Columnstore Parallelism

Sunil Agarwal shows how clustered columnstore indexes take advantage of parallelism:

Do you need to be concerned about that a delta rowgroup is scanned single threaded? The answer is NO for two reasons (a) most columnstore indexes have very few delta rowgroups (b) if you have multiple delta rowgroups, they can be scanned in parallel with one thread per delta rowgroup

I have a beef with (a), at least for SQL Server 2014, but that’s a story for another day.

Sunil has a follow-up post on parallel bulk import:

Recall that on rowstore tables (i.e. the tables organized as rows not as columnstore), SQL Server requires you to specify TABLOCK for parallel bulk import to get minimal logging and locking optimizations. One key difference for tables with clustered columnstore index is that you don’t need TABLOCK for getting locking/logging optimizations for bulk import. The reasons for this difference in behavior is that each bulk import thread can load data exclusively into a columnstore rowgroup. If the batch size < 102400, then the data is imported into a delta rowgroup otherwise a new compressed rowgroup is created and the data is loaded into it. Let us take two following interesting cases to show this bulk import behavior. Assume you are importing 4 data files, each with one bulk import thread, concurrently into a table with clustered columnstore index

The “don’t use TABLOCK” is interesting in comparison to rowstore tables.

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Reserved Memory Allocation Waits And Trace Flag 834

Joe Obbish has another post looking at sub-optimal columnstore index performance: It is possible to see a scalability bottleneck in the form of high average wait time for the RESERVED_MEMORY_ALLOCATION_EXT wait if a highly concurrent workload is run on a server that consumes memory with batch mode operators. I believe that the severity of the bottleneck depends […]

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Columnstore And Merge Replication

Niko Neugebauer tests whether merge replicated tables can use columnstore indexes: Adding this table to the publication will end up with the following, self-explaining error message, being very clear that the Clustered Columnstore Indexes are not supported for the Merge Replication[.] There is no surprise here, as the same Clustered Columnstore Indexes are not supported […]

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