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
on both the column and the query value.
It won't use an index, unless you create a functional
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', 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.
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
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:
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. It is,
however, slightly more efficient than using
lower to get case-insensitive
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
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
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.
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