For the original query: DELETE FROM db_test_table WHERE 'regtime` != 1407657002, Rows_examined: 129071349 suggested that there was no index on regtime column, and 129Million rows were scanned sequentially which will take some time. Did you know how many rows got deleted?
The command “show create table db_test_table\G” will show the table and index definition.
Since the table is memory storage engine, no IO needed, it is not surprising to have the CPU running at full speed. Note that in ‘top’, 100% CPU means one core is fully occupied, not all cores.