SQL Server索引使用问题

Question

Assuming the following:

/*

drop index ix_vouchers_nacsz on dbo.vouchers;
drop index ix_vouchers_nacsz2 on dbo.vouchers;

create index ix_vouchers_nacsz on dbo.Vouchers(
    FirstName, LastName,
    Address, Address2, City,
    State, Zip, Email
);

create index ix_vouchers_nacsz2 on dbo.Vouchers(
    Email, FirstName, LastName,
    Address, Address2, City,
    State, Zip
);

*/

select count(firstname) from vouchers
    with (index(ix_vouchers_nacsz))
where 
    firstname = 'chris' and
    lastname = '' and
    address = '' and
    address2 = '' and
    city = '' and
    state = '' and
    zip = ''

select count(firstname) from vouchers
    with (index(ix_vouchers_nacsz2))
where 
    firstname = 'chris' and
    lastname = '' and
    address = '' and
    address2 = '' and
    city = '' and
    state = '' and
    zip = ''

Why does the second index result in an index scan while the first results in an index seek? What difference does the ordering of the keys make?

Answer 1

The second index starts with the email field, but you're not filtering on email. That makes the index useless.

An index is typically a b-tree that allows you to do a binary search . The b-tree is sorted by its index fields. So if you know the first field, you can look it up quickly. Within the same values of the first field, you can look up the second field very quickly.

It's like a telephone book that's sorted on last name. You can't use it to search for a specific telephone number.

Answer 2

In the case of index "the column ordering in index", "column ordering in where clause" will metter a lot. you could refer following link:

http://ashishkhandelwal.arkutil.com/sql-server/quick-and-short-database-indexes/

•Best Practices to use indexes •How to get best performance form indexes •Clustered index Considerations •Nonclustered Indexes Considerations

I am sure this will help you when planning for index.

Answer 3

Applying the 'phone book' analogy may help in understanding.

The first index is a 'phone book' sorted by FirstName, then Last Name and so on. If your asked to look up Chris in this phone book, then you can find all the Chris' listed together by the index.

In the second index, is a phone book sorted by 'phone numbers' (or email just as easily) then first name, then last name and so on. If your asked to use this phone book to look up listings with the firstname of Chris, your out of luck the phone book is not sorted that way! Of course, if you were asked to look for email address example@example.com and name Chris, then you can find the email address first, and then look for matching names.