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Answer Posted / sumit
Index Scan is nothing but scanning on the data pages from
the first page to the last page. If there is an index on a
table, and if the query is touching a larger amount of
data, which means the query is retrieving more than 50
percent or 90 percent of the data, and then optimizer would
just scan all the data pages to retrieve the data rows. If
there is no index, then you might see a Table Scan (Index
Scan) in the execution plan.
Index seeks are generally preferred for the highly
selective queries. What that means is that the query is
just requesting a fewer number of rows or just retrieving
the other 10 (some documents says 15 percent) of the rows
of the table.
In Index Scan the cost is proportional to the total number
of rows in the table. Thus, a scan is an efficient strategy
if the table is small or if most of the rows qualify for
the predicate.
In Index Seek the cost is proportional to the number of
qualifying rows and pages rather than to the total number
of rows in the table.
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