I recently discovered the default open transaction ceiling of 1024 in MySQL. I have since increased the limit using innodb_extra_rsegments. However, I would like to keep my eye on this moving forward. My MySQL version is 5.1.48. Is there a way to monitor the open transactions at a point in time?
SHOW ENGINE INNODB STATUS will give insight as to how far back your history list is, how many transactions are pending/active and other super useful diagnostics.
Also, this has already been answered: https://stackoverflow.com/questions/3172127/can-i-retrieve-pending-queries-during-an-innodb-transaction