Question

This is pseudo-code:

Select 
    count(score) 
from 
   myTable
join 
   HisTable on myTAble.id = HisTable.id 
where 
   name='aaa'
group by 
   id 
having 
   count(score) > 3 
order by 
   (someField)

My question is about index. Where should I put the non-clustered index and the clustered index?
By which priority? (the aggregate? The order? The where? The join fields?)

Answer 1

In SQL Server, there are several basic considerations:

• the clustering key is vitally important, and it should be chosen with care – it should be narrow, unique, static and possibly ever-increasing (the NUSE principle) – see Kim Tripp’s Ever-increasing clustering key – the Clustered Index Debate……….again! blog post for more detailed information on why those characteristics are important).

  So your clustering key should be narrow – 4-byte INT is ideal, it should be UNIQUE and static, and this is perfectly handled by some kind of an ID – especially if it’s an INT IDENTITY. GUID is significantly worse, variable-width VARCHAR fields are absolutely out of the question

• every foreign key column in a child table referencing a parent table should be part of a nonclustered index. This significantly speeds up JOINs and other operations

• any column used in a WHERE or ORDER BY clause also is a good index candidate.

But mind you: indexing is an art and a balancing act. Each additional index adds more overhead – don’t overdo your indices! Less is often more. Try to

• pick a good clustering key
• put nonclustered indexes on each of your foreign key columns

and then see if your app performs OK. If it does – fine, let it be. If not: try to isolate the offending queries, and try to tune those. Don’t over-do your indexing by tuning each individual query – an index that might benefit one query can severely impact another one. It’s a matter of finding the best set of indices for your entire application.

Read Kim Tripp’s blog posts – all if you have the time. She’s the Queen of Indexing and offers tons of very valuable insights into what to do (and what not to do).