Spec-Zone .ru
спецификации, руководства, описания, API
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Spec-Zone .ru
спецификации, руководства, описания, API
|
When you create a table using one of the standard storage engines (such as MyISAM,
CSV or InnoDB), the table consists of the table
definition and the associated data. When you create a FEDERATED table, the table
definition is the same, but the physical storage of the data is handled on a remote server.
A FEDERATED table consists of two elements:
A remote server with a database table, which
in turn consists of the table definition (stored in the .frm file) and the
associated table. The table type of the remote table may be any type supported by the remote mysqld server, including MyISAM or InnoDB.
A local server with a database table, where
the table definition matches that of the corresponding table on the remote server. The table definition
is stored within the .frm file. However, there is no data file on the
local server. Instead, the table definition includes a connection string that points to the remote
table.
When executing queries and statements on a FEDERATED table on the local server, the
operations that would normally insert, update or delete information from a local data file are instead sent to
the remote server for execution, where they update the data file on the remote server or return matching rows
from the remote server.
The basic structure of a FEDERATED table setup is shown in Figure
14.1, "FEDERATED Table Structure".
When a client issues an SQL statement that refers to a FEDERATED table, the flow of
information between the local server (where the SQL statement is executed) and the remote server (where the data
is physically stored) is as follows:
The storage engine looks through each column that the FEDERATED
table has and constructs an appropriate SQL statement that refers to the remote table.
The statement is sent to the remote server using the MySQL client API.
The remote server processes the statement and the local server retrieves any result that the statement produces (an affected-rows count or a result set).
If the statement produces a result set, each column is converted to internal
storage engine format that the FEDERATED engine expects and can use to
display the result to the client that issued the original statement.
The local server communicates with the remote server using MySQL client C API functions. It invokes mysql_real_query() to send the statement. To read a result set, it uses mysql_store_result() and fetches rows one at a time using mysql_fetch_row().
