MySQL Identifying Blocking Transactions

Recording this incredibly helpful query from Bill Karwin’s post to SO
You can find the source query that’s blocking your DELETE by using the INFORMATION_SCHEMA.LOCK_WAITS and INNODB_TRX tables.

SELECT r.trx_id waiting_trx_id,
       r.trx_mysql_thread_id waiting_thread,
       r.trx_query waiting_query,
       b.trx_id blocking_trx_id,
       b.trx_mysql_thread_id blocking_thread,
       b.trx_query blocking_query
   FROM       information_schema.innodb_lock_waits w
   INNER JOIN information_schema.innodb_trx b  ON
    b.trx_id = w.blocking_trx_id
  INNER JOIN information_schema.innodb_trx r  ON
    r.trx_id = w.requesting_trx_id;

See more information at, under “Example 14.2 Identifying Blocking Transactions”.