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 Indexes, the first group of the waits will suddenly be becoming more important for every single SQL Server user and mixing it together with the Wait Types specific to the internal structures and functions of the Columnstore Indexes makes no sense.

Read on to learn more about these important wait types.

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 on unknown factors in the server’s initial memory state and the rate of memory actually used by queries to run batch mode operations. This blog post shares a reproduction of the issue along with a call to action.

If you use clustered columnstore indexes, check out Joe’s User Voice entry.

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 for the Transactional Replication, but I feel that a great opportunity is lost and the Replication technology are being quite ignored by the emerged technologies, such as In-Memory & Columnstore, where the scenarios of replicating the Data Warehousing data is something that a lot of people can find very useful.

I wish it would be otherwise, and this would allow to bring more customers to use Columnstore Indexes.

Clustered columnstore indexes aren’t possible, but read on to learn whether non-clustered columnstore indexes are supported.

Trace Flag 834 And Columnstore Tables

Joe Obbish shows how trace flag 834 can solve a bottleneck when inserting into tables with clustered columnstore indexes:

In my experience, when we get into a situation with high memory waits caused by too much concurrent CCI activity all queries on the server that use a memory grant can be affected. For example, I’ve seen sp_whoisactive run for longer than 90 seconds.

It needs to be stated that not all CCIs will suffer from this scalability problem. I was able to achieve good scalability with some artificial tables, but all of the real target tables that I tested have excessive memory waits at high concurrency. Perhaps tables which require more CPU to compress naturally spread out their memory requests and the underlying OS is better able to keep up.

Read the whole thing, and also check out Lonny Niederstadt’s comment as it adds pertinent information about TF834.

Batch Mode Memory Fractions

Joe Obbish explains what memory fractions are and how incorrect calculations can lead to tempdb spills:

There’s very little information out there about memory fractions. I would define them as information in the query plan that can give you clues about each operator’s share of the total query memory grant. This is naturally more complicated for query plans that insert into tables with columnstore indexes but that won’t be covered here. Most references will tell you not to worry about memory fractions or that they aren’t useful most of the time. Out of thousands of queries that I’ve tuned I can only think of a few for which memory fractions were relevant. Sometimes queries spill to tempdb even though SQL Server reports that a lot of query memory was unused. In these situations I generally hope for a poor cardinality estimate which leads to a memory fraction which is too low for the spilling operator. If fixing the cardinality estimate doesn’t prevent the spill then things can get a lot more complicated, assuming that you don’t just give up.

Extremely interesting post.

Digging Into The In-Memory Columnstore Location

Niko Neugebauer does some investigation into where, exactly, memory-optimized columnstore data goes:

This is a rather simple blog post that is dedicated to the theme of the In-Memory Columnstore Indexes location. This has been a constant topic of discussion over a long period of time, even during the public events – and there is a need to clear out this topic.

I have assumed that the In-Memory Columnstore structures (Segments, Dictionaries, …) are located in the In-Memory, but there have been voices that I greatly respect, pointing that actually the Columnstore Object Pool is the exact location of any Columnstore structures, and there is nothing better than to take this feature for a ride and see what the SQL Server engine is actually doing.

Niko shows off a couple of useful DMVs along the way, too.

Columnstore Functionality Per Edition

Niko Neugebauer looks at how columnstore indexes differ between SQL Server Standard Edition, Express Edition, and Enterprise Edition:

One rather small (relatively other features, as I imagine), but an incredibly useful improvement was described in Columnstore Indexes – part 109 (“Trivial Plans in SQL Server 2017”) – is the ability to automatically produce Fully Optimised execution plans for the Database, which compatibility level is set to 140.

Running on both instances (Standard & Express), the following script, while altering the compatibility level between 140 (SQL Server 2017) & 130 (SQL Server 2016), will produce different execution plan for the SELECT COUNT_BIG(*) operation – the fast one (with FULL optimisation in 140 compatibility level) and slow one (with TRIVIAL optimisation in 130 compatibility level):

I am happy that this feature has got no Edition dependence, this is a needed improvement that simply increases the value of the offer and can actually be achieved in a lot of different ways, event without parallelism kicking in.

Niko has also helpfully provided a table at the end of the post to summarize his findings.

Columnstore Indexes And Partition Operations

Niko Neugebauer continues his columnstore index series, this time looking at how partitioned tables behave:

Let’s start with a simple test of merging the 2007 partition with the year 2008, by issuing the following command:

It might ready you a reasonably huge surprise, but this command will fail, if you are using the Columnstore Indexes.

The very same command will function without any problem, if we would simply avoid creating Clustered Columnstore Index …
The reason behind this limitation has to do with the fact that Columnstore Indexes do not sort or control the boundaries of the data, and this is biting the total implementation in such operations.

It’s an interesting read, and a little disappointing.

Optimized Bitmaps On Columnstore Indexes

Joe Obbish digs into bitmap filters and clustered columnstore indexes:

The position of the bitmap has changed so that it’s evaluated after the key lookup. That makes sense because the key lookup returns the column to be filtered against. However, the bitmap filter still reduces the estimated number of key lookups from 3000000 to 3000. This is impossible. The filter can only be applied after the key lookup, so it does not make sense for the bitmap to reduce the number of estimated executions of the key lookup.

Performance is significantly worse with the query now requiring 12199107 logical reads from the rowstore table and 13406 CPU time overall. We can see that the query did three million key lookups:

This is a fairly deep post, so you’ll probably want to check out the Paul White post on bitmaps first.

Columnstore Partition Management–Dealing With Non-Empty Partitions

Dmitri Korotkevitch shows a way of dealing with non-empty partitions on columnstore indexes:

The common, by the book approach recommends dropping columnstore index, splitting or merging partitions and recreating the index afterwards. As you can imagine, it would lead to extremely inefficient process with huge amount of unnecessary overhead on large tables. After all, you have to drop and recreate columnstore index, converting table to Heap, while just subset of the partitions needs to be rebuilt. Fortunately, you can minimize the overhead with simple workaround:

  1. Switch partition(s) to split or merge to the separate staging table

  2. Split or merge partition(s) in the main table. You can do that because partitions will be empty after the previous step

  3. Drop columnstore index in the staging table, split/merge partition(s) there and recreate the index afterwards

  4. Switch partition(s) back from staging to the main table.

Read on for a detailed walkthrough of these steps.

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