Will indexing improve varchar(max) query performance, and how to create index
Firstly, I should point out I don't have much knowledge on SQL Server indexes.
My situation is that I have an SQL Server 2008 database table that has a varchar(max) column usually filled with a lot of text.
My ASP.NET web application has a search facility which queries this column for keyword searches, and depending on the number of keywords searched for their may be one or many LIKE '%keyword%' statements in the SQL query to do the search.
My web application also allows searching by various other columns in this table as well, not just that one column. There is also a few joins from other tables too.
My question is, is it worthwhile creating an index on this column to improve performance of these search queries? And if so, what type of index, and will just indexing the one column be enough or do I need to include other columns such as the primary key and other searchable columns?
Answer
The best analogy I've ever seen for why an index won't help '%wildcard%' searches:
Take two people. Hand each one the same phone book. Say to the person on your left:
Tell me how many people are in this phone book with the last name "Smith."
Now say to the person on your right:
Tell me how many people are in this phone book with the first name "Simon."
An index is like a phone book. Very easy to seek for the thing that is at the beginning. Very difficult to scan for the thing that is in the middle or at the end.
Every time I've repeated this in a session, I see light bulbs go on, so I thought it might be useful to share here.