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.

Related Posts

Creating a Columnstore Index

Monica Rathbun shows a scenario where creating a clustered columnstore index can make data retrieval much faster: Using AdventureworksDW2016CTP3 we will work with the FactResellerSalesXL table which has 11.6 million rows in it. The simple query we will use as a demo just selects the ProductKey and returns some aggregations grouping them by the different […]

Read More

Simplifying Columnstore

Monica Rathbun takes us through concepts behind columnstore indexes: Now, I admit when these first were introduced in SQL Server 2012 I found them very intimidating (additionally, you couldn’t update them directly). For me, anytime you say columnstore, my mind tends to set off alarms saying wait stay away, this is too complicated. So, in […]

Read More

Categories

September 2017
MTWTFSS
« Aug Oct »
 123
45678910
11121314151617
18192021222324
252627282930