SQL Server功能效率（如果存在）

Question

I have the following function which returns a bit:

Declare @Ret bit
SET @Ret = 0
IF EXISTS ( Select * from tblExclusion where StatusID = 1 and AccountID = @AccountID )
Begin
SET @Ret = 1
End

Return @Ret

Now there can be multiple entries for the same AccountID in the table or none at all but only one entry will ever have a "1" status if it exists.

I have to be honest I'm not very knowledgeable when it comes to SQL but when called the function seems to take a long time to return. I'm wondering if there is a more efficient way of writing the above.

Thanks in advance.

Answer 1

An index may be necessary, reviewing a sample execution plan will reveal what index would improve.

If you were to modify your query to:

Declare @Ret bit
SET @Ret = 0
IF EXISTS ( Select 1 from tblExclusion where StatusID = 1 and AccountID = @AccountID )
Begin
SET @Ret = 1
End

Return @Ret

An NONCLUSTERED INDEX would be of the format:

USE [DatabaseName]
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[tblExclusion] ([StatusID],[AccountID])
<optional, INCLUDE ([columns within the select,]) >
GO

Types of indexes and how to create them: Create Index

Answer 2

If it takes a long time to run, then I would suspect that there is no index on the column "AccountID". Adding an index on that column will probably significantly improve performance. However, without knowing how tblExclusion is defined, there is no way to be certain of this answer. Also, adding an index to StatusID will help as well, assuming there are a large number of entries for different StatusIDs.

Also, since you only need to test the existence of the record, you don't need to select every column in tblExclusion. You could change "*" to "1" or something, though this will not improve performance significantly.

Answer 3

Try this form

Declare @Ret bitSET @Ret = 0
IF EXISTS ( Select top 1 * from tblExclusion(nolock) where StatusID = 1 and AccountID = @AccountID )
Begin
SET @Ret = 1
End

Return @Ret

Remember the index's and maintenance can make this work slow.

Answer 4

I suggest using select top 1 1 from instead of select * from as in:

Declare @Ret bit
SET @Ret = 0
IF EXISTS (Select top 1 1 from tblExclusion where StatusID = 1 and AccountID = @AccountID)
SET @Ret = 1

Return @Ret

This way you avoid getting unneeded and probably large data.