ColumnStore indexes are a powerful feature in SQL Server that can greatly improve query performance. However, there are certain scenarios where the expected performance improvement may not be achieved. In this blog post, we will discuss some common troubleshooting scenarios and provide solutions to address them.

1. Did the optimizer use the ColumnStore index?

One common issue is when the optimizer does not use the ColumnStore index in the query plan. To determine if the ColumnStore index was used, you can look at the SHOWPLAN. The Storage property will indicate whether the ColumnStore index was used or not. If the optimizer did not choose the ColumnStore index, you can force its usage by using a table hint, such as FROM t WITH (INDEX(myColumnStoreindex)). However, be cautious when using hints, as they can sometimes result in worse performance.

2. Was the query executed using batch processing?

Batch processing can significantly improve query performance. You can check if batch processing was used by looking at the SHOWPLAN. The EstimatedExecutionMode and ActualExecutionMode properties will indicate whether batch processing was chosen. If batch processing was not used, it could be due to insufficient memory or available threads. Optimizing memory and parallel execution settings can help resolve this issue.

3. Did the ColumnStore index provide good compression for the data?

The compression level of a ColumnStore index depends on the data being stored. Columns with a large number of unique values or a wide range of numeric values may not compress as efficiently. Including fewer columns in the ColumnStore index can improve compression and reduce the index size. However, it is generally recommended to include all columns in the index to ensure maximum query performance.

4. Did the query optimizer find a good plan?

The query plan plays a crucial role in query performance. You can analyze the query plan in Showplan to determine if a ColumnStore index was used for star join queries. Look for the presence of Batch Hash Table Build operators and check if the ActualExecutionMode is set to Batch. If the plan is not optimal, you can consider adjusting query hints or optimizing the query itself to improve performance.

By addressing these troubleshooting scenarios, you can optimize the usage of ColumnStore indexes in SQL Server and achieve the desired performance improvements. Remember to carefully analyze the query plans and consider the data characteristics when working with ColumnStore indexes.