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

You aren't specifying an ORDER BY clause in your query, so the optimiser is not being instructed as to the sort order it should be selecting the top 1 from. SQL Server won't just take a random row, it will order the rows by something and take the top 1, and it may be choosing to order by something that is sub-optimal. I would suggest that you add an ORDER BY x clause, where x being the clustered key on that table will probably be the fastest.

This may not solve your problem -- in fact I'm not sure I expect it to from the statistics you've given -- but (a) it won't hurt, and (b) you'll be able to rule this out as a contributing factor.