MySQL :: MySQL 5.1 Reference Manual :: 10.4.3 The BLOB and TEXT Types

MySQL 5.1 Reference Manual :: 10 Data Types :: 10.4 String Types :: 10.4.3 The BLOB and TEXT Types

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10.4.3. 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 10.5, “Data Type Storage Requirements”.

BLOB values are treated as binary strings (byte strings). They have no character set, and sorting and comparison 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, 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 BLOB or 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 5.1.8, “Server SQL Modes”.

Beginning with MySQL 5.1.24, truncation of excess trailing spaces from values to be inserted into TEXT columns always generates a warning, regardless of the SQL mode.

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.

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 7.4.2, “Column Indexes”.
BLOB and TEXT columns cannot have DEFAULT values.

If you use the BINARY attribute with a TEXT data type, the column is assigned the binary collation of the column character set.

LONG and LONG VARCHAR map to the MEDIUMTEXT data type. This is a compatibility feature.

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. 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;
```
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);
```
Instances of BLOB or TEXT columns 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 MEMORY storage engine does not support those data types (see Section 7.5.10, “How MySQL Uses Internal Temporary Tables”). Use of disk incurs a performance penalty, so include BLOB or TEXT columns in the query result only if they are really needed. For example, avoid using SELECT *, which selects all columns.
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 7.5.3, “Tuning Server Parameters”, Section 4.5.1, “mysql — The MySQL Command-Line Tool”, 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 10.5, “Data Type Storage Requirements”

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 11.4, “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/).

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User Comments

Posted by Volnei Puttini on June 22 2007 12:04pm

[Delete] [Edit]

A pratical example of how write and read images into MySQL tables,
using Trolltech Qt4/C++

This example is for who reads/record images in tables
using fields BLOB.

First: Create a table, for example:
CREATE TABLE picture (
ID INTEGER AUTO_INCREMENT,
IMAGE BLOB,
PRIMARY KEY (ID)
) ENGINE=InnoDB;

2) To read a image to a QByteArray

QString fileName = "IMAGE.JPG";

QImage image(filaName);
LBL_IMAGE->setPixmap(QPixmap::fromImage(image)); // Put image into QLabel object (optional)

// load image to bytearray
QByteArray ba;
QFile f(fileName);
if(f.open(QIODevice::ReadOnly))
{
ba = f.readAll();
f.close();
}

// Writing the image into table
QSqlDatabase::database().transaction();
QSqlQuery query;
query.prepare( "INSERT INTO picture ( IMAGE ) VALUES (:IMAGE)" );
query.bindValue(":IMAGE", ba);
query.exec();
if( query.lastError().isValid()) {
qDebug() << query.lastError().text();
QSqlDatabase::database().rollback();
} else
QSqlDatabase::database().commit();

3) Now, recovery the field with the image

int idx = 1; // The records ID to recover

QSqlDatabase::database().transaction();
QSqlQuery query;
query.prepare("SELECT ID, IMAGE FROM picture WHERE ID=:ID");
query.bindValue(":ID", idx);
query.exec();
query.next();
if( query.lastError().isValid()) {
qDebug() << query.lastError().text();
QSqlDatabase::database().rollback();
} else {
QByteArray ba1 = query.value(1).toByteArray();
QPixmap pic;
pic.loadFromData( ba1);

// Show the image into a QLabel object
LBL_IMAGE->setPixmap(pic);
QSqlDatabase::database().commit();
}

This example works fine and I use it frequently.

Thanks.

Posted by Bryce Nesbitt on April 4 2008 4:36pm

[Delete] [Edit]

On MS Windows the "no DEFAULT" rule is an error, while on other platforms it is often a warning. While not a bug, it's possible to get trapped by this if you write code on a lenient platform, and later run it on a strict platform:

mysql> show warnings;

+---------+------+------------------------------------------------------+
| Level   | Code | Message                                              |
+---------+------+------------------------------------------------------+
| Warning | 1101 | BLOB/TEXT column 'abcdef' can't have a default value |
+---------+------+------------------------------------------------------+

Posted by [name withheld] on April 27 2008 11:17am

[Delete] [Edit]

I struggled for some time to utilize mysql's blob column to store images and especially large files with good performance in and out. I found this tutorials implementation very useful: http://www.dreamwerx.net/phpforum/?id=1

Posted by Rajiv Kapoor on December 9 2008 9:46am

[Delete] [Edit]

Following way we can store blob data in a table using MYSQL:
INSERT INTO PICTABLE (MYID, PIC) VALUES (3, LOAD_FILE('/PHP/ME.JPG'));

Posted by Kristian Köhntopp on October 2 2009 3:19pm

[Delete] [Edit]

Simon Mudd is right, but there are several things that must come together to make this bad:

1. You must have a query that has an EXPLAIN which includes 'using temporary'. If 'using temporary' is shown in your EXPLAIN plan, then a temporary table is being created either in MEMORY or as MyISAM table on disk. MySQL prefers MEMORY, but there are situations where it is forced to go to disk.

2. You must have a query which includes any TEXT or BLOB type in the column list, that is in the part of the query between SELECT and FROM. The actual size of the column or its content do not matter - even a TINYTEXT that is empty is enough.

Since the MEMORY storage engine cannot represent any TEXT or BLOB types at all, this forces MySQL to realize the table as an on-disk MyISAM table.

How to diagnose:

1. Run show session status:

kris@localhost [test_world]> show session status like 'Created_tmp%tables';

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 3     |
| Created_tmp_tables      | 7     |
+-------------------------+-------+

2 rows in set (0.00 sec)

2. Execute the query. Make sure it is not cached:

kris@localhost [test_world]> select sql_no_cache * from kris group by countrycode order by population;
...
232 rows in set (0.00 sec)

3. Check show status again:

kris@localhost [test_world]> show session status like 'Created_tmp%tables';

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 4     |
| Created_tmp_tables      | 8     |
+-------------------------+-------+

2 rows in set (0.00 sec)

As you can see the Created_tmp_tables counter increased by one (in MySQL 5.0 is increases by two because the SHOW STATUS itself creates an in-memory tmp table which is being counted). If the table goes to disk as MyISAM instead of being a MEMORY Table, Created_tmp_disk_tables is also incremented by one, as seen here. This is slow.

The test table I used is using the MySQL world database and

mysql> create table kris as select * from City;
mysql> alter table kris modify column name text;
mysql> alter table kris add primary key (id);

The test query shown above is 'using temporary' because I group by one column and order by another, forcing MySQL to use a temporary table.

Had I been using the same query on the original City table from the world database, a tmp table would have been needed as well, but it would have been created as a MEMORY table as the original name column is a CHAR(35). So Created_tmp_tables is being bumped by one, but Created_tmp_disk_tables is not.

Had I been leaving off the SQL_NO_CACHE, the query cache would have been catching repeated executions of the same query in testing and the counters would not have been moving at all except for the very first test.

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10.4.3. The BLOB and TEXT Types

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10.4.3. The `BLOB` and `TEXT` Types