The Ultimate Guide to High Availability with MariaDB

VARCHAR

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Syntax

[NATIONAL] VARCHAR(M) [CHARACTER SET charset_name] [COLLATE collation_name]

Description

A variable-length string. M represents the maximum column length in characters. The range of M is 0 to 65,535. The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used. For example, utf8 characters can require up to three bytes per character, so a VARCHAR column that uses the utf8 character set can be declared to be a maximum of 21,844 characters.

MariaDB stores VARCHAR values as a one-byte or two-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A VARCHAR column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.

Note: MariaDB 5.1 and later follow the standard SQL specification, and do not remove trailing spaces from VARCHAR values.

VARCHAR(0) columns can contain 2 values: an empty string or NULL. Such columns cannot be part of an index. The CONNECT storage engine does not support VARCHAR(0).

VARCHAR is shorthand for CHARACTER VARYING. NATIONAL VARCHAR is the standard SQL way to define that a VARCHAR column should use some predefined character set. MariaDB uses utf8 as this predefined character set, as does MySQL 4.1 and up. NVARCHAR is shorthand for NATIONAL VARCHAR.

Currently, all MariaDB collations are of type PADSPACE, meaning that VARCHAR (as well as CHAR and TEXT values) are compared without regard for trailing spaces. This does not apply to the LIKE pattern-matching operator, which takes into account trailing spaces.

If a unique index consists of a column where trailing pad characters are stripped or ignored, inserts into that column where values differ only by the number of trailing pad characters will result in a duplicate-key error.

Examples

The following are equivalent:

VARCHAR(30) CHARACTER SET utf8
NATIONAL VARCHAR(30)
NVARCHAR(30)
NCHAR VARCHAR(30)
NATIONAL CHARACTER VARYING(30)
NATIONAL CHAR VARYING(30)

Trailing spaces:

CREATE TABLE strtest (v VARCHAR(10));
INSERT INTO strtest VALUES('Maria   ');

SELECT v='Maria',v='Maria   ' FROM strtest;
+-----------+--------------+
| v='Maria' | v='Maria   ' |
+-----------+--------------+
|         1 |            1 |
+-----------+--------------+

SELECT v LIKE 'Maria',v LIKE 'Maria   ' FROM strtest;
+----------------+-------------------+
| v LIKE 'Maria' | v LIKE 'Maria   ' |
+----------------+-------------------+
|              0 |                 1 |
+----------------+-------------------+

Difference between VARCHAR and TEXT

  • VARCHAR columns can be fully indexed. TEXT columns can only be indexed over a specified length.

For storage engine developers

  • Internally in the full length of the VARCHAR ciolumn is allocated inside each TABLE objects record[] structure. As there are 3 such buffers, each open table will 3 times max-length-to-store-varchar bytes of memory.
  • TEXT and BLOB columns are stored with a pointer (4 or 8 bytes) + a 1-4 byte length. The TEXT data is only stored once. This means that internally TEXT uses less memory for each open table but has instead the additional overhead that each TEXT object needs to be allocated & freed for each row access (with some caching in between).

See Also

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