Question

I’m in visual studio, looking to create a composite index on 2 columns for several tables. There are 2 columns: UserID is in all tables and acts as the foreign key; then, each table has its own key to refer to the parts of the object, such as phone, address… Like this:

TablePhones:
PhoneID  | UserID | PhonePrefix | PhoneNumber | PhoneExtention

TableAddresses:
AddressID | UserID | AddressStreet1 | AddressStreet2 | AddressCity...

Note that users can have more than 1 address and more than 1 phone number.

I’m using linq to sql and the where clauses queries to get the objects look like this:

read queries:
where x.UserID == TheUserID

update/delete queries:
where x.UserID == TheUserID && x.PhoneID = ThePhoneID

At the moment, the primary keys are on PhoneID and AddressID and I’m looking to create composite indexes on PhoneID/UserID and AddressID/UserID. Is the order of the columns in the database fine as it is or should I move UserID in first position for all tables.

Thanks for suggestions.

Answer 1

Order of columns in table doesn’t matter; at least for SQLServer. The important thing is in which order fields are listed in an index. Queries with conditions on leading column[s] will very benefit from the index.

If your primary key is clustered, you can create index on only userID, no need for composite key. Anyway, it will have a reference to clustered key.