When is a full table scan better than index scan?

Question

What are some scenarios (in SQL Server) in which a full table scan is better than an index scan?

Answer 1

When selectivity of records is very high full table scan is always better than going through index scan.

http://www.techipost.com/single-index-versus-full-table-scan/

Edit by gbn:

Example, the optimiser may decide that it's easier to scan the table/clustered index if it would require many key lookups (eg non-clustered index to clustered indexs for non-key data).

Or you don't have many distinct values in the indexed columns

Answer 2

When the

• table is small enough there is no practical difference
• statistically, you'd return most rows anyway

The 2nd case needs qualifying

• An index scan will replace an index seek if an index is covering
• An index seek or scan with many rows that requires key/bookmark lookups will be expensive and a table scan could be better

Finally

• An index scan and a table scan are pretty much the same for clustered indexes

Answer 3

If the table

• is very small
• the cost of doing an index scan and then a number of bookmark lookups into the base table is more expensive than a full table scan

Answer 4

Better in what way?

The optimizer may pick a table scan if an index is not covering, because an index scan would still have to be augmented with a bookmark lookup.

In those cases, a poorly indexed table is no better than an unindexed table.

http://www.sqlservercentral.com/articles/Performance+Tuning/bookmarklookups/1899/