% 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

why is LIKE '%...' not good? you can't use any index and must scan the entire table.

here is a good example:

go to the phone book and find me all names that match '%ch'. That will take quite a while since you are not able to use the clustered index, and must scan the entire book!

Given the data 'abcdefg'

WHERE Column1 LIKE '%cde%'  --can't use an index

WHERE Column1 LIKE 'abc%' --can use and index

WHERE Column1 Like '%defg' --can't use an index, but see note below

Note: If you have important queries that require '%defg', you could use a persistent computed column where you REVERSE() the column and then index it. Your can then query on:

WHERE Column1Reverse Like REVERSE('defg')+'%' --can use the persistent computed column's index

to add a persisted computed column (that reverses the string) and index on it, use this code:

ALTER TABLE YourTable ADD ReversedYourString  AS REVERSE(YourString) PERSISTED 

CREATE NONCLUSTERED INDEX IX_YourTable_ReversedYourString 
ON YourTable (ReversedYourString)