SQL Server Clustered Index - Order of Index Question

Question

I have a table like so:

keyA keyB data

keyA and keyB together are unique, are the primary key of my table and make up a clustered index.

Answer 1

As others have said, the ordering is based on how you specify it in the index creation script (or PK constraint). One thing about clustered indexes though is that there is a lot to keep in mind.

You may get better overall performance by using your clustered index on something other than the PK. For example, if you are writing a financial system and reports are almost always based on date and time of an activity (all activity for the past year, etc.) then a clustered index on that date column might be better. As HLGEM says, sorting can also be affected by your selection of clustered index.

Clustered indexes can also affect inserts more than other indexes. If you have a high volume of inserts and your clustered index is on something like an IDENTITY column then there could be contention problems for that particular part of the disk since all of the new rows are being inserted into the same place.

For small look-up tables I always just put the clustered index on the PK. For high-impact tables though it's a good idea to spend the time thinking about (and testing) various possible clustered indexes before choosing the best one.