Whenever you run a normal 'select *', a fetch task is created rather than a mapreduce task which just dumps the data as it is without doing anything on it.

when you write select * from table_name, the whole file is viewed, while if you select column, a map only job is launched, no reduce will be launched for it, since we are selecting the whole column .

Select * from table_name; -->  will not launch a MR JOB
Select column from table_name; -->  will launch a M JOB (map only job)

Select MAX(column_name) from table_name; --> will launch a MR JOB

We hope it will help you understand.

If you are still facing any issue feel free to revert.

We will be glad assisting you.

Please note if you are not happy with the response on this ticket, please escalate it to escalations@edureka.in.
We assure you that we will get back to you within 24 hours

Md Israil
edureka! Support Team

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