Re: indices not used

[Rick James]

Q1:
const,const -- implies it is using only c1 and c2. Strange. Please show me what this says:
SHOW CREATE TABLE products;
(If you are in some UI, you can paste that into the screen that lets you type SQL.)

Maybe c2 is NULLable?

For the query shown, the ALTER I gave you won't do any good. It would be better to do
ALTER TABLE product ORDER BY c1, c2, c3, factor;

Q2:
Let me know if there is a diff between disabling keys and not.

I would not bother running the alter more than weekly unless there is a lot of churn.