Statistics – Page 2 – Curated SQL

Metadata-Based Counting and Filtered Indexes

Published 2023-12-21 by Kevin Feasel

That’s great when you want to count the whole table without size-of-entire-table reads. It gets more complicated if you need to retrieve the count of rows that meet – or don’t meet – some criteria. Sometimes an index can help, but not always, depending on how complex the criteria might be.

For me, counting more efficiently typically means I take off my shoes.

One other note is, if you just need a guesstimate, or if the cardinality of that column you’re splitting by is fairly low, you could also look at the histogram, especially if there’s a statistic on the column (or columns) you’re interested in. It’s rare that I think to go that way, but it is one of the tools the optimizer itself uses, so it’s fair game.

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Manual Stats Updates in SQL Server

Published 2023-09-08 by Kevin Feasel

Matthew McGiffen can’t wait for SQL Server to update those stats:

Having up to date statistics is vital for getting the best performance out of your queries. Even though SQL Server automatically updates statistics in the background for you (When do statistics get updated?), you may find there are times when you want to manage updating them yourself.

Click through to learn several techniques for stats updates.

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Statistics and Ascending Keys in SQL Server

Published 2023-07-24 by Kevin Feasel

Matthew McGiffen talks stats:

The Ascending Key Problem relates to the most recently inserted data in your table which is therefore also the data that may not have been sampled and included in the statistics histograms. This sort of issue is one of the reasons it can be critical to update your statistics more regularly than the built-in automatic thresholds.

We’ll look at the problem itself, but also some of the mitigations that you can take to deal with it within SQL Server.

Read on to see how this behaved prior to SQL Server 2014’s new cardinality estimator and what has changed since then.

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When Statistics Updates Happen

Published 2023-06-28 by Kevin Feasel

Matthew McGiffen gives us the numbers:

SQL Server has had the ability to automatically update statistics since version 7.0. Nonetheless for a long part of my career working with SQL Server, whenever a performance issue raised its head everyone’s knee-jerk response would be “Update Statistics!” In most cases though the people shouting that didn’t really understand what the “Statistics” were, or what mechanisms might already be in place for keeping them up to date.

Of course SQL Server isn’t perfect and sometimes it is helpful for human intelligence to intervene. But to provide intelligent intervention one has to understand how things work.

Read on to learn what triggers automatic stats updates in various versions of SQL Server.

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An Overview of Statistics in SQL Server

Published 2023-06-22 by Kevin Feasel

Matthew McGiffen has a primer for us:

Statistics are vitally important in allowing SQL Server to find the most efficient way to execute your queries. In this post we learn more about them, what they are and how they are used.

Read on for plenty of information about stats. One thing I’d emphasize in this is that, for the most part, auto-generated stats are fine. The only time I’ve ever seen any value in creating my own stats is in multi-column statistics (which auto-generated stats doesn’t do), and even that’s of fairly limited value, at least in my experience. I’m sure that there’s a point in which hand-crafting your own statistics makes a tiny but noticeable marginal difference, but the systems I’ve worked with tend not to be anywhere near that point. There are usually much bigger fish to fry.

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Troubleshooting High I/O Usage on Azure SQL DB

Published 2023-01-13 by Kevin Feasel

Etienne Lopes troubleshoots a strange issue:

After the downsizing (to GeneralPurpose: Standard-series (Gen5), 2 vCores) occasionally there were timeouts in the application for a very specific task (the command timeout property in the application was set to 30 seconds). Other times the very same task would execute immediately, as it should always, since the underlying query was actually quite simple: a SELECT to a single, although large table (58 GB) but with a predicate that would always result in a perfect index seek to return never more than 300 rows. Furthermore each time there were timeouts, there were also momentary I/O spikes up to 100%:

Read on to learn more about what caused this problem and how Etienne was able to resolve it.

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Auto-Drop Stats in SQL Server 2022

Published 2022-12-20 by Kevin Feasel

Dennes Torres takes out the trash:

Auto-Create statistics is a well-known feature critical for the database performance. When one statistic is auto created, changing the table structure is not blocked by the presence of the statistic. An auto-created statistics is also dropped automatically when a schema change happens.

On the other hand, if the statistic is created by the user, any schema change will be blocked by the presence of the statistic.

The Auto-Drop setting on a statistic is a new SQL Server 2022 feature to change this behaviour.

Click through for a demonstration.

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Filtered Statistics and Table Performance

Published 2022-07-11 by Kevin Feasel

Guy Glantser provides a use case for filtered statistics:

Let’s say you have a very large table on a SQL Server 2012 Standard Edition instance. This means: old cardinality estimator and no partitioning. The table has a DATETIME column, which is ever-increasing, and it contains 5 years of data (it has to, due to regulations). The auto-update statistics kicks in only every 4 days, more or less, even when trace flag 2371 is enabled. The problem is that users usually query the table for the last day, and only rarely need to access older data. Since auto-update statistics uses a very small sample rate for very large tables, the result is not very accurate. The bottom line of all this is that most of the time you get a poor execution plan, because the optimizer estimates very few rows, while in fact there are many rows. What can you do?

I’m not sure I’ve ever used filtered statistics but it is good to know such a thing exists.

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Trivial Plans and Stats Updates

Published 2022-03-08 by Kevin Feasel

Lonny Niederstadt lays out the harshness of reality:

OK. SQL Server trivial plans for rowstore table INSERT. And related optimizer stats interaction.
TL;DR cached trivial plans for INSERT can be surprisingly stubborn. If a query matches to one, it won’t perform or queue a stats update even if the stats are stale. If the stats have been updated and would otherwise warrant a per-index plan – but there is a matching cached trivial plan for a per-row plan… outta luck. Might hafta DBCC FREEPROCCACHE or add OPTION(RECOMPILE) hint to make sure a cached trivial plan doesn’t prevent a per-index update for an INSERT when you really want one.

Read on for a dive into the topic.

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Querying Stats Data with a DMF

Published 2022-02-08 by Kevin Feasel

Grant Fritchey wants queryable data:

We’ve always been able to look at statistics with DBCC SHOW_STATISTICS. You can even tell SHOW_STATISTICS to only give you the properties, STAT_HEADER, or histogram, HISTOGRAM. However, it’s always come back in a format that you can’t easily consume in T-SQL. From SQL Server 2012 to everything else, you can simply query sys.dm_db_stats_properties to get that same header information, but in a consumable fashion.

Read on for a quick post showing a couple of things you can do with the DMF.

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Category: Statistics