Do I need to rebuild my SQL server database indexes?

Question

I used the built-in performance tuning tool and built indexes for my database. Since then I changed some of the queries from "select top 10..." to "select top 50".

My question is do I need to rebuild my SQL server database indexes?

Answer 1

You don't need to rebuild your indexes if all you've changed is top 10 for top 50. The queries are exactly equal regarding execution plan (and thus will benefit from existing indexes), the only difference is that you are retrieving some extra rows from the resultset.

If you are noticing a big performance drop after that change, the problem must lie elsewhere. For example, I've seen the need to rebuild a clustered index for performance reasons after adding data in an order different to the natural order of the index. But this is because of the new data, not because a top 10 changed to top 50.

EDIT: Sam Saffron comments that execution plans can change between top 10 and top 50 queries, to make sure this isn't happening to you check both execution plans and see if they vary.

Answer 2

Though you do not need to rebuild indexes just because querying patterns have changes, you may have to rethink indexes.

For example, say you have a table with a non-clustered index on a particular column. Whenever you need to pull out a row with data that is not in the non-clustered index, SQL will have to go to the underlying table to pull out the data.

When you are pulling 1 row it usually will be cheapest to go to the non-clustered index and then hit the main table, however at some point if your are pulling lots of rows it may become cheaper just to scan the whole table.

bottom line If querying patterns change you may want to review your indexes, it is possible that an index that worked great for a top 10 query, will no longer be picked for a top 50 query.