10.8.4 Obtaining Execution Plan Information for a Named Connection

To obtain the execution plan for an explainable statement executing in a named connection, use this statement:

EXPLAIN [options] FOR CONNECTION connection_id;

EXPLAIN FOR CONNECTION returns the EXPLAIN information that is currently being used to execute a query in a given connection. Because of changes to data (and supporting statistics) it may produce a different result from running EXPLAIN on the equivalent query text. This difference in behavior can be useful in diagnosing more transient performance problems. For example, if you are running a statement in one session that is taking a long time to complete, using EXPLAIN FOR CONNECTION in another session may yield useful information about the cause of the delay.

connection_id is the connection identifier, as obtained from the INFORMATION_SCHEMA PROCESSLIST table or the SHOW PROCESSLIST statement. If you have the PROCESS privilege, you can specify the identifier for any connection. Otherwise, you can specify the identifier only for your own connections. In all cases, you must have sufficient privileges to explain the query on the specified connection.

If the named connection is not executing a statement, the result is empty. Otherwise, EXPLAIN FOR CONNECTION applies only if the statement being executed in the named connection is explainable. This includes SELECT, DELETE, INSERT, REPLACE, and UPDATE. (However, EXPLAIN FOR CONNECTION does not work for prepared statements, even prepared statements of those types.)

If the named connection is executing an explainable statement, the output is what you would obtain by using EXPLAIN on the statement itself.

If the named connection is executing a statement that is not explainable, an error occurs. For example, you cannot name the connection identifier for your current session because EXPLAIN is not explainable:

mysql> SELECT CONNECTION_ID();
+-----------------+
| CONNECTION_ID() |
+-----------------+
|             373 |
+-----------------+
1 row in set (0.00 sec)

mysql> EXPLAIN FOR CONNECTION 373;
ERROR 1889 (HY000): EXPLAIN FOR CONNECTION command is supported
only for SELECT/UPDATE/INSERT/DELETE/REPLACE

The Com_explain_other status variable indicates the number of EXPLAIN FOR CONNECTION statements executed.