hadoop - 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;

DOES. What is the general principle used to decide when to use map reduce (by hive)?

Answer 1

In general, any sort of aggregation, such as min/max/count is going to require a MapReduce job. This isn't going to explain everything for you, probably.

Hive, in the style of many RDBMS, has an EXPLAIN keyword that will outline how your Hive query gets translated into MapReduce jobs. Try running explain on both your example queries and see what it is trying to do behind the scenes.