Why is doing a top(1) on an indexed column in SQL Server slow?

Question

I\'m puzzled by the following. I have a DB with around 10 million rows, and (among other indices) on 1 column (campaignid_int) is an index.

Now I have 700k rows wher

Answer 1

but since I'm specifying 'top(1)' it means: give me any row. Why would it first crawl through the 700k rows just to return one? – reinier 30 mins ago

Sorry, can't comment yet but the answer here is that SQL server is not going to understand the human equivalent of "Bring me the first one you find" when it hears "Top 1". Instead of the expected "Give me any row" SQL Server goes and fetches the first of all found rows. Only time it knows that is after fetching all rows first, then discarding the rest. Very thorough but in your case not really fast.

Main issue as other said are your statistics and selectivity of your index. If you have another unique field in your table (like an identity column) then try an combined index on campaignid_int first, unique column second. As you only query on campaignid_int it has to be the first part of the key. Sounds worth a try as this index should have a higher selectivity thus the optimizer can use this better than doing an index crawl.