FEDERATED tables may be replicated to other slaves, but you must ensure that the slave servers are able to use the user/password combination that is defined in the CONNECTION string (or the row in the mysql.servers table) to connect to the remote server.

The remote server must be a MySQL server. Support by FEDERATED for other database engines may be added in the future.

The remote table that a FEDERATED table points to must exist before you try to access the table through the FEDERATED table.

It is possible for one FEDERATED table to point to another, but you must be careful not to create a loop.

A FEDERATED table does not support indexes per se. Because access to the table is handled remotely, it is the remote table that supports the indexes. Care should be taken when creating a FEDERATED table since the index definition from an equivalent MyISAM or other table may not be supported. For example, creating a FEDERATED table with an index prefix on VARCHAR, TEXT or BLOB columns will fail. The following definition in MyISAM is valid:

CREATE TABLE `T1`(`A` VARCHAR(100),UNIQUE KEY(`A`(30))) ENGINE=MYISAM;

The key prefix in this example is incompatible with the FEDERATED engine, and the equivalent statement will fail:

CREATE TABLE `T1`(`A` VARCHAR(100),UNIQUE KEY(`A`(30))) ENGINE=FEDERATED
  CONNECTION='MYSQL://127.0.0.1:3306/TEST/T1';

If possible, you should try to separate the column and index definition when creating tables on both the remote server and the local server to avoid these index issues.

Internally, the implementation uses SELECT, INSERT, UPDATE, and DELETE, but not HANDLER.

The FEDERATED storage engine supports SELECT, INSERT, UPDATE, DELETE, TRUNCATE, and indexes. It does not support ALTER TABLE, or any Data Definition Language statements that directly affect the structure of the table, other than DROP TABLE. The current implementation does not use prepared statements.

FEDERATED accepts INSERT ... ON DUPLICATE KEY UPDATE statements, but if a duplicate-key violation occurs, the statement fails with an error.

Performance on a FEDERATED table when performing bulk inserts (for example, on a INSERT INTO ... SELECT ... statement) is slower than with other table types because each selected row is treated as an individual INSERT statement on the FEDERATED table.

Transactions are not supported.

Before MySQL 5.1.21, for a multiple-row insert into a FEDERATED table that refers to a remote transactional table, if the insert failed for a row due to constraint failure, the remote table would contain a partial commit (the rows preceding the failed one) instead of rolling back the statement completely. This occurred because the rows were treated as individual inserts.

As of MySQL 5.1.21, FEDERATED performs bulk-insert handling such that multiple rows are sent to the remote table in a batch. This provides a performance improvement and enables the the remote table to perform improvement. Also, if the remote table is transactional, it enables the remote storage engine to perform statement rollback properly should an error occur. This capability has the following limitations:

There is no way for the FEDERATED engine to know if the remote table has changed. The reason for this is that this table must work like a data file that would never be written to by anything other than the database system. The integrity of the data in the local table could be breached if there was any change to the remote database.

When using a CONNECTION string, you cannot use an '@' character in the password. You can get round this limitation by using the CREATE SERVER statement to create a server connection.

The INSERT_ID and TIMESTAMP options are not propagated to the data provider.

Any DROP TABLE statement issued against a FEDERATED table drops only the local table, not the remote table.

FEDERATED tables do not work with the query cache.

User-defined partitioning is not supported for FEDERATED tables. Beginning with MySQL 5.1.15, it is no longer possible to create such tables at all.