efficiency of wildcards at both ends

I'm curious how is wildcards at both ends implemented, e.g. LIKE '%str%'

How efficient is it if that's the only search criteria against a large table? how much does indexing the column help and roughly how much more space is needed for the index?

if the answers are too long, please point me to the relavant text =D 

thanks
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Zhongshi (Sam) Jiang
sammyjiang721@gmail.com