Question

I have created a table

create table #temp(a int, b int, c int)

I have 2 indexes on this table:

1. Non clustered non unique index on c
2. Clustered Index on a

When I try to execute the following query:

select b from #temp where c = 3

I see that the system goes for index scan. This is fine, because the non clustered index doesn’t have b as the key value. Hence it does an index scan from column a.

But when I try to execute the below query:-

select b from #temp where c= 3 and a = 3

I see that the execute plan has got only index seek. No scan. Why is that?

Neither the clustered index nor the nonclustered index as b as one of the columns?

I was hoping an index scan.

Please clarify

Answer 1

If you have a as your clustering key, then that column is included in all non-clustered indices on that table.

So your index on c also includes a, so the condition

where c= 3 and a = 3

can be found in that index using an index seek. Most likely, the query optimizer decided that doing a index seek to find a and c and a key lookup to get the rest of the data is faster/more efficient here than using an index scan.

BTW: why did you expect / prefer an index scan over an index seek? The index seek typically is faster and uses a lot less resources – I would always strive to get index seeks over scans.