WPTC realtime backups and 6h hour backups will make use of the wptc_query_recorder table.
WPTC will record all the SQL queries in realtime to the wptc_query_recorder table,
This table contents will get cleared after every successful backups, sometimes this table will become big in size due to large number of unwanted SQL queries, which may be avoided by excluding the contents of the unnecessary tables via WPTC plugin settings.
If your wptc_query_recorder table becomes big, then kindly stop the backup and then restart the backup manually from the WPTC plugin settings, which will clear the query_recorder_table.
If the above method fails, then you can Truncate the wptc_query_recorder table via PHPmyAdmin.