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

I recently had the same issue and it's really quite simple to solve (at least in some cases).

If you add an ORDER BY-clause on any or some of the columns that's indexed it should be solved. That solved it for me at least.

Answer 2

Your query does not work as you expect, because Sql Server keeps statistics about your index and in this particular case knows that there are a lot of duplicate rows with the identifier 3835, hence it figures that it would make more sense to just do a full index (or table) scan. When you test for an ID which resolves to only one row, it uses the index as expected, i.e. performs an index seek (the execution plan should verify this guess).

Possible solutions ? Make the index composite, if you have anything to compose it with, that is, e.g. compose it with the date the message was sent (if I understand your case correctly) and then select the top 1 entry from the list with the specified id ordered by the date. Though I'm not sure whether this would be better (for one, a composite index takes up more space) - just a guess.

EDIT: I just tried out the suggestion of making the index composite by adding a date column. If you do that and specify order by date in your query, an index seek is performed as expected.