A Clustered Index Scan is similar to a Table Scan. IE: The data is being read row-by-row

A SCAN happens when the optimizer determines that 1) all/most rows need to be returned (so it would be a waste of time reading the index keys), 2) the index is not selective enough, and the optimizer thinks it needs to read all/most of the index. 3) the index stats are found to be out-of-date 4) when the query adds functions to a column’s data, obscuring the columns data from the optimizer.

An (index) SCAN suggests that because a normally sufficient index is NOT sufficient at the moment – more data may be being returned than (normally) needed – suggesting more filtering may be needed – perhaps in the WHERE clause.

A CLUSTERED index seek is beneficial because the data can be read from the index, without having to look at the table.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s