limit does not work if applied to outer most block and it is not a query
insert into T1 select ... from T2 limit 10;
will be executed as a map-job only.
which means that the total number of rows in T1 is bounded by 10 * number of mappers.
We need another map-reduce job to fix this or the move task needs to be modified.
For now, the simpler fix of another map-reduce job should be ok, in future we can further optimize it by modifying the move task