How to make mysql use index for where in?

Question

MySQL

Actually at request select * from entries where a in (a1, a2, a3, a4)
In explain shows using where; using index, I would like to see only using index.
Accordingly, requests are executed for 10 seconds, despite the fact that there is a unique index on field a.

Answer 1

Sometimes MySQL does not use an index, even if one is available. One circumstance under which this occurs is when the optimizer estimates that using the index would require MySQL to access a very large percentage of the rows in the table. (In this case, a table scan is likely to be much faster because it requires fewer seeks.)

Answer 2

You can try index hints: https://dev.mysql.com/doc/refman/5.7/en/index-hint...