Consider the following scenario:

  • You have a Microsoft SQL Server 2016 or 2017 database which contains columnstore indexes.

  • You enable Read Committed Snapshot Isolation (RCSI) at database level, or the database is acting as a readable secondary replica in an Availability Group (RCSI is enabled by default in this case).

  • You run read (SELECT) queries against columnstore index as INSERT statements occur on the same index.

In this scenario, you may notice poor performance in SQL Server 2016 and 2017. Additionally, you may experience severe blocking and query waits on QUERY_TASK_ENQUEUE_MUTEX and ROWGROUP_VERSION.


Cumulative update information:

This issue is fixed in the following cumulative updates for SQL Server:

       Cumulative Update 16 for SQL Server 2017

       Cumulative Update 3 for SQL Server 2016 Service Pack 2

Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:

Latest cumulative update for SQL Server 2017

Latest cumulative update for SQL Server 2016

On-Demand Hotfix Information:

This issue is fixed in the following on-demand hotfix for SQL Server:

      On-demand hotfix for SQL Server 2017 CU15


As a workaround for this issue, you can drop the columnstore index on the table.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.


Learn about the terminologythat Microsoft uses to describe software updates.

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