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11.4.3. The BLOB and TEXT Types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types are TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB. These differ only in the maximum length of the values they can hold. The four TEXT types are TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT. These correspond to the four BLOB types and have the same maximum lengths and storage requirements. See Section 11.5, “Data Type Storage Requirements”. No lettercase conversion for TEXT or BLOB columns takes place during storage or retrieval.

BLOB columns are treated as binary strings (byte strings). TEXT columns are treated as non-binary strings (character strings). BLOB columns have no character set, and sorting and comparison are based on the numeric values of the bytes in column values. TEXT columns have a character set, and values are sorted and compared based on the collation of the character set.

If a 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 BLOB columns.

When not running in strict mode, if you assign a value to a BLOB or TEXT column that exceeds the data type's maximum length, the value is truncated to fit. If the truncated characters are not spaces, a warning is generated. You can cause an error to occur and the value to be rejected rather than to be truncated with a warning by using strict SQL mode. See Section 5.2.5, “The Server SQL Mode”.

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. BLOB and TEXT differ from VARBINARY and VARCHAR in the following ways:

  • For indexes on BLOB and TEXT columns, you must specify an index prefix length. For CHAR and VARCHAR, a prefix length is optional. See Section 7.4.3, “Column Indexes”.

  • BLOB and TEXT columns cannot have DEFAULT values.

LONG and LONG VARCHAR map to the MEDIUMTEXT data type. This is a compatibility feature. If you use the BINARY attribute with a TEXT data type, the column is assigned the binary collation of the column character set.

MySQL Connector/ODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and 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. This value can be changed using the --max_sort_length=N option when starting the mysqld server. See Section 5.2.2, “Server System Variables”.

    You can make more bytes significant in sorting or grouping by increasing the value of max_sort_length at 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;
    

    Another way to use GROUP BY or ORDER BY on a BLOB or TEXT column containing long values when you want more than max_sort_length bytes to be significant is to convert the column value into a fixed-length object. The standard way to do this is with the SUBSTRING function. For example, the following statement causes 2000 bytes of the comment column to be taken into account for sorting:

    mysql> SELECT id, SUBSTRING(comment,1,2000) FROM t
        -> ORDER BY SUBSTRING(comment,1,2000);
    
  • The maximum size of a BLOB or TEXT object 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 allow you to change the client-side max_allowed_packet value. See Section 7.5.2, “Tuning Server Parameters”, Section 8.5, “mysql — The MySQL Command-Line Tool”, and Section 8.10, “mysqldump — A Database Backup Program”.

Each BLOB or 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 BLOB or TEXT columns. You may find MySQL's string handling functions useful for working with such data. See Section 12.3, “String Functions”. For security and other reasons, it is usually preferable to do so using application code rather than allowing application users the FILE privilege. You can discuss specifics for various languages and platforms in the MySQL Forums (http://forums.mysql.com/).


 
 
  Published under the terms of the GNU General Public License Design by Interspire