KB3167159 - FIX: Query runs slowly when SQL Server uses hash aggregate in the query plan

Applies To
SQL Server 2014 Enterprise - duplicate (do not use) SQL Server 2014 Developer - duplicate (do not use) SQL Server 2014 Enterprise Core - duplicate (do not use) SQL Server 2014 Standard - duplicate (do not use) SQL Server 2016 Developer - duplicate (do not use) SQL Server 2016 Enterprise - duplicate (do not use) SQL Server 2016 Enterprise Core - duplicate (do not use) SQL Server 2016 Standard - duplicate (do not use)

Symptoms

Your query may run slowly if the following conditions are true:

  • The query uses aggregates such as group by.
  • SQL Server uses hash aggregate in the query plan.
  • The estimated number of distinct values is much lower than the actual number of distinct values for the aggregate.

When this issue occurs, you will see a hash warning Event if you do Profiler or Extended Event tracing.

Cause

This issue occurs because SQL Server underestimates the amount of memory grant needed. This results in intermiedate results spilling to tempdb.

Resolution

The issue was first fixed in the following cumulative update of SQL Server:

Note You have to enable trace flag 9410 to activate this fix.

Recommendation: Install the latest cumulative update for SQL Server

Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. We recommend that you download and install the latest cumulative updates for SQL Server:

      

References

Learn about the terminology that Microsoft uses to describe software updates.