(mysql.info) federated-description

(mysql.info) federated-storage-engine
 
 14.7.1 Description of the `FEDERATED' Storage Engine
 ----------------------------------------------------
 
 When you create a `FEDERATED' table, the server creates a table format
 file in the database directory. The file begins with the table name and
 has an `.frm' extension. No other files are created, because the actual
 data is in a remote table. This differs from the way that storage
 engines for local tables work.
 
 For local database tables, data files are local. For example, if you
 create a `MyISAM' table named `users', the `MyISAM' handler creates a
 data file named `users.MYD'. A handler for local tables reads, inserts,
 deletes, and updates data in local data files, and rows are stored in a
 format particular to the handler. To read rows, the handler must parse
 data into columns. To write rows, column values must be converted to
 the row format used by the handler and written to the local data file.
 
 With the MySQL `FEDERATED' storage engine, there are no local data
 files for a table (for example, there is no `.MYD' file). Instead, a
 remote database stores the data that normally would be in the table.
 The local server connects to a remote server, and uses the MySQL client
 API to read, delete, update, and insert data in the remote table. Data
 retrieval is initiated via a `SELECT * FROM TBL_NAME' SQL statement. To
 read the result, rows are fetched one at a time by using the
 `mysql_fetch_row()' C API function, and then converting the columns in
 the `SELECT' result set to the format that the `FEDERATED' handler
 expects.
 
 The flow of information is as follows:
 
   1. SQL calls issued locally
 
   2. MySQL handler API (data in handler format)
 
   3. MySQL client API (data converted to SQL calls)
 
   4. Remote database -> MySQL client API
 
   5. Convert result sets (if any) to handler format
 
   6. Handler API -> Result rows or rows-affected count to local
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