Table 8.4. Character Types
||variable-length with limit|
||fixed-length, blank padded|
||variable unlimited length|
Table 8.4shows the general-purpose character types available inPostgreSQL.
SQLdefines two primary character types:
n_is a positive integer. Both of these types can store strings up to
n_characters (not bytes) in length. An attempt to store a longer string into a column of these types will result in an error, unless the excess characters are all spaces, in which case the string will be truncated to the maximum length. (This somewhat bizarre exception is required by theSQLstandard.) If the string to be stored is shorter than the declared length, values of type
characterwill be space-padded; values of type
character varyingwill simply store the shorter string.
If one explicitly casts a value to
n), then an over-length value will be truncated to_
n_characters without raising an error. (This too is required by theSQLstandard.)
n)are aliases for
characterwithout length specifier is equivalent to
character varyingis used without length specifier, the type accepts strings of any size. The latter is aPostgreSQLextension.
In addition,PostgreSQLprovides the
texttype, which stores strings of any length. Although the type
textis not in theSQLstandard, several other SQL database management systems have it as well.
Values of type
characterare physically padded with spaces to the specified width
n, and are stored and displayed that way. However, trailing spaces are treated as semantically insignificant and disregarded when comparing two values of type
character. In collations where whitespace is significant, this behavior can produce unexpected results; for example
SELECT 'a '::CHAR(2) collate "C" < E'a\n'::CHAR(2)returns true, even though
Clocale would consider a space to be greater than a newline. Trailing spaces are removed when converting a
charactervalue to one of the other string types. Note that trailing spaces_are_semantically significant in
textvalues, and when using pattern matching, that is
LIKEand regular expressions.
The storage requirement for a short string (up to 126 bytes) is 1 byte plus the actual string, which includes the space padding in the case of
character. Longer strings have 4 bytes of overhead instead of 1. Long strings are compressed by the system automatically, so the physical requirement on disk might be less. Very long values are also stored in background tables so that they do not interfere with rapid access to shorter column values. In any case, the longest possible character string that can be stored is about 1 GB. (The maximum value that will be allowed for_
n_in the data type declaration is less than that. It wouldn't be useful to change this because with multibyte character encodings the number of characters and bytes can be quite different. If you desire to store long strings with no specific upper limit, use
character varyingwithout a length specifier, rather than making up an arbitrary length limit.)
There is no performance difference among these three types, apart from increased storage space when using the blank-padded type, and a few extra CPU cycles to check the length when storing into a length-constrained column. While
n)has performance advantages in some other database systems, there is no such advantage inPostgreSQL; in fact
n)is usually the slowest of the three because of its additional storage costs. In most situations
character varyingshould be used instead.
Refer toSection 220.127.116.11for information about the syntax of string literals, and toChapter 9for information about available operators and functions. The database character set determines the character set used to store textual values; for more information on character set support, refer toSection 23.3.
Example 8.1. Using the Character Types
CREATE TABLE test1 (a character(4)); INSERT INTO test1 VALUES ('ok'); SELECT a, char_length(a) FROM test1; -- (1) a | char_length ------+------------- ok | 2 CREATE TABLE test2 (b varchar(5)); INSERT INTO test2 VALUES ('ok'); INSERT INTO test2 VALUES ('good '); INSERT INTO test2 VALUES ('too long'); ERROR: value too long for type character varying(5) INSERT INTO test2 VALUES ('too long'::varchar(5)); -- explicit truncation SELECT b, char_length(b) FROM test2; b | char_length -------+------------- ok | 2 good | 5 too l | 5
There are two other fixed-length character types inPostgreSQL, shown inTable 8.5. The
nametype exists_only_for the storage of identifiers in the internal system catalogs and is not intended for use by the general user. Its length is currently defined as 64 bytes (63 usable characters plus terminator) but should be referenced using the constant
Csource code. The length is set at compile time (and is therefore adjustable for special uses); the default maximum length might change in a future release. The type
"char"(note the quotes) is different from
char(1)in that it only uses one byte of storage. It is internally used in the system catalogs as a simplistic enumeration type.
Table 8.5. Special Character Types
||1 byte||single-byte internal type|
||64 bytes||internal type for object names|