Re: JOIN with GROUP BY, without PRIMARY KEY

[KimSeong Loh]

Is B.gr your primary key?

MySQL optimiser choose the 1st option because there is only 1 row to read from table B, due to the where B.gr = 1, I expect this is the primary key in table B, so this is very efficient for the join.

The 2nd option with the force index, reads table A first, this will allow it to eliminate the temporary and sorting, but may need to read more rows in the join, actually all the rows in table A, it read from the index in this case, the whole index tree.