INSERT INTO bench (SELECT * FROM temp JOIN (select CONCAT(call_src_realm,'-',SUBSTRING(start_time,1,7)) as rym from temp limit 15000) tmp limit 15000);

This query runs really fast .. 1.84 seconds

 INSERT INTO bench (SELECT * FROM temp JOIN (select CONCAT(call_src_realm,'-',SUBSTRING(start_time,1,7)) as rym from temp) tmp);

This query runs really slow..actually ive never had it finish.. after waiting like 10 minutes.  Whats wierd is there are only 17,000 rows.. so i dont know why its crapping out.