Friday, March 23, 2012

Increase performance by adding NONCLUSTERED INDEX

Got a call from a client reporting an issue that is randomly occurred. after looking into the log file, I found one method took over a min to complete. The code shows only a simple select statement with where clause on "sentby" column. This column is a "nvarchar" and the table itself has over 1 million records.

Here is what I learned when I trying to fix this issue:

1. use "varchar" instead of "nvarchar". only use "nvarchar" when you have different languages in the same column! if you look at the sql execution plan ( sql -> display estimated ececution plan), search "nvarchar" does an index scan and "varchar" does index seek. index seek is orders of magnitude faster than a scan!

2. no index is set on "sentby" column. It won't be an issue in the small table (sql does index scan on small table), but it will impact performance when data grows. I used the script below to add a NONCLUSTERED INDEX on this column.


after the index is created, the 1 min search time is down to under a second.