Columnstore Segment Elimination Pains

Niko Neugebauer shows us two pain points with row group (segment) elimination on clustered columnstore indexes:

Still this gives us processing of just 1 Segment with 253 lob logical reads vs 6 Segments and 2669 lob logical reads in the original query.

But wait, that’s not all! It can get significantly worse!
Let’s use the inequality search with the OR condition, to see what happens – should be safe, right ?

Spoilers:  it’s not safe.  I’ve been burned on the Min + Max issue that Niko shows, where you’d think that it could eliminate all segments except for the first and last, but that’s not how the process works today.

Related Posts

Trickle Insertion With Clustered Columnstore Indexes

Sunil Agarwal provides a pattern for trickle loading clustered columnstore indexes: A traditional scenario of loading data into CCI is a nightly load from one or more data files containing millions of rows. Recommended technique is to load the data with batchsize >= 102400 as explained https://blogs.msdn.microsoft.com/sqlserverstorageengine/2014/07/27/clustered-column-store-index-bulk-loading-the-data/. However, we are seeing many scenarios where data source is parallel […]

Read More

Wait Stats Related To Columnstore Indexes

Niko Neugebauer has some documentation on important wait stats around columnstore: I split the known wait types into the following distinct groups: – Batch Execution Mode (HTBUILD, HTDELETE, HTMEMO, HTREINIT, HTREPARTITION, PWAIT_QRY_BPMEMORY, BPSORT) – Columnstore Indexes (ROWGROUP_VERSION, ROWGROUP_OP_STATS, SHARED_DELTASTORE_CREATION, COLUMNSTORE_BUILD_THROTTLE, COLUMNSTORE_MIGRATION_BACKGROUND_TASK, COLUMNSTORE_COLUMNDATASET_SESSION_LIST) With an appearance in the next SQL Server version of the Batch Execution Mode for the RowStore […]

Read More

Categories

August 2017
MTWTFSS
« Jul Sep »
 123456
78910111213
14151617181920
21222324252627
28293031