Question

I’ve got a table that looks like this (I wasn’t sure what all might be relevant, so I had Toad dump the whole structure)

CREATE TABLE [dbo].[TScore] (
[CustomerID] int NOT NULL,
[ApplNo] numeric(18, 0) NOT NULL,
[BScore] int NULL,
[OrigAmt] money NULL,
[MaxAmt] money NULL,
[DateCreated] datetime NULL,
[UserCreated] char(8) NULL,
[DateModified] datetime NULL,
[UserModified] char(8) NULL,
CONSTRAINT [PK_TScore]
PRIMARY KEY CLUSTERED ([CustomerID] ASC, [ApplNo] ASC)
);

And when I run the following query (on a database with 3 million records in the TScore table) it takes about a second to run, even though if I just do: Select BScore from CustomerDB..TScore WHERE CustomerID = 12345, it is instant (and only returns 10 records) — seems like there should be some efficient way to do the Max(ApplNo) effect in a single query, but I’m a relative noob to SQL Server, and not sure — I’m thinking I may need a separate key for ApplNo, but not sure how clustered keys work.

SELECT BScore   
FROM CustomerDB..TScore (NOLOCK)
WHERE ApplNo = (SELECT Max(ApplNo)
               FROM CustomerDB..TScore sc2 (NOLOCK)
               WHERE sc2.CustomerID = 12345)

Thanks much for any tips (pointers on where to look for optimization of sql server stuff appreciated as well)

Answer 1

When you filter by ApplNo, you are using only part of the key. And not the left hand side. This means the index has be scanned (look at all rows) not seeked (drill to a row) to find the values.

• If you are looking for ApplNo values for the same CustomerID:

Quick way. Use the full clustered index:

SELECT BScore   
FROM CustomerDB..TScore
WHERE ApplNo = (SELECT Max(ApplNo)
               FROM CustomerDB..TScore sc2
               WHERE sc2.CustomerID = 12345)
AND CustomerID = 12345

This can be changed into a JOIN

SELECT BScore   
FROM
     CustomerDB..TScore T1 
     JOIN
     (SELECT Max(ApplNo) AS MaxApplNo, CustomerID 
      FROM CustomerDB..TScore sc2
      WHERE sc2.CustomerID = 12345
     ) T2 ON T1.CustomerID = T2.CustomerID AND T1.ApplNo= T2.MaxApplNo

• If you are looking for ApplNo values independent of CustomerID, then I’d look at a separate index. This matches your intent of the current code

CREATE INDEX IX_ApplNo ON TScore (ApplNo) INCLUDE (BScore);

Reversing the key order won’t help because then your WHERE sc2.CustomerID = 12345 will scan, not seek

Note: using NOLOCK everywhere is a bad practice