How does Hive decide when to use map reduce and when not to?

Question

As a simple example,

select * from tablename;

DOES NOT kick in map reduce, while

select count(*) from tablename;
         


        

Answer 1

Whenever we fire a query like select * from tablename, Hive reads the data file and fetches the entire data without doing any aggregation(min/max/count etc.). It'll call a FetchTask rather than a mapreduce task.

This is also an optimization technique in Hive. hive.fetch.task.conversion property can (i.e. FETCH task) minimize latency of map-reduce overhead.

This is like we are reading a hadoop file : hadoop fs -cat filename

But if we use select colNames from tablename, it requires a map-reduce job as it needs to extract the 'column' from each row by parsing it from the file it loads.