Why is Select Count(*) slower than Select * in hive

Question

When i am running queries in VirtualBox Sandbox with hive. I feel Select count(*) is too much slower than the Select *.

Can an

Answer 1

This is because the DB is using clustered primary keys so the query searches each row for the key individually, row by agonizing row, not from an index.

• Run optimize table. This will ensure that the data pages are physically stored in sorted order. This could conceivably speed up a range scan on a clustered primary key.

• create an additional non-primary index on just the change_event_id column. This will store a copy of that column in index pages which be much faster to scan. After creating it, check the explain plan to make sure it's using the new index