Tag: DBA

Headed Serial

Getting it out of the way, yes, we can create this nonclustered row store index to store our data in oh-so-perfect order for the windowing function:

CREATE INDEX row_store 
ON dbo.Comments
(
    UserId, 
    CreationDate
);

Keeping in mind that sort direction matters in how you write your query and define your index, this particular topic has, in my mind, been done to death.

Look ma, no Sort! Whoopie.

We also get serial plans. For the row store query, it’s about twice as fast, even single-threaded.

For the column store query, it’s about twice as slow.

Headed Parallel

Here are some interesting things. You’re going to want to speak to the manager.

Let’s force these to go parallel. For science.

Not only is the parallel version of the row mode plan a full second slower, but… look at that batch mode plan.

Look at it real close. There’s a sort before the Window Aggregate, despite reading from the same nonclustered index that the row mode plan uses.

But the row mode plan doesn’t have a Sort in it. Why? Because it reads ordered data from the index, and the batch mode plan doesn’t.

This must be hard to do, with all the support added for Batch Mode stuff, to still not be able to do an ordered scan of the data.

For those of you keeping track at home: yes, we are sorting sorted data.

Column-Headed

Let’s try an equivalent column store index:

CREATE COLUMNSTORE INDEX column_store_ordered 
ON dbo.Comments
(
    UserId, 
    CreationDate
)

Both have to sort, but both are fast and parallel. Yes, Sorting is annoying. Unfortunately, we can’t do this Sort in the application.

But hey, look how fast those index scans are. Choo-choo, as a wise man once said.

And of course, since we have to sort anyway, we’d be better off creating a wide nonclustered column store index on the table, so it would be more generally useful to more queries. You only get one per table, so it’s important to choose wisely.

If you have queries using window functions where performance is suffering, it might be wise to considered nonclustered column store indexes as a data source for them. Beyond just tricking the optimizer into using Batch Mode, the data compression really helps.

Elsewise

But there’s something else to consider, here: the plans with Sorts in them require memory.

It’s not much here — about 570MB — but in situations where more columns are needed for the query, they could get much larger.

And you know what can happen then. We’ll look at that in tomorrow’s post.

Thanks for reading!

Ramble On

Prior to SQL Server 2012, this blog post would be wrong. If you’re still on a version prior to 2012, ignore this blog post.

In fact, ignore every blog post and go upgrade those servers. Go on. Get out of here. Show’s over.

After SQL Server 2012, AKA the only versions that currently matter, this blog post is largely correct, and leans more towards correctness as you get to closer to SQL Server 2019.

Sliding scale correctness. Get on board.

Say It Slow

When windowing functions don’t have a Partition By, the parallel zone ends much earlier on than it does with one.

That doesn’t mean it’s always slower, though. My general experience is the opposite, unless you have a good supporting index.

But “good supporting index” is for tomorrow. You’re just going to have to deal with that.

WITH Comments AS 
(
    SELECT
        ROW_NUMBER() OVER
        (
            ORDER BY 
                c.CreationDate
        ) AS n
    FROM dbo.Comments AS c
)
SELECT 
    c.*
FROM Comments AS c
WHERE c.n = 0
OPTION(USE HINT('QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_140'));

With just an Order By in the windowing function, our query plan looks about like so:

Immediately after sorting data, our parallel streams are gathered. This is the end of our parallel zone, and it will occur regardless of if you’re filtering on the windowing function or now. I’m filtering on it here because I don’t want to spend any time retuning rows to SSMS.

Here’s an example of when a parallel zone is started again later:

WITH Comments AS 
(
    SELECT
        ROW_NUMBER() OVER
        (
            ORDER BY 
                c.CreationDate
        ) AS n
    FROM dbo.Comments AS c
)
SELECT 
    c.*
FROM Comments AS c
WHERE c.n > 100000000
OPTION(USE HINT('QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_140'));

Note that there’s no Top in this plan prior to the Filter.

You’ll see a Top generally when you filter on the windowing function with an equality or less-than predicate. Greater than seems to most often not end up with a Top in the plan.

Margaret Batcher

If we let Batch Mode run free, things turn out a little bit different.

WITH Comments AS 
(
    SELECT
        ROW_NUMBER() OVER
        (
            ORDER BY 
                c.CreationDate
        ) AS n
    FROM dbo.Comments AS c
)
SELECT 
    c.*
FROM Comments AS c
WHERE c.n = 0;

The Window Aggregate operator is within the parallel zone, unlike the Segment and Sequence Project operators in the Row Mode plan.

If we reuse the greater-than query from above while allowing Batch Mode to be used, we get a fully parallel plan.

Paddington

Of course, Partition By adds work, especially in Row Mode, and especially without a supporting index.

The nearly 11 second Row Mode plan compared to the 1.6 second Batch Mode plan doesn’t leave a lot of room for arguing.

It’s also worth noting here that Batch Mode Sorts (at least currently) will always sort on a single thread, unless it’s the child of a Window Aggregate operator, like in the above plan.

Tomorrow, we’ll look at how indexing can improve things, but not just row store indexes!

Everyone knows about those P(artition By) O(rder By) C(overing) indexes, but does that attention to ordering matter as much with column store indexes?

Tune in to find out!

Thanks for reading.

Kickball

To start things off, we’re going to talk about query plan patterns related to windowing functions.

There are several things to consider with windowing function query plans:

• Row vs Batch mode
• With and Without Partition By
• Index Support for Partition and Order By
• Column SELECTion
• Rows vs Range/Global aggregates

We’ll get to them in separate posts, because there are particulars about them that would make covering them all in a single post unwieldy.

Anyway, the first one is pretty simple, and starting simple is about my speed.

Row Mode

I’m doing all of this work in SQL Server 2019, with the database in compatibility level 150. It makes my life easier.

First, here’s the query we’ll be using. The only difference will be removing the hint to allow for Batch Mode later on.

WITH Comments AS 
(
    SELECT
        ROW_NUMBER() OVER
        (
            PARTITION BY
                c.UserId
            ORDER BY 
                c.CreationDate
        ) AS n
    FROM dbo.Comments AS c
)
SELECT 
    c.*
FROM Comments AS c
WHERE c.n = 0
OPTION(USE HINT('QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_140'));

Part of the row mode query plan will look like this:

The two operators that generate the numbering are the Segment and Sequence Project.

We’ll talk about the Sort later on. For now, we can see the segment “grouping” by UserId, and the Sequence Project description notes that it works over an ordered set.

You can probably guess why we need the Sort here.

Group is in quotes up there, because technically there’s no grouping. The “Segment1002” column in the Output List of the Segment is a computed column that marks the beginning and ending of each set of values. Likewise, the Sequence Project outputs “Expr1001”, which in this case is the calculated row number.

Batch Mode

In Batch Mode, there are three operators associated with windowing functions that get replaced with a single operator: the Window Aggregate.

The operators that get replaced are two we’ve already seen — Segment and Sequence Project, along with one we’ll see in a future post, the Window Spool.

We still need to Sort data for it without a supporting index. Gosh, those indexes sure are magickal.

The details of the Window Aggregate do still show a sequence generated, but we no longer see the “grouping”.

Baby Steps

Batch Mode kicks the pantalones off of Row Mode when it comes to window functions, but that’s not really the point of the post.

If you’re using a relatively modern version of SQL Server and also windowing functions, you should look at various ways to get Batch Mode processing alongside them.

Unless you’re on Standard Edition, probably.

In the next entry in this series, we’ll look at how the absence and presence of Partition By changes parallelism.

Thanks for reading!