MySQL: How to explain this report of `show engine innodb status`?

when ran displays the InnoDB status of the engineI found a simple query running for 9401 seconds.

--- TRANSACTION 846912376553, ACTIVE 9401 sec reading of the starting index
mysql tables in use 1, locked 0
4 lock structures, 1184 heap size, 1 line lock (s), undo log entries 2
MySQL thread identifier 501910, OS thread handle 0x62ddd67f6700, request identifier 113423579292 appserver01.phx.sf.priv 10.14.17.23 kuber production statistics
SELECT 1 AS A 'PARTICIPANTS' WHERE` PARTICIPANTS`.`ID_User = 19811 AND `PARTICIPANTS`.`channel_id` = 8668 LIMIT 1
The read view of Trx will not see that trx with id> = 846912376554, sees <846912366834
  1. What is this sql waiting for?

  2. What will cause a simple query?