Question

I’m creating an index in sql server 2005 and the discussion with a coworker is if it makes a difference between index key columns being id and date vs date then id.

Is there a fundamental difference in the way the index would be created in either scenario?

Would it make a difference in other versions of SQL server?

Thanks

Answer 1

Yes, definitely. Does anyone ever query the table for JUST date or JUST id? An index of date,id can be used to look up just date, but not just id, and vice-versa

Using date,id:

Jan 1     4  
Jan 1     7  
Jan 2     6  
Jan 2     9  
Jan 2     33  
Jan 3     23  
Jan 4     1

Using id,date:

1     Jan 4  
4     Jan 1  
6     Jan 2  
7     Jan 1  
9     Jan 2  
23    Jan 3  
33    Jan 2 

If your WHERE clause or a JOIN in your query is using both date and id, then either index is fine. But you can see that if you’re doing a lookup just by date, the first index is useful for that, but the second one is totally random.

In a more general sense, an index on A, B, C, D is going to be useful for queries on A,B,C,D, OR A,B,C OR A,B OR just A.