Why is Select Count(*) slower than Select *

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This topic contains 2 replies, has 3 voices, and was last updated by  Tom Hanlon 2 years, 1 month ago.

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  • #14459

    Just got my VirtualBox Sandbox running and am going through the tutorials (which are amazing btw!) and had a question that I was hoping folks out here could help me with.

    I ran the Hive query (in Tutorial 1) for running a count on the NYSE_Stocks and then just on listing all the stocks (Select *)..and I am a bit puzzled on how the Select count(*) is slower than the Select *. Also, I noticed that the Select count(*) did not spawn any MR jobs.

    Can anyone explain what is going on behind the covers for both these queries and why one is slower than the other?

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  • #14761

    Tom Hanlon

    There are three types of operations that a hive query can perform.

    In order of cheapest and fastest to more expensive and slower here they are.

    A hive query can be a metadata only request.

    Show tables, describe table are examples. In these queries the hive process performs a lookup in the metadata server. The metadata server is a SQL database, probably MySQL, but the actual DB is configurable.

    A hive query can be an hdfs get request.
    Select * from table, would be an example. In this case hive can return the results by performing an hdfs operation. hadoop fs -get, more or less.

    A hive query can be a Map Reduce job.

    Hive has to ship the jar to hdfs, the jobtracker queues the tasks, the tasktracker execute the tasks, the final data is put into hdfs or shipped to the client.

    The Map Reduce job has different possibilities as well.

    It can be a Map only job.
    Select * from table where id > 100 , for example all of that logic can be applied on the mapper.

    It can be a Map and Reduce job,
    Select min(id) from table;
    Select * from table order by id ;

    It can also lead to multiple map Reduce passes, but I think the above summarizes some behaviors.



    Carter Shanklin

    In Hive an aggregation like count requires a map-reduce job while select * is streamed locally out of a single process.

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