Columnstore indexes are a powerful feature in SQL Server that can greatly improve query performance for large data sets. However, there may be situations where using a columnstore index is not ideal for certain queries. In this article, we will discuss how to ignore a columnstore index in selective queries.

One of the common scenarios where you may want to ignore a columnstore index is when a specific query that used to run efficiently becomes slow after creating the index. This can happen because the columnstore index is optimized for large-scale data processing, but may not be the best choice for every query.

To ignore a columnstore index in a query, you can use the query hint IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX. This hint tells the SQL Server Engine to use any other index that is best suited for the query, after ignoring the columnstore index.

Here is an example script that demonstrates how to use the IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX hint:

USE AdventureWorks

-- Create a 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]

-- Create a clustered index
CREATE CLUSTERED INDEX [CL_MySalesOrderDetail] ON [dbo].[MySalesOrderDetail] ([SalesOrderDetailID])

-- Insert sample data into the table
INSERT INTO [dbo].[MySalesOrderDetail]
SELECT S1.*
FROM Sales.SalesOrderDetail S1
GO 100

-- Create a columnstore index
CREATE NONCLUSTERED COLUMNSTORE INDEX [IX_MySalesOrderDetail_ColumnStore] ON [MySalesOrderDetail] (UnitPrice, OrderQty, ProductID)
GO

-- Run a query that uses the columnstore 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

-- Run the same query with the IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX hint
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

-- Clean up the database
DROP INDEX [IX_MySalesOrderDetail_ColumnStore] ON [dbo].[MySalesOrderDetail]
GO
TRUNCATE TABLE dbo.MySalesOrderDetail
GO
DROP TABLE dbo.MySalesOrderDetail
GO

It is important to note that the IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX hint should be used sparingly and with caution. It is recommended to test the query with and without the hint, and consult with your database administrator to determine the best approach for your specific scenario.

In conclusion, columnstore indexes are a valuable tool for optimizing query performance in SQL Server. However, there may be cases where ignoring the columnstore index is necessary to improve the performance of specific queries. By using the IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX hint, you can instruct the SQL Server Engine to use an alternative index that is better suited for the query.

Have you started using SQL Server 2012 for your validation and development? We would love to hear your experiences and insights.