14.7.2 How to Use FEDERATED Tables

The procedure for using FEDERATED tables is very simple. Normally, you have two servers running, either both on the same host or on different hosts. (It is possible for a FEDERATED table to use another table that is managed by the same server, although there is little point in doing so.)

First, you must have a table on the remote server that you want to access by using a FEDERATED table. Suppose that the remote table is in the federated database and is defined like this:

CREATE TABLE test_table (
    id     INT(20) NOT NULL AUTO_INCREMENT,
    name   VARCHAR(32) NOT NULL DEFAULT '',
    other  INT(20) NOT NULL DEFAULT '0',
    PRIMARY KEY  (id),
    INDEX name (name),
    INDEX other_key (other)
)
ENGINE=MyISAM
DEFAULT CHARSET=latin1;

The example uses a MyISAM table, but the table could use any storage engine.

Next, create a FEDERATED table on the local server for accessing the remote table:

CREATE TABLE federated_table (
    id     INT(20) NOT NULL AUTO_INCREMENT,
    name   VARCHAR(32) NOT NULL DEFAULT '',
    other  INT(20) NOT NULL DEFAULT '0',
    PRIMARY KEY  (id),
    INDEX name (name),
    INDEX other_key (other)
)
ENGINE=FEDERATED
DEFAULT CHARSET=latin1
CONNECTION='mysql://fed_user@remote_host:9306/federated/test_table';

(Before MySQL 5.0.13, use COMMENT rather than CONNECTION.)

The basic structure of this table should match that of the remote table, except that the ENGINE table option should be FEDERATED and the CONNECTION table option is a connection string that indicates to the FEDERATED engine how to connect to the remote server.

Note

You can improve the performance of a FEDERATED table by adding indexes to the table on the host. The optimization will occur because the query sent to the remote server will include the contents of the WHERE clause and will be sent to the remote server and subsequently executed locally. This reduces the network traffic that would otherwise request the entire table from the server for local processing.

The FEDERATED engine creates only the test_table.frm file in the federated database.

The remote host information indicates the remote server to which your local server connects, and the database and table information indicates which remote table to use as the data source. In this example, the remote server is indicated to be running as remote_host on port 9306, so there must be a MySQL server running on the remote host and listening to port 9306.

The general format of the connection string in the CONNECTION option is as follows:

scheme://user_name[:password]@host_name[:port_num]/db_name/tbl_name

Only mysql is supported as the scheme value at this point; the password and port number are optional.

Sample connection strings:

CONNECTION='mysql://username:password@hostname:port/database/tablename'
CONNECTION='mysql://username@hostname/database/tablename'
CONNECTION='mysql://username:password@hostname/database/tablename'

The use of CONNECTION for specifying the connection string is nonoptimal and is likely to change in future. Keep this in mind for applications that use FEDERATED tables. Such applications are likely to need modification if the format for specifying connection information changes.

Because any password given in the connection string is stored as plain text, it can be seen by any user who can use SHOW CREATE TABLE or SHOW TABLE STATUS for the FEDERATED table, or query the TABLES table in the INFORMATION_SCHEMA database.