Improvement

This improves the Qualification Limit for REORGANIZE to remove deleted rows from a rowgroup in Clustered Columnstore Index (CCI) in SQL Server 2019.

For better maintanence of deleted rows in Clustered Columnstore Index, Trace Flag (TF) 11631 and TF 11634 are introduced. To know more about the Trace Flags, see DBCC TRACEON - Trace Flags (Transact-SQL).

More information

By default when ALTER INDEX REORGANIZE is executed the qualifying limit to remove delete rows from a rowgroup is that the specific rowgroup has 10% of the maximum possible rows in a rowgroup deleted. Since the maximum possible rows in a row group is 1 million rows, the qualification limit is 10%, which is 100K rows deleted in a row group before the REORGANIZE removes the rows from the rowgroup. To know more about reorganize and rebuild indexes see, Defragmenting indexes by rebuilding or reorganizing the index.

The threshold limit has few concerns:

  • For few groups which are of full size of 1M rows, reaching 100K deleted rows may take a long time and until then the deleted rows are not cleared.

  • For smaller rowgroups, the percentage of deleted rows will be huge but still the deleted rows will not be considered done.

For further details, see New Trace Flags for better maintenance of deleted rows in Columnstore Index

Resolution

This improvement is included in the following cumulative update for SQL Server:

Each new cumulative update for SQL Server contains all the hotfixes and security fixes that were in the previous build. We recommend that you install the latest build for your version of SQL Server:

Latest cumulative update for SQL Server 2019

References

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

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