Question

Changing and finding stuff in a database containing a few dozen tables with around half a million rows in the big ones I’m running into timeouts quite often.

Some of these timeouts I don’t understand. For example I got this table:

CREATE TABLE dbo.[VPI_APO]
 (
  [Key] bigint IDENTITY(1,1) NOT NULL CONSTRAINT [PK_VPI_APO] PRIMARY KEY,
  [PZN] nvarchar(7) NOT NULL,
  [Key_INB] nvarchar(5) NOT NULL,
 ) ON [PRIMARY]
GO

ALTER TABLE dbo.[VPI_APO] ADD CONSTRAINT [IX_VPI_APOKey_INB] UNIQUE NONCLUSTERED 
 (
  [PZN],
  [Key_INB]
 ) ON [PRIMARY]
GO

I often get timeouts when I search an item in this table like this (during inserting high volumes of items):

SELECT [Key] FROM dbo.[VPI_APO] WHERE ([PZN] = @Search1) AND ([Key_INB] = @Search2)

These timeouts when searching on the unique constraints happen quite often. I expected unique constraints to have the same benefits as indices, was I mistaken? Do I need an index on these fields, too?
Or will I have to search differently to benefit of the constraint?

I’m using SQL Server 2008 R2.

Answer 1

A unique constraint creates an index. Based on the query and the constraint defined you should be in a covering situation, meaning the index provides everything the query needs without a need to back to the cluster or the heap to retrieve data. I suspect that your index is not sufficiently selective or that your statistics are out of date. First try updating the statistics with sp_updatestats. If that doesn’t change the behavior, try using UPDATE STATISTICS VPI_APO WITH FULL SCAN. If neither of those work, you need to examine the selectivity of the index using DBCC SHOW_STATISTICS.