Why using IN(…) when selecting on indexed fields, will kill the performance of SELECT query?

Question

Avoid using IN(...) when selecting on indexed fields, It will kill the performance of SELECT query.

I found this here: https://wikis.ora

Answer 1

Prior to MySQL 5.0 it seems that mySQL would only use a single index for a table. So, if you had a SELECT * FROM tbl WHERE (a = 6 OR b = 33) it could chooose to use either the a index or the b index, but not both. Note that it says fields, plural. I suspect the advice comes from that time and the work-around was to union the OR results, like so:

SELECT * FROM tbl WHERE (a = 6)
UNION
SELECT * FROM tbl WHERE (b = 33)