SQL Server: Index columns used in like?

Question

Is it a good idea to index varchar columns only used in LIKE opertations? From what I can read from query analytics I get from the following query:

SELECT * FROM ClientUsers WHERE Email LIKE '%niels@bosmainter%'

I get an "Estimated subtree cost" of 0.38 without any index and 0.14 with an index. Is this a good metric to use for anlayzing if a query has been optimized with an index?

Balcer

This is very problematic when searching large tables with a data tables type scenario where all coloumns need to do a .contains search – Psychodiagnostics

Answer 1

Given the data 'abcdefg'

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

WHERE Column1 LIKE 'abc%' --can use an 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

Platypus

I like your solution, though I'd call it an ugly hack ;) – Locative

I've used this ugly hack on a system where the original designer made numbers char(x) with leading zeros and other keys letter+ leading zeros. try telling users they have to enter in the leading zeros and or letter+ leading zeros! ha. they want to enter in 203, not G000000203, so this reverse() "hack" will help there. – Platypus

@Platypus referenced this answer here: dba.stackexchange.com/questions/91340/… – Entitle

I wouldn't call it a hack but a very nice solution. – Stickney

It's a beautiful ugly hack – Foretop

Not working like me expect. Ex: I want search "have" in "I have a machine". And not working – Phenomenal

Answer 2

In my experience the first %-sign will make any index useless, but one at the end will use the index.

Locative

Close enough - LIKE will use the prefix on an index but you can't resolve a wildcard through an index so that's as far as it will go. – Caseous

So yay or nay for indexes on these columns? – Balcer

If you always query the column with a leading %, the index will never be used. The answer thus depends on how you query your data. – Pulver

NOT COMPLETELY TRUE, you CAN hit an index on queries that use LIKE '%xyz', but you need to do a little extra work, see my answer for an explanation – Platypus

Answer 3

To answer the metrics part of your question: The type of index/table scan/seek being performed is a good indicator for knowing if an index is being (properly) used. It's usually shown topmost in the query plan analyzer.

The following scan/seek types are sorted from worst (top) to best (bottom):

• Table Scan
• Clustered Index Scan
• Index Scan
• Clustered Index Seek
• Index Seek

As a rule of thumb, you would normally try to get seeks over scans whenever possible. As always, there are exceptions depending on table size, queried columns, etc. I recommend doing a search on StackOverflow for "scan seek index", and you'll get a lot of good information about this subject.

Hexa