What Prevents Columnstore Row Group Merge

Niko Neugebauer shows two major scenarios in which the Row Group Merge operator will not work as expected, leaving you with an unexpectedly large number of row groups:

Let’s take a look at the most evil and uncorrectable pressure within SQL Server for the Columnstore Indexes – the Dictionary Pressure. For any final user without a bit of the internal knowledge of the Row Group sizes, Dictionaries & Pressures, it is always a huge surprise to find out that they can’t have those perfectly sized Row Groups with 1.048.576 rows.

Let us set up a table producing a good Dictionary Pressure and load 1.1 Million Rows into it, expecting to get just 2 Row Groups, while perfectly being aware that the result shall be quite different:

Niko’s dictionary pressure example is a good reason not to include textual columns on columnstore indexes.

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Preventing Issues With Columnstore Indexes

Kevin Chant has a post covering issues in his experience with the creation of columnstore indexes: Error due to online statement It’s a very common error caused usually by somebody copying a rowstore “Create Index” command. The “online=on” option you use with rowstore indexes does not work with creating columnstore indexes yet. For the record the […]

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Avoid Key Lookups On Clustered Columnstore Indexes

Joey D’Antoni points out a potential big performance problem with clustered columnstore indexes: In the last year or so, with a large customer who makes fairly heavy use of this pattern, I’ve noticed another concern. Sometimes, and I can’t figure out what exactly triggers it, the execution plan generated, will do a seek against the […]

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