MySQL query WHERE IN + ORDER BY, which EXPLAIN is better and how to avoid filesort?
Given the structure of multicolumn B-tree index it is not viable to do a sort on position when using IN on category_id. But as the test data suggest the position is not "global" but seems to have a meaning only for given category_id. So as I suggested in comments, it is instead possible to ORDER BY (category_id, position) - that can use two-column index on those columns to get it without filesort. http://sqlfiddle.com/#!9/aabaa/7

Because of the longtext column it is not possible to turn this into an index-only scan (text columns can be only indexed on prefix, not on entire value, as index key length is limited).

Using varchar fields as primary key has some drawbacks, but it is not "wrong" by itself so just a little suggestion - would it make sense in your desing to use some integer ids instead?