% in the beginning of like clause

Question

I heard that its not advised to use % in the beginning of LIKE clause in SQL Server due to performance reasons.Why is this is so?

Some more details on this will help

Answer 1

%foo basically says "All strings ending with 'foo'". In order to filter out those, SQL server has to scan all table (in worst-case scenario) and check each and every string. This is why it's so expensive.