Sliding Window Partitioning And Columnstore Indexes

Dmitri Korotkevitch walks through setting up sliding window partitioning on tables with columnstore indexes:

The biggest difference resides in partition function split and merge behavior. With B-Tree indexes, you can split and merge non-empty partitions. SQL Server would split or merge the data automatically, granted with the schema-modification (Sch-M) table lock held in place. Other sessions would be unable to access the table but at least split and merge would work.

This is not the case with columnstore indexes where you would get the error when you try to split or merge non-empty partitions. There are several reasons for this limitation. Without diving very deep into columnstore index internals, I could say that the only option of doing split of merge internally is rebuilding columnstore index on affected partitions. Fortunately, you can split and merge empty columnstore partitions, which allow you to workaround the limitation and also implement Sliding Window pattern and use partitioning to purge the data.

With SQL Server 2017, the logic gets a little simpler, as you can directly truncate partitions instead of shuffling them off to a separate table.

Partitioning Tables

Eleni Markou shows how to partition tables in Postgres, SQL Server, and Google’s BigQuery:

When it comes to Microsoft SQL Server things are a bit different as this database system does not support dynamic partitions and so partitioning the table manually can be a huge maintenance issue.

That being said, in order to create a partitioned table a similar procedure to the one previously presented must be followed. This time we will create a monthly partition.

Read on for scripts for each.

Max And Min Partition Values

Ken Kaufman explains a major performance problem when trying to get maximum (or minimum) values from a partitioned table:

Now that I rambled a bit you want to know why when using a partitioned table does grabbing the min and max of the primary key take sooooo long, and how do you fix it.  Theoretically you would expect SQL to perform the following steps in grabbing the Max Id

  1.         Grab the Max Id from each partition using a seek
  2.         Hold the results in  temp storage
  3.         Get the Max ID from the  temp storage, and return that result.

 

However SQL doesn’t do that, it actually scans each partition and finds the max id after it has examined every record in each partition.  This is very inefficient, and could kill a query that depends on this value, as well as impact a busy server low on physical resources.    So what we need to do, is manually write the code to perform the steps that SQL Server should actually be doing.

Read on for one workaround Ken uses to deal with this inefficiency.

Discovering Partition Schemes

Kennie Nybo Pontoppidan shows us how to find the partition scheme for a particular table:

I needed to query SQL Servers metadata about partitioned tables, especially the column and the partition scheme used partitioning. The former is quite nicely documented in the SQL Server documentation (see link below), but the latter is not (yet). I have written the team about this, hopefully the documentation will be updated. Until then, I wrote this blog post to help others searching for an answer to this.

Click through for the script.

Switching In Identity Columns

Kenneth Fisher shows a way of working around the difficulty of adding an identity column to an existing table:

A friend had an interesting problem today. A really big table (multiple millions of rows) and no primary key. He then ran into an issue where he had to have one. The easiest thing is to create a new int column with an identity column right? Unfortunately in this case because of the size of the table, the log growth on adding an identity column was too much. So what to do?

Well, it would be nice if we could add an int column, populate it in chunks, then make it an identity column. Unfortunately, you can’t add identity to an existing column.

Read on for the answer.

Finding Partition Boundaries

Kenneth Fisher shows how to find the min and max values for a partition:

So what does it do? Per BOL

Returns the partition number into which a set of partitioning column values would be mapped for any specified partition function in SQL Server 2016.

So it basically tells us which partition any given row is in. This can be particularly handy at times. For example, if you want to know the min and max values of a column per partition.

Read on for a couple scripts which use $Partition.

Partitioning Nullable Columns

Kenneth Fisher looks at what happens when you use a nullable column as a partition key:

So to start with how does partitioning handle a NULL? If you look in the BOL for the CREATE PARTITION FUNCTION you’ll see the following:

Any rows whose partitioning column has null values are placed in the left-most partition unless NULL is specified as a boundary value and RIGHT is indicated. In this case, the left-most partition is an empty partition, and NULL values are placed in the following partition.

So basically NULLs are going to end up in the left most partition(#1) unless you specifically make a partition for NULL and are using a RIGHT partition. So let’s start with a quick example of where NULL values are going to end up in a partitioned table (a simple version).

Click through to see Kenneth’s proof and the repercussions of making that partitioning column nullable.

Partitioned Columnstore Tables

Denny Cherry makes an important point about dealing with columnstore tables:

ColumnStore indexes are all the rage with data warehouses. They’re fast, they’re new(ish) and they solve all sorts of problems when dealing with massive amounts of data.  However they can cause some issues as well if you aren’t very careful about how you setup your partitions on the ColumnStore index.  This is because, you can’t split a ColumnStore partition once it contains data.

Now, if everything is going according to plan you create your partitions well in advance and there’s no issues.

However, if everything hasn’t gone according to plan and someone forgets to create the partitions and you end up with rows in the final partition, you can’t create any more partitions because you can’t split the partition.

Ideally, you get those ducks in a row first.  Keep reading for a repro script and a couple potential workarounds.

Columnstore Partitioning In SQL Server 2016

Niko Neugebauer demonstrates some performance improvements to partitioned columnstore indexes in SQL Server 2016:

266ms was the partitioned table under SQL Server 2016 (compatibility level 120) while 353ms of the total elapsed time was obtained on SQL Server 2014! This represents a solid 25% improvement

All execution plans will have the same iterators, but will differ on the overall estimated cost (the non-partitioned queries will be way lower than the partitioned ones), as well as the distribution of the estimated costs within the execution plan, but as for the rest – it will be quite similar, like the one shown on the image below:

These improvements were swamped by the aggregate predicate pushdown improvements in 2016, at least in Niko’s example, but I’ll take a free 25%-33% performance improvement.

Partitioning Basics

Kim Tripp explains partitioning:

So, you’d still need to determine if this is the right approach. But, the main point – partitioning really isn’t designed to give incredible gains to your queries. It’s meant to be better for data management and maintenance. However, some partitioning designs can lead to query performance benefits too.

This is a nice introduction and makes a good point:  performance benefits to partitioning are incidental to the real benefit, which is simplicity of administration.

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