how does MySQL implement the "group by"?
- by user188916
I read from the MySQL Reference Manual and find that when it can take use of index,it just do index scan,other it will create tmp tables and do things like filesort.
And I also read from other article that the "Group By" result will sort by group by columns by default,if "order by null" clause added,it won't don filesort.
The difference can be…