MySQL 5.7 Release Notes
A thread can have any of the following
Command values:
This is a thread on a replication source for sending binary log contents to a replica.
The thread is executing a change user operation.
The thread is closing a prepared statement.
A replica is connected to its source.
A replica is connecting to its source.
The thread is executing a create database operation.
This thread is internal to the server, not a thread that services a client connection.
The thread is generating debugging information.
The thread is a delayed insert handler.
The thread is executing a drop database operation.
The thread is executing a prepared statement.
The thread is fetching the results from executing a prepared statement.
The thread is retrieving information for table columns.
The thread is selecting a default database.
The thread is killing another thread.
The thread is retrieving long data in the result of executing a prepared statement.
The thread is handling a server ping request.
The thread is preparing a prepared statement.
The thread is producing information about server threads.
The thread is executing a statement.
The thread is terminating.
The thread is flushing table, logs, or caches, or resetting status variable or replication server information.
The thread is registering a replica server.
The thread is resetting a prepared statement.
The thread is setting or resetting a client statement execution option.
The thread is shutting down the server.
The thread is waiting for the client to send a new statement to it.
The thread is producing server status information.
Unused.