索引如何减慢select语句的速度？

I am having quite a difficult time at finding why adding an index on the foreign key of a table is slowing down the view of my colleague. This view is composed of several packed views with outer join and inner join. I tried to remove them one by one to figure out where the problem was, but I cannot say, it doesn't seem to come from a particular view but more from them all.I am having quite a difficult time at finding w