MySQL 5.7 Release Notes
Returns the Performance Schema thread ID assigned to a given
connection ID, or the thread ID for the current connection if
the connection ID is NULL.
in_connection_id BIGINT UNSIGNED: The ID of the connection for which to return the thread ID. This is a value of the type given in thePROCESSLIST_IDcolumn of the Performance Schemathreadstable or theIdcolumn ofSHOW PROCESSLISToutput.