index file and KeysCached

[魏金仙]

for each sstable, there is an index file, which is loaded in memory to locate a 
particular key's offset efficiently.
and for each CF, KeysCached can be set to cache keys' location.
could you pls tell me the difference between the two?
I'm wondering whether it's necessary to set "KeysCached" for a CF or not.
Thanks for your time.