11.3.4 The BLOB and TEXT Types
BLOB is a binary large object that can hold a variable amount of data. The four
BLOB types are
MEDIUMBLOB , and
LONGBLOB . These differ only in the maximum length of the values they can hold. The four
TEXT types are
MEDIUMTEXT , and
LONGTEXT . These correspond to the four
BLOB types and have the same maximum lengths and storage requirements. See Section 11.7, “Data Type Storage Requirements”.
BLOB values are treated as binary strings (byte strings). They have the
binary character set and collation, and comparison and sorting are based on the numeric values of the bytes in column values.
TEXT values are treated as nonbinary strings (character strings). They have a character set other than
binary , and values are sorted and compared based on the collation of the character set.
If strict SQL mode is not enabled and you assign a value to a
TEXT column that exceeds the column's maximum length, the value is truncated to fit and a warning is generated. For truncation of nonspace characters, you can cause an error to occur (rather than a warning) and suppress insertion of the value by using strict SQL mode. See Section 5.1.10, “Server SQL Modes”.
Truncation of excess trailing spaces from values to be inserted into TEXT columns always generates a warning, regardless of the SQL mode.
BLOB columns, there is no padding on insert and no bytes are stripped on select.
TEXT column is indexed, index entry comparisons are space-padded at the end. This means that, if the index requires unique values, duplicate-key errors will occur for values that differ only in the number of trailing spaces. For example, if a table contains
'a' , an attempt to store
'a ' causes a duplicate-key error. This is not true for
In most respects, you can regard a
BLOB column as a VARBINARY column that can be as large as you like. Similarly, you can regard a
TEXT column as a VARCHAR column.
TEXT differ from VARBINARY and VARCHAR in the following ways:
If you use the
BINARY attribute with a
TEXT data type, the column is assigned the binary (
_bin ) collation of the column character set.
LONG VARCHAR map to the
MEDIUMTEXT data type. This is a compatibility feature.
MySQL Connector/ODBC defines
BLOB values as
TEXT values as
TEXT values can be extremely long, you might encounter some constraints in using them:
- Only the first max_sort_length bytes of the column are used when sorting. The default value of max_sort_length is 1024. You can make more bytes significant in sorting or grouping by increasing the value of max_sort_length at server startup or runtime. Any client can change the value of its session max_sort_length variable:
mysql> SET max_sort_length = 2000; mysql> SELECT id, comment FROM t -> ORDER BY comment;
TEXTcolumns in the result of a query that is processed using a temporary table causes the server to use a table on disk rather than in memory because the
MEMORYstorage engine does not support those data types (see Section 8.4.4, “Internal Temporary Table Use in MySQL”). Use of disk incurs a performance penalty, so include
TEXTcolumns in the query result only if they are really needed. For example, avoid using SELECT *, which selects all columns.
The maximum size of a
TEXTobject is determined by its type, but the largest value you actually can transmit between the client and server is determined by the amount of available memory and the size of the communications buffers. You can change the message buffer size by changing the value of the max_allowed_packet variable, but you must do so for both the server and your client program. For example, both mysql and mysqldump enable you to change the client-side max_allowed_packet value. See Section 5.1.1, “Configuring the Server”, Section 4.5.1, “mysql — The MySQL Command-Line Client”, and Section 4.5.4, “mysqldump — A Database Backup Program”. You may also want to compare the packet sizes and the size of the data objects you are storing with the storage requirements, see Section 11.7, “Data Type Storage Requirements”
TEXT value is represented internally by a separately allocated object. This is in contrast to all other data types, for which storage is allocated once per column when the table is opened.
In some cases, it may be desirable to store binary data such as media files in
TEXT columns. You may find MySQL's string handling functions useful for working with such data. See Section 12.7, “String Functions and Operators”. For security and other reasons, it is usually preferable to do so using application code rather than giving application users the FILE privilege. You can discuss specifics for various languages and platforms in the MySQL Forums (http://forums.mysql.com/).