLASS="TYPE"
>citext</TT
>'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.
</P
></LI
><LI
><P
> As of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.1, you can attach a
<TT
CLASS="LITERAL"
>COLLATE</TT
> specification to <TT
CLASS="TYPE"
>citext</TT
> columns or data
values. Currently, <TT
CLASS="TYPE"
>citext</TT
> operators will honor a non-default
<TT
CLASS="LITERAL"
>COLLATE</TT
> specification while comparing case-folded strings,
but the initial folding to lower case is always done according to the
database's <TT
CLASS="LITERAL"
>LC_CTYPE</TT
> setting (that is, as though
<TT
CLASS="LITERAL"
>COLLATE "default"</TT
> were given). This may be changed in a
future release so that both steps follow the input <TT
CLASS="LITERAL"
>COLLATE</TT
>
specification.
</P
></LI
><LI
><P
> <TT
CLASS="TYPE"
>citext</TT
> is not as efficient as <TT
CLASS="TYPE"
>text</TT
> because the
operator functions and the B-tree comparison functions must make copies
of the data and convert it to lower case for comparisons. It is,
however, slightly more efficient than using <CODE
CLASS="FUNCTION"
>lower</CODE
> to get
case-insensitive matching.
</P
></LI
><LI
><P
> <TT
CLASS="TYPE"
>citext</TT
> 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 <TT
CLASS="TYPE"
>text</TT
> type and
manually use the <CODE
CLASS="FUNCTION"
>lower</CODE
> 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 <TT
CLASS="TYPE"
>citext</TT
> and explicitly casting the column to <TT
CLASS="TYPE"
>text</TT
>
when you want case-sensitive comparison. In either situation, you will
need two indexes if you want both types of searches to be fast.
</P
></LI
><LI
><P
> The schema containing the <TT
CLASS="TYPE"
>citext</TT
> operators must be
in the current <TT
CLASS="VARNAME"
>search_path</TT
> (typically <TT
CLASS="LITERAL"
>public</TT
>);
if it is not, the normal case-sensitive <TT
CLASS="TYPE"
>text</TT
> operators
will be invoked instead.
</P
></LI
></UL
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146883"
>F.7.5. Author</A
></H2
><P
> David E. Wheeler <CODE
CLASS="EMAIL"
><<A
HREF="mailto:david@kineticode.com"
>david@kineticode.com</A
>></CODE
>
</P
><P
> Inspired by the original <TT
CLASS="TYPE"
>citext</TT
> module by Donald Fraser.
</P
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