Re: index cannot be used when passing 'int' to 'varchar'

[Rick James]

> they both using index?!
Yes. The second one is "Using index" because all the fields in the SELECT are in the index! Add a second column to the table, and "Using index" will vanish (because of the "*"). You can see part of this by doing
EXPLAIN EXTENDED SELECT SQL_NO_CACHE * from employees.ttt where emp_no=19967;
SHOW WARNINGS; -- immediately after the EXPLAIN EXTENDED.
The WARNING will say something like
SELECT `employees`.`ttt`.`emp_no` FROM `employees`.`ttt` WHERE `employees`.`ttt`.`emp_no` = 19967.

A table with a single column and many rows is virtually useless. Some of the anomalies you are encountering are from the artificial nature of the benchmark.

Perhaps the only thing you have 'proven' is that
where emp_no=19967;
is not evaluated the way you expect. I suspect that is evaluated as
WHERE CONVERT(emp_no, UNSIGNED) = 19967
MySQL cannot efficiently use an index when the column is hidden in a function call. Hence, the need to scan the entire index.

<opinion>
Benchmarks often lead to confusion and misinformation.
</opinion>