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13.7.7.4 KILL Syntax

KILL [CONNECTION | QUERY] processlist_id

Each connection to mysqld runs in a separate thread. You can kill a thread with the KILL processlist_id statement.

Thread processlist identifiers can be determined from the ID column of the INFORMATION_SCHEMA PROCESSLIST table, the Id column of SHOW PROCESSLIST output, and the PROCESSLIST_ID column of the Performance Schema threads table. The value for the current thread is returned by the CONNECTION_ID() function.

KILL permits an optional CONNECTION or QUERY modifier:

  • KILL CONNECTION is the same as KILL with no modifier: It terminates the connection associated with the given processlist_id, after terminating any statement the connection is executing.

  • KILL QUERY terminates the statement the connection is currently executing, but leaves the connection itself intact.

The ability to see which threads are available to be killed depends on the PROCESS privilege:

  • Without PROCESS, you can see only your own threads.

  • With PROCESS, you can see all threads.

The ability to kill threads and statements depends on the CONNECTION_ADMIN and SUPER privileges:

  • Without CONNECTION_ADMIN or SUPER, you can kill only your own threads and statements.

  • With CONNECTION_ADMIN or SUPER, you can kill all threads and statements, except that to affect a thread or statement that is executing with the SYSTEM_USER privilege, your own session must additionally have the SYSTEM_USER privilege.

You can also use the mysqladmin processlist and mysqladmin kill commands to examine and kill threads.

When you use KILL, a thread-specific kill flag is set for the thread. In most cases, it might take some time for the thread to die because the kill flag is checked only at specific intervals:

  • During SELECT operations, for ORDER BY and GROUP BY loops, the flag is checked after reading a block of rows. If the kill flag is set, the statement is aborted.

  • ALTER TABLE operations that make a table copy check the kill flag periodically for each few copied rows read from the original table. If the kill flag was set, the statement is aborted and the temporary table is deleted.

    The KILL statement returns without waiting for confirmation, but the kill flag check aborts the operation within a reasonably small amount of time. Aborting the operation to perform any necessary cleanup also takes some time.

  • During UPDATE or DELETE operations, the kill flag is checked after each block read and after each updated or deleted row. If the kill flag is set, the statement is aborted. If you are not using transactions, the changes are not rolled back.

  • GET_LOCK() aborts and returns NULL.

  • If the thread is in the table lock handler (state: Locked), the table lock is quickly aborted.

  • If the thread is waiting for free disk space in a write call, the write is aborted with a disk full error message.

Warning

Killing a REPAIR TABLE or OPTIMIZE TABLE operation on a MyISAM table results in a table that is corrupted and unusable. Any reads or writes to such a table fail until you optimize or repair it again (without interruption).