Earlier I wrote about SQL SERVER – Fundamentals of Columnstore Index and very first question I received in email was as following.

“We are using SQL Server 2012 CTP3 and so far so good. In our data warehouse solution we have created 1 non-clustered columnstore index on our large fact table. We have very unique situation but your article did not cover it. We are running few queries on our fact table which is working very efficiently but there is one query which earlier was running very fine but after creating this non-clustered columnstore index this query is running very slow. We dropped the columnstore index and suddenly this one query is running fast but other queries which were benefited by this columnstore index it is running slow.

Any workaround in this situation?”

In summary the question in simple words “How can we ignore using columnstore index in selective queries?”

Very interesting question – you can use I can understand there may be the cases when columnstore index is not ideal and needs to be ignored the same. You can use the query hint IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX to ignore the columnstore index. SQL Server Engine will use any other index which is best after ignoring the columnstore index.

Here is the quick script to prove the same.

We will first create sample database and then create columnstore index on the same. Once columnstore index is created we will write simple query. This query will use columnstore index. We will then show the usage of the query hint.

USE AdventureWorks
GO
-- Create New Table
CREATE TABLE [dbo].[MySalesOrderDetail](
[SalesOrderID] [int] NOT NULL,
[SalesOrderDetailID] [int] NOT NULL,
[CarrierTrackingNumber] [nvarchar](25) NULL,
[OrderQty] [smallint] NOT NULL,
[ProductID] [int] NOT NULL,
[SpecialOfferID] [int] NOT NULL,
[UnitPrice] [money] NOT NULL,
[UnitPriceDiscount] [money] NOT NULL,
[LineTotal] [numeric](38, 6) NOT NULL,
[rowguid] [uniqueidentifier] NOT NULL,
[ModifiedDate] [datetime] NOT NULL
) ON [PRIMARY]
GO
-- Create clustered index
CREATE CLUSTERED INDEX [CL_MySalesOrderDetail] ON [dbo].[MySalesOrderDetail]
( [SalesOrderDetailID])
GO
-- Create Sample Data Table
-- WARNING: This Query may run upto 2-10 minutes based on your systems resources
INSERT INTO [dbo].[MySalesOrderDetail]
SELECT S1.*
FROM Sales.SalesOrderDetail S1
GO 100
-- Create ColumnStore Index
CREATE NONCLUSTERED COLUMNSTORE INDEX [IX_MySalesOrderDetail_ColumnStore]
ON [MySalesOrderDetail]
(UnitPrice, OrderQty, ProductID)
GO


Now we have created columnstore index so if we run following query it will use for sure the same index.

-- Select Table with regular Index
SELECT ProductID, SUM(UnitPrice) SumUnitPrice, AVG(UnitPrice) AvgUnitPrice,
SUM(OrderQty) SumOrderQty, AVG(OrderQty) AvgOrderQty
FROM [dbo].[MySalesOrderDetail]
GROUP BY ProductID
ORDER BY ProductID
GO

We can specify Query Hint IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX as described in following query and it will not use columnstore index.

-- Select Table with regular Index
SELECT ProductID, SUM(UnitPrice) SumUnitPrice, AVG(UnitPrice) AvgUnitPrice,
SUM(OrderQty) SumOrderQty, AVG(OrderQty) AvgOrderQty
FROM [dbo].[MySalesOrderDetail]
GROUP BY ProductID
ORDER BY ProductID
OPTION (IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX)
GO

Let us clean up the database.

-- Cleanup
DROP INDEX [IX_MySalesOrderDetail_ColumnStore] ON [dbo].[MySalesOrderDetail]
GO
TRUNCATE TABLE dbo.MySalesOrderDetail
GO
DROP TABLE dbo.MySalesOrderDetail
GO

Again, make sure that you use hint sparingly and understanding the proper implication of the same. Make sure that you test it with and without hint and select the best option after review of your administrator. Here is the question for you – have you started to use SQL Server 2012 for your validation and development (not on production)? It will be interesting to know the answer.

Reference: Pinal Dave (http://blog.SQLAuthority.com)

Filed under: PostADay, SQL, SQL Authority, SQL Index, SQL Optimization, SQL Performance, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology