When reviewing execution plans I often look for table scans as an indicator of an area which performance can be improved. A table scan or a clustered index scan is when the query optimizer has to search the data entirely to find the row you are requesting. The reason this can be costly is when you have a scan on a large table SQL has to load all the data into the buffer this will slow performance and tax your resources. On smaller tables SQL Server may determine that a table scan is faster then an index seek, it can keep the entire data set in the buffer without issue. A table scan would be caused by a query with no where clause. SELECT COLUMN1 from TABLE1. It can also be caused by a query with a WHERE clause which is not on the clustered index. SELECT COLUMN1 from TABLE1 where NOTPKEY=5.
Table Scan
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