> If you does the query in the same order, the primary's key index is used. By
> example:
...
> - title, start_date, username ---> don't use the primary's key index
> because the column "username" is not the first column.
I find this a strange example. Sure, if you have
SELECT <whatever>
FROM MyTitle MT
JOIN MyMainTable MMT
ON MT.Title = MMT.Title
LEFT JOIN MyUserName MUN
ON MMT.UserName = MUN.UserName
WHERE MMT.Start_Date = '9.9.2011'
then your example is very true, the MyUserName table has to be after
MyMainTable in the plan, and hence, our index in question cannot be used.
However, the order of the fields in
SELECT <whatever>
FROM MyMainTable
WHERE title = 'Red Nose of Courage'
AND Start_Date = '9.9.1991'
AND UserName = 'Firebird'
should be irrelevant. Of course, order matters if you're talking about using an
index in ORDER BY, but indexes are generally great for limiting rows through
selection (WHERE) or linking (JOIN) and not very useful for ordering a result
set (it might be different for desktop databases, but with client/server
databases I think you always ought to return only records that you're actually
going to use).
Set
