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The citext module provides a case-insensitive
character string type, citext. Essentially, it internally calls
lower when comparing values. Otherwise, it behaves almost
exactly like text.
The standard approach to doing case-insensitive matches
in PostgreSQL has been to use the lower
function when comparing values, for example
SELECT * FROM tab WHERE lower(col) = LOWER(?);
This works reasonably well, but has a number of drawbacks:
It makes your SQL statements verbose, and you always have to remember to
use lower on both the column and the query value.
It won't use an index, unless you create a functional index using
lower.
If you declare a column as UNIQUE or PRIMARY KEY, the implicitly generated index is case-sensitive. So it's useless for case-insensitive searches, and it won't enforce uniqueness case-insensitively.
The citext data type allows you to eliminate calls
to lower in SQL queries, and allows a primary key to
be case-insensitive. citext is locale-aware, just
like text, which means that the comparison of upper case and
lower case characters is dependent on the rules of
the LC_CTYPE locale setting. Again, this behavior is
identical to the use of lower 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.
Here's a simple example of usage:
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';
The SELECT statement will return one tuple, even though the nick column was set to larry and the query was for Larry.
In order to emulate a case-insensitive collation as closely as possible, there are citext-specific versions of a number of the comparison operators and functions. So, for example, the regular expression operators ~ and ~* exhibit the same behavior when applied to citext: they both compare case-insensitively. The same is true for !~ and !~*, as well as for the LIKE operators ~~ and ~~*, and !~~ and !~~*. If you'd like to match case-sensitively, you can always cast to text before comparing.
Similarly, all of the following functions perform matching case-insensitively if their arguments are citext:
regexp_replace()
regexp_split_to_array()
regexp_split_to_table()
replace()
split_part()
strpos()
translate()
For the regexp functions, if you want to match case-sensitively, you can specify the "c" flag to force a case-sensitive match. Otherwise, you must cast to text before using one of these functions if you want case-sensitive behavior.
citext's behavior depends on the LC_CTYPE setting of your database. How it compares values is therefore determined when initdb is run to create the cluster. 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 citext'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.
citext is not as efficient as text 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 lower to get
case-insensitive matching.
citext 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 text type and
manually use the lower 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 most of
the time and case-sensitive infrequently, consider storing the data
as citext and explicitly casting the column to text
when you want case-sensitive comparison. In either situation, you
will need two indexes if you want both types of searches to be fast.
The schema containing the citext operators must be in the current search_path (typically public); if it is not, a normal case-sensitive text comparison is performed.
David E. Wheeler <david@kineticode.com>
Inspired by the original citext module by Donald Fraser.