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14.3. The MERGE Storage Engine

The MERGE storage engine, also known as the MRG_MyISAM engine, is a collection of identical MyISAM tables that can be used as one. “Identical” means that all tables have identical column and index information. You cannot merge MyISAM tables in which the columns are listed in a different order, do not have exactly the same columns, or have the indexes in different order. However, any or all of the MyISAM tables can be compressed with myisampack. See Section 8.7, “myisampack — Generate Compressed, Read-Only MyISAM Tables”. Differences in table options such as AVG_ROW_LENGTH, MAX_ROWS, or PACK_KEYS do not matter.

When you create a MERGE table, MySQL creates two files on disk. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format, and an .MRG file contains the names of the tables that should be used as one. The tables do not have to be in the same database as the MERGE table itself.

Starting with MySQL 5.0.36 the underlying table definitions and indexes must conform more closely to the definition of the MERGE table. Conformance will be checked when the merged tables are opened, not when the MERGE table is created. This means that changes to the definitions of tables within a MERGE may cause a failure when the MERGE table is accessed.

You can use SELECT, DELETE, UPDATE, and INSERT on MERGE tables. You must have SELECT, UPDATE, and DELETE privileges on the MyISAM tables that you map to a MERGE table.


The use of MERGE tables entails the following security issue: If a user has access to MyISAM table t, that user can create a MERGE table m that accesses t. However, if the user's privileges on t are subsequently revoked, the user can continue to access t by doing so through m. If this behavior is undesirable, you can start the server with the new --skip-merge option to disable the MERGE storage engine. This option is available as of MySQL 5.0.24.

If you DROP the MERGE table, you are dropping only the MERGE specification. The underlying tables are not affected.

To create a MERGE table, you must specify a UNION=(list-of-tables) clause that indicates which MyISAM tables you want to use as one. You can optionally specify an INSERT_METHOD option if you want inserts for the MERGE table to take place in the first or last table of the UNION list. Use a value of FIRST or LAST to cause inserts to be made in the first or last table, respectively. If you do not specify an INSERT_METHOD option or if you specify it with a value of NO, attempts to insert rows into the MERGE table result in an error.

The following example shows how to create a MERGE table:

mysql> CREATE TABLE t1 (
    ->    message CHAR(20)) ENGINE=MyISAM;
mysql> CREATE TABLE t2 (
    ->    message CHAR(20)) ENGINE=MyISAM;
mysql> INSERT INTO t1 (message) VALUES ('Testing'),('table'),('t1');
mysql> INSERT INTO t2 (message) VALUES ('Testing'),('table'),('t2');
mysql> CREATE TABLE total (
    ->    message CHAR(20), INDEX(a))

The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

Note that the a column is indexed as a PRIMARY KEY in the underlying MyISAM tables, but not in the MERGE table. There it is indexed but not as a PRIMARY KEY because a MERGE table cannot enforce uniqueness over the set of underlying tables.

In MySQL 5.0.36 and higher, when a table that is part of a MERGE table is opened, the following checks are applied before opening each table. If any table fails the conformance checks, then the operation that triggered the opening of the table will fail. The conformance checks applied to each table are:

After creating the MERGE table, you can issue queries that operate on the group of tables as a whole:

mysql> SELECT * FROM total;
| a | message |
| 1 | Testing |
| 2 | table   |
| 3 | t1      |
| 1 | Testing |
| 2 | table   |
| 3 | t2      |

To remap a MERGE table to a different collection of MyISAM tables, you can use one of the following methods:

MERGE tables can help you solve the following problems:

The disadvantages of MERGE tables are:

Additional resources

14.3.1. MERGE Table Problems

The following are known problems with MERGE tables:

  • If you use ALTER TABLE to change a MERGE table to another storage engine, the mapping to the underlying tables is lost. Instead, the rows from the underlying MyISAM tables are copied into the altered table, which then uses the specified storage engine.

  • REPLACE does not work.

  • You cannot use REPAIR TABLE, OPTIMIZE TABLE, DROP TABLE, ALTER TABLE, DELETE without a WHERE clause, TRUNCATE TABLE, or ANALYZE TABLE on any of the tables that are mapped into an open MERGE table. If you do so, the MERGE table may still refer to the original table, which yields unexpected results. The easiest way to work around this deficiency is to ensure that no MERGE tables remain open by issuing a FLUSH TABLES statement prior to performing any of those operations.

    The unexpected results include the possibility that the operation on the MERGE table will report table corruption. However, if this occurs after operations on the underlying MyISAM tables such as those listed in the previous paragraph (REPAIR TABLE, OPTIMIZE TABLE, and so forth), the corruption message is spurious. To deal with this, issue a FLUSH TABLES statement after modifying the MyISAM tables.

  • DROP TABLE on a table that is in use by a MERGE table does not work on Windows because the MERGE storage engine's table mapping is hidden from the upper layer of MySQL. Windows does not allow open files to be deleted, so you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table.

  • A MERGE table cannot maintain uniqueness constraints over the entire table. When you perform an INSERT, the data goes into the first or last MyISAM table (depending on the value of the INSERT_METHOD option). MySQL ensures that unique key values remain unique within that MyISAM table, but not across all the tables in the collection.

  • In MySQL 5.0.36 and later, the definition of the MyISAM tables and the MERGE table are checked when the tables are accessed (for example, as part of a SELECT or INSERT statement). The checks ensure that the definitions of the tables and the parent MERGE table definition match by comparing column order, types, sizes and associated indexes. If there is a difference between the tables then an error will be returned and the statement will fail.

    Because these checks take place when the tables are opened, any changes to the definition of a single, including column changes, ocolumn ordering and engine alterations will cause the statement to fail.

    In MySQL 5.0.35 and earlier:

    • When you create or alter MERGE table, there is no check to ensure that the underlying tables are existing MyISAM tables and have identical structures. When the MERGE table is used, MySQL checks that the row length for all mapped tables is equal, but this is not foolproof. If you create a MERGE table from dissimilar MyISAM tables, you are very likely to run into strange problems.

    • Similarly, if you create a MERGE table from non-MyISAM tables, or if you drop an underlying table or alter it to be a non-MyISAM table, no error for the MERGE table occurs until later when you attempt to use it.

    • Because the underlying MyISAM tables need not exist when the MERGE table is created, you can create the tables in any order, as long as you do not use the MERGE table until all of its underlying tables are in place. Also, if you can ensure that a MERGE table will not be used during a given period, you can perform maintenance operations on the underlying tables, such as backing up or restoring them, altering them, or dropping and recreating them. It is not necessary to redefine the MERGE table temporarily to exclude the underlying tables while you are operating on them.

  • The order of indexes in the MERGE table and its underlying tables should be the same. If you use ALTER TABLE to add a UNIQUE index to a table used in a MERGE table, and then use ALTER TABLE to add a non-unique index on the MERGE table, the index ordering is different for the tables if there was already a non-unique index in the underlying table. (This happens because ALTER TABLE puts UNIQUE indexes before non-unique indexes to facilitate rapid detection of duplicate keys.) Consequently, queries on tables with such indexes may return unexpected results.

  • If you encounter an error message similar to ERROR 1017 (HY000): Can't find file: 'mm.MRG' (errno: 2) it generally indicates that some of the base tables are not using the MyISAM storage engine. Confirm that all tables are MyISAM.

  • There is a limit of 232 (~4.295E+09)) rows to a MERGE table, just as there is with a MyISAM, it is therefore not possible to merge multiple MyISAM tables that exceed this limitation. However, you build MySQL with the --with-big-tables option then the row limitation is increased to (232)2 (1.844E+19) rows. See Section, “Typical configure Options”. Beginning with MySQL 5.0.4 all standard binaries are built with this option.

  • The MERGE storage engine does not support INSERT DELAYED statements.

As of MySQL 5.0.44, if a MERGE table cannot be opened or used because of a problem with an underlying table, CHECK TABLE displays information about which table caused the problem.

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