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.
Consider using nondeterministic collations (see Section 24.2.2.4) 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.
This module is considered “trusted”, that is, it can be
installed by non-superusers who have CREATE privilege
on the current database.
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 matching of upper case and
lower case characters is dependent on the rules of
the database's LC_CTYPE 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', 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 ( 'Bjørn', sha256(random()::text::bytea) );
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.
citext performs comparisons by converting each string to lower
case (as though lower were called) and then comparing the
results normally. Thus, for example, two strings are considered equal
if lower would produce identical results for them.
In order to emulate a case-insensitive collation as closely as possible,
there are citext-specific versions of a number of string-processing
operators and functions. So, for example, the regular expression
operators ~ and ~* exhibit the same behavior when
applied to citext: they both match 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 cast the operator's arguments to text.
Similarly, all of the following functions perform matching
case-insensitively if their arguments are citext:
regexp_match()
regexp_matches()
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 case-folding behavior depends on
the LC_CTYPE 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 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.
As of PostgreSQL 9.1, you can attach a
COLLATE specification to citext columns or data
values. Currently, citext operators will honor a non-default
COLLATE specification while comparing case-folded strings,
but the initial folding to lower case is always done according to the
database's LC_CTYPE setting (that is, as though
COLLATE "default" were given). This may be changed in a
future release so that both steps follow the input COLLATE
specification.
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. Also, only
text can support B-Tree deduplication. However,
citext is 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 behavior 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, the normal case-sensitive text operators
will be invoked instead.
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
case mapping and case
folding for this reason. Use nondeterministic collations
instead of citext to handle that correctly.
David E. Wheeler <david@kineticode.com>
Inspired by the original citext module by Donald Fraser.