Columnstore Index Reorganization

Sunil Agarwal has a couple of posts on columnstore index defragmentation in SQL Server 2016.

Part 1:

Let us now look at how you can use REORGANIZE command to defragment your columnstore index. Note, this command is only supported for clustered columnstore index (CCI) and nonclustered columnstore index for disk-based tables. In the example below, I create an empty table and then create a clustered columnstore index and finally I load 300k rows.  SQL Server 2016 loads data from staging table into CCI in parallel when you specify TABLOCK hint. The machine I ran this test on has 4 logical processors so the 300k rows got divided into 75k each between 4 threads. Since each thread was loading < 102400 rows, the columnstore index ends up with 4 delta rowgroups as shown below.

Part 2:

A compressed rowgroup is considered as fragmented when any of the following two conditions is met

  • Less than 1 million rows but the trim_reason ( please refer to ) is other than DICTIONARY_SIZE. If the size of a compressed rowgroup is reduced because it has reached the maximum dictionary size, then it can’t be further reduced

  • It has nonzero deleted rows that exceeds a minimum threshold.

I just got finished with a first draft of a script to determine whether reorganizing a clustered columnstore index partition would be worthwhile, so this is great timing.  I hope to make my script available soon, after I incorporate Sunil’s heuristics.

Related Posts

Optimized Bitmaps On Columnstore Indexes

Joe Obbish digs into bitmap filters and clustered columnstore indexes: The position of the bitmap has changed so that it’s evaluated after the key lookup. That makes sense because the key lookup returns the column to be filtered against. However, the bitmap filter still reduces the estimated number of key lookups from 3000000 to 3000. […]

Read More

How Non-Clustered Index Key Columns Are Stored

Kendra Little walks through page-level details on a non-clustered index: Just like in the root page and the intermediate pages, the FirstName and RowID columns are present. Also in the leaf: CharCol, our included column appears! It was not in any of the other levels we inspected, because included columns only exist in the leaf of a […]

Read More


March 2016
« Feb Apr »