Statistics are vital in allowing SQL Server to execute your queries in the most performant manner. Having a deeper understanding of how Statistics work really helps when you are performance tuning

I talked a few posts ago about Automatic Sample Sizes for Statistics Updates. From SQL 2016 CU4 we’ve been able to override that. You can manually update a statistics object specifying a given sample rate, and specify that that sample rate is what will be used for any subsequent automatic updates on the same object.

You can’t change the default sample rate globally, but this functionality is actually more useful. It means that if you find that you have a particular statistics object where the automatic sampling rate is too low, and that has a negative effect on query performance, then you can choose a sampling rate specific for that object.

Let’s take a quick look at doing this in practice.

Here’s some reused SQL from the previous post to create a table and throw in a million rows:

--Create table for Testing
CREATE TABLE dbo.Test(
   Id INT IDENTITY(1,1) CONSTRAINT PK_Test PRIMARY KEY CLUSTERED, 
   TextValue VARCHAR(20) NULL
);

--Create index on TextValue
CREATE INDEX IX_Test_TextValue ON dbo.Test(TextValue);

--Insert a bunch of rows
INSERT INTO dbo.Test(TextValue) 
SELECT TOP 1000000 'blah'
FROM sys.objects a, sys.objects b, sys.objects c, sys.objects d; 

I haven’t updated the stats yet. As we can see if I view them:

--View the statistics
DBCC SHOW_STATISTICS('dbo.Test', IX_Test_TextValue) WITH STAT_HEADER;

The stats haven’t any information yet but there is still one thing to notice. See that final column Persisted Sample Percent? That was added to support this functionality.

The stats will only get updated by the auto stats mechanism when I run a query against the table and the optimiser is interested in them but sees they’re stale. From zero to a million rows should be stale enough. Let’s run a quick query:

--Random Query
SELECT * 
FROM dbo.Test
WHERE TextValue = 'not blah’;'

And view the stats again:

We can see the stats got updated and the table was big enough that SQL decided to sample rather than scan the whole index. About 42% of the rows got sampled. Persisted Sample Percent is set to zero, i.e. it is not set.

Let’s say that I want to make sure that this statistics object always updates with a full scan.

What I can now do is manually update the statistics and specific that the sample percentage should be persisted:

--Update the stats and persist the sample rate
UPDATE STATISTICS dbo.Test IX_Test_TextValue 
WITH FULLSCAN, PERSIST_SAMPLE_PERCENT = ON;

Let’s check that did what we wanted by viewing the stats:

Great, we can see all the rows were sampled and the Persisted Sample Percent is now set to 100.

Now let’s see what happens when another auto update kicks in. I insert another million rows with the same SQL from above. Then I run the select query again. The optimiser notices the statistics are now stale and so it updates them and recompiles the plan for the query. We can see this when we check the stats yet again:

SQL has done exactly what we wanted, it performed the auto stats update in the background and it used the 100% persisted sample rate.

Got a problem or embarking on a SQL Server project and want some help and advice? I’m available for consulting – please get in touch or check out my services page to find out what I can do for you.

Statistics are vital in allowing SQL Server to execute your queries in the most performant manner. Having a deep understanding of how the SQL Server Optimizer interacts with Statistics really helps when you are performance tuning

One thing that can be useful when looking at an execution plan is to understand what statistics objects the optimizer used to come up with the plan. In this post we look at how that can be achieved using the undocumented traceflag 8666 which can be used to save internal debugging informational into the plan XML – including details of the Statistics objects used. 

There are actually a few other trace flags that do similar things but this seems the simplest and the one that works across the most versions of SQL Server. It looks like it was introduced in SQL 2008 and works on all versions up to and including SQL 2016 – but it doesn’t work on SQL 2005.

Note that this technique only works for plans generated once the trace flag is on, so you can’t view the additional information for existing plans in your cache:

Here’s a quick example using it against the AdventureWorks2012 database. You need to make sure you’ve selected the option to show the actual plan, then run the query as below:

--Turn the trace flag on
DBCC traceon (8666);

--Run the query
SELECT *
FROM Person.Person
WHERE LastName LIKE 'Smith%';

--Turn the trace flag off again
DBCC traceoff (8666);

This query produces the following plan:

If you right-click over the SELECT operator and select properties you see the following:

You’ll notice an extra field “Internal Debugging Information” which contains a bunch of unformatted XML. Rather than grabbing that string and formatting it to make it readable, you can right-click back on the plan itself and select “Show Execution Plan XML…” to view the same information in a more friendly format.

If you scroll down through the XML generated you will get to the following section:

<ModTrackingInfo>

   <Field FieldName=“wszStatName” FieldValue=“_WA_Sys_00000007_693CA210” />

   <Field FieldName=“wszColName” FieldValue=“LastName” />

   <Field FieldName=“m_cCols” FieldValue=“1” />

   <Field FieldName=“m_idIS” FieldValue=“2” />

   <Field FieldName=“m_ullSnapShotModCtr” FieldValue=“19972” />

   <Field FieldName=“m_ullRowCount” FieldValue=“19972” />

   <Field FieldName=“ullThreshold” FieldValue=“4494” />

   <Field FieldName=“wszReason” FieldValue=“heuristic” />

</ModTrackingInfo>

<ModTrackingInfo>

   <Field FieldName=“wszStatName” FieldValue=“IX_Person_LastName_FirstName_MiddleName” />

   <Field FieldName=“wszColName” FieldValue=“LastName” />

   <Field FieldName=“m_cCols” FieldValue=“1” />

   <Field FieldName=“m_idIS” FieldValue=“7” />

   <Field FieldName=“m_ullSnapShotModCtr” FieldValue=“19972” />

   <Field FieldName=“m_ullRowCount” FieldValue=“19972” />

   <Field FieldName=“ullThreshold” FieldValue=“4494” />

   <Field FieldName=“wszReason” FieldValue=“heuristic” />

</ModTrackingInfo>

Each ModTrackingInfo node displays the information about one statistics object that has been referenced. The wszStatName shows us the name of the Statistics object. We have two in this case, _WA_Sys_00000007_693CA210 and IX_Person_LastName_FirstName_MiddleName. The first is an auto-generated one on the LastName column (I happen to know this was generated before I created the index referenced). Interestingly SQL has looked at both – even though they contain pretty much the same info and one isn’t really required.

Why is this useful?

Well, apart from simply the interesting aspect of seeing some of what is going on in the background when SQL decides what plan to use for your query, it is often the case with poor performing queries that there is bad cardinality estimation going on. Sometimes this might be because the statistics aren’t accurate and would benefit from being updated. You can use this technique to see what statistics objects are being used for the estimation (where it is not just obvious) and then you can look at the objects themselves and see whether they might want refreshing, or whether ther may be other issues with your data – such as a skewed distribution – that make esimates difficult.

I see growing opinion in the SQL world that the refreshing of statistics is often more important than rebuilding indexes to reduce fragmentation – the latter operation will also refresh the statistics but is a lot more resource intensive. You also will commonly have statistics on columns that are not indexed, and the threshold for them to be refreshed can be pretty large when you have a lot of data. You can see more information about manually refreshing statistics in my previous posts on the subject:

When do Statistics Get Updated?

Manually updating Statistics

Automatic Sample Sizes for Statistics Updates

Got a problem or embarking on a SQL Server project and want some help and advice? I’m available for consulting – please get in touch or check out my services page to find out what I can do for you.

Statistics are vital in allowing SQL Server to execute your queries in the most performant manner. Having a deep understanding of how Statistics work really helps when you are performance tuning

I mentioned in my previous post about manually updating statistics that you can specify whether they’re updated using a full scan, or you can specify an amount of data to sample, either a percentage of the table size, or a fixed number of rows. You can also choose not to specify this, and SQL Server will decide for you whether to do a full scan, or to sample a certain amount of data.

I thought it would be interesting to look at what the sample sizes are that SQL will choose to use, depending on the amount of data in your table. Note that this occurs if you update statistics without specifying how they should be sampled as below:

UPDATE STATISTICS dbo.Test _WA_Sys_00000002_3AD6B8E2;

This is also the behaviour you will get when SQL updates statistics through the auto-stats mechanism. The fact that auto-stats may sample at a lower rate than is optimal for a given table and the queries against it is another reason you may choose to perform manual statistics updates.

To test this, I created a table and progressively pumped data in. Then after I inserted each batch of rows:

• I Ran a stats update capturing the CPU time taken
• Checked the statistics to see what sample size was used in the update
• Checked the size of the index

Here’s some of the code I used for the test:

--Drop table if exists
IF (OBJECT_ID('dbo.Test')) IS NOT NULL DROP TABLE dbo.Test;

--Create table for Testing
CREATE TABLE dbo.Test(
   Id INT IDENTITY(1,1) CONSTRAINT PK_Test PRIMARY KEY CLUSTERED, 
   TextValue VARCHAR(20) NULL
);

--Create index on TextValue
CREATE INDEX IX_Test_TextValue ON dbo.Test(TextValue);

--Insert a bunch of rows
INSERT INTO dbo.Test(TEXTValue) 
SELECT TOP 100000 'blah'
FROM sys.objects a, sys.objects b, sys.objects c, sys.objects d;  

--Update statistics without specifying how many rows to sample
SET STATISTICS TIME ON;
UPDATE STATISTICS dbo.Test IX_Test_TextValue;
SET STATISTICS TIME OFF;

--View the statistics
DBCC SHOW_STATISTICS('dbo.Test', IX_Test_TextValue) WITH STAT_HEADER;

--Check the size of the index
SELECT
    i.name                  AS IndexName,
    SUM(s.used_page_count)   AS Pages
FROM sys.dm_db_partition_stats  AS s 
JOIN sys.indexes                AS i
ON s.[object_id] = i.[object_id] AND s.index_id = i.index_id
WHERE i.name = 'IX_Test_TextValue'
GROUP BY i.name

The results of my testing are shown in the below table:

You can see that we have a full sample being taken for the statistics updates up to 4000,000 records (896 pages) but that once the table size hits 500,000 sampling is happening. If you look at the number of pages you will see we now have over 1,000 pages, 1000 pages being about 8MB of data, which is the threshold that sampling kicks in.

I wasn’t able to find a nice neat formula to determine the sampling rate based on table size, but if we look at the above figures there are still some insights to be gained. The main one is that you’ll notice that even as we double the table size, the number of rows sampled doesn’t go up by much. For instance from 500,000 to a million rows, only 10,000 more rows are sampled. This also means that even for pretty large tables, the update isn’t taking long – another reason why it’s worth leaving auto stats updates enabled and running synchronously with queries – they’re generally not going to take that long.

Another insight is that the percentage of rows sampled drops off very quickly. As the sample size doesn’t really increase that much even when the table size doubles – the percentage sampled has almost halved each time.

Got a problem or embarking on a SQL Server project and want some help and advice? I’m available for consulting – please get in touch or check out my services page to find out what I can do for you.

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.

Cardinality Estimation

Cardinality is a term originally from Mathematics, generally defined as “The number of objects in a given set or grouping”. In SQL we’re continually dealing with sets so this becomes a very relevant topic, which in our context is just the “number of rows”.

When you have a query across multiple tables there any many ways in which SQL Server could decide to physically go about getting you the results you want. It could query and join the tables in any order and it could use different methods for matching records from the various tables you have joined together. It also needs to know how much memory to allocate for the operation – and to do that it needs to have an idea of the amount of data generated at each stage of processing.

A lot of this requires cardinality estimation, and SQL Server uses something called Statistics objects to perform that calculation.

Let’s look at a simple example:
SELECT *
FROM Person.Person p
INNER JOIN Person.[Address] a
ON p.AddressId = a.AddressId
WHERE p.LastName = 'Smith'
AND a.City = 'Bristol'

When it comes to gathering the results for this query there are a number of ways the database engine could go about it. For instance:

a) It could find all the records in the Person table with a LastName of Smith, look each of their addresses up and return only the ones who live in Bristol.
b) It could find all the Addresses in Bristol, look up the people associated with each address and return only the ones called Smith.
c) It could grab the set of people called Smith from the People table, grab all the addresses in Bristol, and only finally match up the records between those two sets.

Which of those operations is going to be most efficient depends very much on the number of records returned from each table. Let’s say that we have a million people called Smith, but there’s only one address in our whole database that is in Bristol (and let’s say that address does actually belong to someone called Smith).

In the first method above I would grab the million Smiths and then look their address up one by one in the address table until I found the one that lived in Bristol.

If I used method b) though, I would find the one matching address first, and then I would simply look up the owner of that address. Clearly in this rather contrived example, that’s going to be a lot quicker. So if SQL knows ahead of time roughly how many records to expect from each part of the query, hopefully it can make a good decision about how to get the data.

But how can it work out how many rows will be returned without actually running the query?

Statistics

That’s where statistics objects come in. SQL Server maintains in the background data that equates to a histogram showing the distribution of the data in certain columns within a table. It does this any time you create an index – statistics will be generated on the columns the index is defined against, but it also does it any time it determines that it would be useful. So if SQL encounters a Where clause on Person.LastName – and that column isn’t involved in a useful index, SQL is likely to generate a statistics object to tell it about the distribution of data in that column.

I say “likely to” because it actually depends on the settings of your SQL instance. Server configuration is beyond the scope of this post but suffice to say you can let SQL automatically create Statistics objects – or not. You can let it automatically update them when the data has changed by more than a given threshold – or not. And you can specify whether updates to statistics should happen asynchronously or synchronously – i.e. in the latter case if your query determines that statistics needs updating then it will kick that off and wait until the update is complete before processing the query.

It’s generally recommended that auto creation and updating is on, and async updating is off.

Viewing Statistics Objects
Let’s have a look at some actual statistics and see what they hold. There are a couple of ways of doing this, the first is through SSMS. If you look under a table in the object browser you will see a Statistics folder which holds any statistics objects relating to that table:

In the above example you can see some that have friendly names, these are Statistics that  are related to an actual index that has been defined on the table and they have the same name as the Index – e.g. IX_Address_StateProvinceId.

You’ll also see we have some prefixed _WA_Sys and then with some random numbers following. These are statistics objects that SQL has created automatically on columns that aren’t indexed – or at least they weren’t indexed at the time the Statistics objects were created.

You can open these up with a double-click and see what’s inside:

This is the General tab you open up to. You’ll see it tells you what table the Statistics are for and what column(s). There are options for adding columns and changing the order – but you never really need to do this – as well as information to tell you when the statistics were last updated, and a check box if you want to update them now.

In the details tab there’s a lot more info:

I don’t find this the easiest display format to work with though, so rather than delving into what everything means here let’s look at the other way you can view statistics, which is by running the following command:

DBCC SHOW_STATISTICS('Person.Address', '_WA_Sys_00000004_164452B1')

The format is straightforward, you just specify the table you are interested in the Statistics for, and the actual name of the Statistics object you want. You can see the same information as if you double-clicked on it, but the results are output in the results pane like any other query and are (I think) a lot easier to read. Allegedly there will soon be a third way in SQL Server to view Statistics as DBCC commands are considered a bit “clunky” – but we don’t know what that will look like yet.

The command outputs three resultsets:

This post is just an introduction to statistics – and generally you don’t need to know that much, it’s just handy to understand the basics. So let’s just run over the key bits of information you can see above:

First of all in the first recordset – otherwise know as the…

Stats Header

Rows – is the number of rows in your table

Rows Sampled – this is how many rows were sampled to generate the statistics. SQL can generate or update statsitics using sampling rather than reading all the rows. In this case you’ll see it did actually read the whole table.

Steps – If you imagine the statistics as a bar chart – this is the number of bars on the chart. Statistics objects have a maximum of 200 steps so if you have more distinct values in your column than that they will be grouped into steps.

Density – This is supposed to be the probability of a row having a particular value (calculated as 1 / Number of Distinct values in column). According to books online “This Density value is not used by the query optimizer and is displayed for backward compatibility with versions before SQL Server 2008.”  I am using SQL 2012, and this number is just plain incorrect so don’t use it…

Recordset Number 2: The Density Vector

All Density – this is the accurate version of the Density statistic described above. So your probability of a given row having a specific value is about 0.0017. That’s a little less than one in 500. I happen to know there are 575 different Cities in the table so that makes sense. Sometimes SQL will use this value to form a plan – if it knows you’re going to search this table for a specific City and it doesn’t know that City when it makes the plan, then it could guess that about 1/500th of the rows will match your criteria.

Average Length – Is what it says on the can. The average length of data in this column.

Columns – The names of any column measured in this statistics objects. You can have statistics across multiple columns but I’m not going to cover that in this post. In this case it tells us these statistics are based on the “City” column.

Recordset Number 3: The Histogram

This last recordset shows the distribution of the data, and is what you could effectively use to to draw a graph of the relative frequencies of different groups of values. Each row represents a step – or bar on the bar chart – and as mentioned above there can be a maximum of 200 steps. So you can see, a statistics object is quite lightweight, even for a massive table.

RANGE_HI_KEY – This upper limit of each step, so each step contains all the values bigger than the RANGE_HI_KEY of the last step, right up to and including this value.

RANGE_ROWS – This is how many rows in the table fall in this range – not including the number that match the RANGE_HI_KEY itself.

EQ_ROWS – The number of rows equal to the HI_KEY

DISTINCT_RANGE_ROWS – The number of different values in the range that there is data for (excluding the HI_KEY).

AVERAGE_RANGE_ROWS – The average number of rows for a given value within the range.

That’s a whistle-stop tour of the Statistics SQL Server holds on your data.

The algorithms that SQL then uses to calculate the number of rows for a given part of your query are pretty transparent when there’s just one column involved. If we look at the above example and let’s say you wanted to look up the rows where the City is “Abingdon” – the statistics tell us there is 1 matching row and that’s the figure SQL will use for cardinality estimation. Where a value is within a range then it will use a calculation based on the AVERAGE_RANGE_ROWS.

When there’s multiple columns involved it’s more complicated there are various algorithms and assumptions that come into play. If you’re interested in digging deeper, one very good resource is the Whitepaper on the 2014 Cardinality Estimator written by Joe Sack: https://www.sqlskills.com/blogs/joe/optimizing-your-query-plans-with-the-sql-server-2014-cardinality-estimator/

Conclusions

The main takeaway from this should just be the understand the extent – and limitations – of the information about the distribution of your data that SQL holds in the background.

If, when you’re tuning queries, you notice that the estimated row counts don’t match the actual, then this could be encouraging SQL to form a bad plan for the query. In these cases you might want to investigate what’s going on with the statistics.

Maybe your query is written in a way that it can’t use statistics effectively, one example of this can be where you store constant values in variables, then query using that variable in a WHERE clause. SQL will then optimise based on the average, rather than on your actual value.

Maybe the plan is based on one data value that has a very different cardinality to the one currently being queried. For instance when you first run a stored procedure, the plan is formed based on the parameters passed. Those parameters could have a cardinality that is quite different to those used in later executions.

Maybe the statistics are out of date and need refreshing. Stats get updated when approximately 20% of the data in the table changes, for a large table this can be a big threshold, so current statistics may not always hold good information about the data you are querying for.

Or maybe SQL is doing as good a job as it can with the information it has at its disposal. You might need to work out how you can give it a little extra help.

Here are some other recent posts about Statistics you may find useful:

When do Statistics get updated?

Rowcount estimates when there are no Statistics

Got a problem or embarking on a SQL Server project and want some help and advice? I’m available for consulting – please get in touch or check out my services page to find out what I can do for you.