MySQL index for MIN and MAX

Question

Could anyone clarify this point from the official MySQL documentation

Indexes are used ... To find the MIN() or MAX() value for a specific indexed column

Answer 1

SELECT MIN(b), MAX(b) FROM tbl WHERE a = 12;

loves

INDEX(a, b)

both columns, in that order.

The query looks in the index for a = 12, grabs the first (a,b) pair to get MIN(b) and grabs the last pair to get MAX(b).

The statement about "replacing with a constant" is confusing because it is going too deep into the details of how it first figures out how to perform the query (which happens to get the min and max), then proceeds to execute what is left of the query (nothing is left).

More generally, the optimal index is usually one that starts with all the WHERE columns compared to constants with =. After that it gets complex, so let me give another tip:

A "covering" index is one that has all the columns mentioned in the SELECT (a and b in my example).

Sorry, I don't seem to be clearer than the manual.

Answer 2

In their example they are not trying to find the min and max values of the column called key_col. They are trying to find the min and max of the second key in a multi-column key. The issue is further complicated since the query is limiting its search to the subset of the multi-column key set that is defined by having the key_part1 = 10.

10, 2, 3
10, 9, 5
10, 7, 7
10, 3, 1
10, 2, 5
11, 3, 0
11, 0, 3

So in this example the query selects only the rows where the first column is 10. Then it finds that among the rows with 10 in the first column the min for the second column is 2 and the max is 9. Notice that 0 in the second column is not chosen as the min because the first column does not match on 10. Hope that helps.