also add the distribute by clause as I wrote below otherwise each reducer will write to 1173 partitions which guarantees OOM exceptions. ( ORC keeps some memory for every task. )
Also really no idea why he distributes by the second column ( _col1 ) He shouldn't add a reducer to a simple SELECT * from. Which column is that DT? Then everything is ok.
From your example, you seem to be using Tez. Check this article https://community.hortonworks.com/articles/22419/hive-on-tez-performance-tuning-determining-reducer.... which has more detail on how reducers can be controlled.
This is different from how it works in mapreduce. hive.exec.reducers.byte.per.reducer specifies who much goes to each reducer which determines number of reducers.