为什么优化器使用“ ALL”类型而不是“ index”类型？

Question

I'm currently learning execution plan in MySql .
And I use dump data from here.

https://dev.mysql.com/doc/employee/en/sakila-structure.html

I added new index to from_date field of dept_emp table, named ix_fromdate .
And I typed like below

EXPLAIN
SELECT *
FROM dept_emp
WHERE from_date>='1985-01-01'  

Execution plain result like below

I understand that rows value is closer amount of this table rows amount and optimizer didn't use range index.
And.. I think, type should be index , but ALL .
Why they did table full scan ?
There's index for ix_fromdate , why they didn't use it though?
I know index full scan is better than table full scan ...
Do I know it wrong?

Answer 1

• Even if you have INDEX(from_date) ?
• But more than about 20% of the table satisfies WHERE from_date>='1985-01-01' ?

Then the Optimizer decides that it is probably faster to simply scan through the data, filtering as it goes. The alternative (using the index) could be costly because of bouncing between the index BTree and the data BTree.

Use a newer date; the Optimizer may change its mind and use the index.