I’ve been asked a lot of questions about updating statistics in SQL Server over the years. And I’ve asked a lot of questions myself! Here’s a rundown of all the practical questions that I tend to get about how to maintain these in SQL Server.
I don’t dig into the internals of statistics and optimization in this post. If you’re interested in that, head on over and read the fahhhbulous white paper, Statistics Used by the Query Optimizer in SQL Server 2008. Then wait a couple days and chase it with it’s charming cousin, Optimizing Your Query Plans with the SQL Server 2014 Cardinality Estimator.
I’m also not talking about statistics for memory optimized tables in this article. If you’re interested in those, head over here.
Quick Links to Jump Around This Article
- General Advice on Statistics Maintenance
- Which Free Third Party Tools Help Update Statistics?
- What are Statistics?
- What Creates Column Statistics?
- What are Index Statistics?
- Do I Need to Create Statistics Manually?
- Do I Need to Worry About Statistics Taking Up Too Much Storage in My Database?
- How Does Auto-Update Work for Statistics?
- Should I Turn On Trace Flag 2371 to Control Automatic Statistics Updates?
- Should I Turn on Trace Flag 7471 for Concurrent Statistics Update?
- Should I Update Statistics Asynchronously?
- How Often Should You Manually Update Statistics?
- Should I Use a Maintenance Plan to Update Statistics?
- Can Statistics Be Created or Updated using Parallelism?
- When Should You Update Statistics with FULLSCAN?
- Should I Update Statistics with sp_updatestats?
- Why Doesn’t Updating Statistics Help if Queries Use a Linked Server?
- How do Duplicate Statistics Get Created?
- Do I Need to Drop Duplicate Statistics?
- What are Multi-Column Statistics?
- What Can Go Wrong if I Create Multi-Column Statistics?
- What are Filtered Statistics, and Do I Need Them?
- How Can I Tell How Many Times a Statistic Has Been Used to Compile a Query?
- Can I Fake Out/ Change the Content of Statistics without Changing the Data?
- What are Temporary Statistics, and When Would I Need Them?
- Do I Need to Update Statistics on Temporary Tables?
- How Can I Tell if Statistics are Making a Query Slow?
General Advice on Statistics Maintenance
Back when I read philosophy, I found Aristotle a bit annoying because he talked so much about “moderation”. Doing too little is a vice. Doing too much was a vice. Where’s the fun in that?
Unfortunately, Aristotle was right when it comes to statistics maintenance in SQL Server. You can get in trouble by doing too little. And you can get in trouble by doing too much.
⇒ Be a little proactive: If you have millions of rows in some of your tables, you can get burned by doing no statistics maintenance at all if query performance stats to get slow and out of date statistics are found to be the cause. This is because you didn’t do any proactive work at all.
⇒ If you’re too proactive, you’ll eventually be sorry: If you set up statistics maintenance too aggressively, your maintenance windows can start to run long. You shouldn’t run statistics maintenance against a database at the same time you’re checking for corruption, rebuilding indexes, or running other IO intensive processes. If you have multiple SQL Servers using shared storage, that maintenance may hit the storage at the same time. And what problem were you solving specifically?
⇒ The moderate approach:
- Start with a SQL Server Agent job that updates statistics and index maintenance as part of a single operation/script run. Most folks begin running this once a week. If you have a nightly maintenance window, that can work as well.
- Only run statistics updates more frequently when it’s clear that you need to, and then customize the more frequent job to only update the specific statistics that are problematic. Only use FULLSCAN on individual stats if you must, and document which queries require it.
- If your tables have millions of rows and you run into estimate problems for recent data, consider using Trace Flag 2371 to increase the frequency of statistics updates for these tables instead of manually updating statistics throughout the day. (SQL Server 2008R2 SP1 +, not needed in SQL Server 2016.)
- If you have a batch process or ETL that loads significant amounts of data, consider having it update statistics on impacted tables at the end of the process. Exception: if the job creates or rebuilds indexes at the end of its run, the statistics related to those indexes are already updated with FULLSCAN and do not require any maintenance.
- Beware statistics updates that need to run frequently throughout the day: if this is your only option to fix a problem, you are applying a reactive fix which consumes IO and the query is periodically still slow. This is rarely a solid long term solution. Investigate stabilizing execution plans via indexes and query tuning instead.
Which Free Third Party Tools Help Update Statistics?
One widely used free script is Ola Hallengren’s SQL Server Index and Statistics Maintenance script.
If you’re managing lots of SQL Server instances and want ultimate customization, there is a free version of Minion Reindex.
What are Statistics?
Statistics are small, lightweight objects that describe the distribution of data in a SQL Server table. The SQL Server query optimizer uses statistics to estimate how many rows will be returned by parts of your query. This heavily influences how it runs your queries.
For example, imagine you have a table named agg.FirstNameByYear and you run this query:
SQL Server needs to estimate how many rows will come back for FirstNameId=74846. Is it 10 million rows, or 2? And how are the rows for Gender distributed?
The answers to both of these questions impact what it does to GROUP those rows and SUM the NameCount column.
Statistics are lightweight little pieces of information that SQL Server keeps on tables and indexes to help the optimizer do a good job.
What Creates Column Statistics?
If the agg.FirstNameByYear table was freshly created when we ran our query, it would have no column statistics.
By default, the SQL Server optimizer will see that no statistics exists, and wait while two column statistics are created on the FirstNameId and Gender columns. Statistics are small, and are created super fast– my query isn’t measurably any faster when I run it a second time.
Here’s what the statistics look like on the table in Object Explorer. Notice the artisanally crafted names.
If you want to verify which column is in each auto-created statistic, you can do that with this query:
SELECT s.name, s.auto_created, s.user_created, c.name as colname FROM sys.stats AS s JOIN sys.stats_columns as sc on s.stats_id=sc.stats_id and s.object_id=sc.object_id JOIN sys.columns as c on sc.object_id=c.object_id and sc.column_id=c.column_id WHERE s.object_id=OBJECT_ID('agg.FirstNameByYear') and s.auto_created = 1 ORDER BY sc.stats_column_id; GO
Sure enough, here are our statistics, and they are on Gender and FirstNameId. These are not considered ‘user created’ even though our user query was the cause of them being auto-generated. (“User created” means someone ran a CREATE STATISTICS command.)
SQL Server can now use the statistic on Gender and the statistic on FirstNameId for future queries that run.
What are Index Statistics?
Whenever you create an index in SQL Server, it creates a statistic associated with that index. The statistic has the same name of the index. Our agg.FirstNameByYear table has a clustered primary key, and here is the statistic that was created along with that index:
If columns are important enough to index, SQL Server assumes that it’s also important to estimate how many rows would be returned by that index when you query it. You can’t drop statistics associated with indexes (unless you drop the index).
Do I Need to Create Statistics Manually?
Nope! SQL Server does a really good job creating single-column statistics automatically.
Statistics will continue to be created on single columns when queries run as long as the “Auto Create Statistics” database property remains on. You can check that setting on your databases with the query:
SELECT is_auto_create_stats_on FROM sys.databases; GO
You should leave auto_create_stats_on set to 1 unless an application is specifically designed to manually create its own statistics. (That is pretty much limited to weirdoes like SharePoint.)
In rare situations, manually creating a multi-column statistic or a filtered statistic can improve performance… but keep reading to find out what those are and why it’s rare to require them.
Do I Need to Worry About Statistics Taking Up Too Much Storage in My Database?
Auto-created statistics are incredibly small, and you only get one per column in a table. Even if you have a statistic on every column in the table, this is a very small amount of overhead.
Statistics take up a negligible amount of space compared to indexes.
How Does Auto-Update Work for Statistics?
With default database settings, the SQL Server optimizer looks at how many changes have occurred for a given column statistic as part of query optimization. If it looks like a significant amount of rows in the column have changed, SQL Server updates the statistic, then optimizes the query. Because why optimize a query on bad data estimates?
The thresholds for when auto-update statistics kicks in are a little complicated.
For SQL Server 2005 – SQL Server 2014 (with no trace flags)
- If the table has 1-500 rows, if 500 rows have changed, statistics are considered not fresh enough for optimization
- If the table has 500+ rows, 500 rows + 20% of total rows in the table are the threshold
Note that the statistics don’t auto-update when the rows are changed. It’s only when a query is being compiled and the statistics would actually be used to make an optimization decision that the update is kicked off. Erin Stellato proves that in her post here.
Should I Turn On Trace Flag 2371 to Control Automatic Statistics Updates?
Trace Flag 2371 makes the formula for large tables more dynamic. When tables have more than 25,000 rows, the threshold for automatic statistics update becomes more dynamic, and this adjusts as the rowcount goes up. See a graph of the adjusting threshold in this post from the SAP team. (I think we know which team really felt some pain and wanted this trace flag to exist, because the trace flag was announced on their blog!)
Trace flag 2371 is available in SQL Server 2008R2 SP1-SQL Server 2014.
SQL Server 2016 automatically uses this improved algorithm. Woot! So if you’re using SQL Server 2016, you don’t need to decide. Erik Darling tested out the behavior in 2016 and wrote about it here.
Prior to 2016, here’s a quick rundown of pros and cons of TF2371:
Benefits to turning on Trace Flag 2371
- This trace flag is documented in KB 2754171
- This has been used by SAP for a while, and has become the default behavior in SQL Server 2016. That seems like a big vote of confidence.
Risks of turning on Trace Flag 2371
- The trace flag is instance-wide (“global”). You can’t change this behavior for a specific database.
- The documentation in KB 2754171 covers its behind a little conspicuously. They advise you that “If you have not observed performance issues due to outdated statistics, there is no need to enable this trace flag.” (Unless you run SAP.)
- This trace flag didn’t win the popularity contest to make it into the main Books Online list of trace flags.
- If you have this trace flag on and you need to open a support ticket, you may have to spend time explaining why you have this one and jumping through extra hoops to get a problem figured out.
Overall, this is a low risk trace flag. But in general it does NOT pay off to enable trace flags “just in case” for most people.
Should I Turn on Trace Flag 7471 for Concurrent Statistics Update?
Trace Flag 7471 is a global trace flag released in SQL Server 2014 SP1 Cumulative Update 6.
This trace flag changes the locking behavior associated with updating statistics. With the trace flag on, if you set up jobs to concurrently update statistics on the same table, they won’t block each other on metadata related to statistics updates.
Jonathan Kehayias has a great post on TF 7471 on SQL Performance.com. He demos the trace flag in a video for the SQL Skills Insider Newsletter where he shows the changes in locking behavior this flag introduces. Download the video in WMV format or MOV format to watch.
Benefits to turning on Trace Flag 7471
- This trace flag is documented in KB 3156157.
- Parikshit Savjani explains the trace flag on the MSSQL Tiger team blog
Risks of turning on Trace Flag 7471
- Running concurrent statistics updates against the same table can use more IO and CPU resources. Depending on what you’re doing, that might not be awesome.
- Running concurrent parallel statistics updates against very large tables can use more workspace memory resources than executing them serially. Running low on workspace memory can cause queries to have to wait for memory grants to even get started (RESOURCE_SEMAPHORE waits). (See more on this in Jonathan Kehayias’ post here.)
- Microsoft’s testing showed “TF 7471 can increase the possibility of deadlock especially when creation of new statistics and updating of existing statistics are executed simultaneously.” (source)
- The trace flag is instance-wide (“global”). You can’t change this behavior for a specific database.
- The documentation in KB 3156157 is pretty minimal.
- This is a new trace flag introduced in a cumulative update, and it isn’t very widely used yet.
- This trace flag also didn’t win the popularity contest to make it into the main Books Online list of trace flags.
- If you have this trace flag on and you need to open a support ticket, you may have to spend time explaining why you have this one and jumping through extra hoops to get a problem figured out.
Any trace flag can have unintended side effects. If I had a really good reason to run concurrent statistics updates against one table after exhausting other options to avoid the maintenance, I’d consider using this. I’d also only turn it on for that specific maintenance window, and turn it off afterward. (Edit: I wrote this before Microsoft blogged about the trace flag, but it turns out it’s exactly what the recommend in their blog post due to the deadlocking issue called out in “risks”.)
Should I Update Statistics Asynchronously?
Remember how I said that when SQL Server is optimizing a query, it smells the statistics to see if they’re still fresh, and then waits to update them if they’re smelly?
You can tell it not to wait to update them, and just use the smelly stats. It will then optimize the query. The stats will still be updated for any queries that come later, but boy, I hope those statistics were good enough for that first query!
You control this with the ‘Auto Update Stats Async’ setting. You can query this setting on your databases like this:
SELECT is_auto_update_stats_async_on FROM sys.databases; GO
Asynchronous statistics updates are usually a bad choice. Here’s why:
- Statistics update is typically incredibly quick
- It’s usually a greater risk that the first query will get a bad plan, rather than it having to wait
How Often Should You Manually Update Statistics?
If you know that a lot of data is changing in a table, you may want to manually update statistics. Possibly:
- A lot of data is changing, but it’s below the 20%+500 rows limit of where auto-update kicks in because the table has hundreds of millions of rows
- A small amount of data is changing in a large table, but it’s frequently queried
Data Loading: Update Statistics After Each Load
A classic example of a stats problem is “recent” data from an ETL. You have a table with 10 million rows. 500,000 rows of data are loaded for the most recent batch. Important queries are looking at the table for rows with LoadDate > two hours ago.
Statistics won’t update automatically for queries, because < 2,000,500 rows have changed.
Those queries will estimate that there is one row. (To be safe. Wouldn’t wanna estimate zero!) That’s a huge mis-estimation from 500,000, and you might end up with some very bad query plans. Gail Shaw wrote a great post on this– it’s called the “ascending date” problem.
In this kind of data load situation, it is helpful to run UPDATE STATISTICS against the entire table where the data has loaded at the end of the load. This is typically a lightweight command, but in very large tables, UPDATE STATISTICS may be run against specific indexes or columns that are sensitive to recent data to speed up the process. (Exception: if the job creates or rebuilds indexes at the end of its run, the statistics related to those indexes are already updated with FULLSCAN and do not require any maintenance.)
If you have found optimization problems and you can’t change your data load to manually update statistics, Trace Flag 2371 might help.
What if a Lot of Data Changes Through the Day Based on User Interactions?
Most databases are small. They don’t have millions and millions of rows. And most of those databases are just fine without any manual update of statistics.
If you’ve got tables with millions of rows, and you’ve found that statistics aren’t updating fast enough, you can consider:
- Turning on Trace Flag 2371 to make auto-update statistics run more frequently on tables with large amounts of rows
- Using a third party script to handle statistics maintenance along with your index maintenance (see the list above)
Should I Use a Maintenance Plan to Update Statistics?
No. You really shouldn’t.
Maintenance Plans are really dumb when it comes to statistics. If you run UPDATE STATISTICS against a table, index, or statistic, by default it will use a sampling. It won’t look at every row.
Maintenance Plans don’t do that. Instead, they require you to either:
- Scan every row in the table, for every statistic in the table. So if you’ve got a 100 million row table with 50 column statistics and no nonclustered indexes, it’ll scan the 100 million row table fifty times.
- Specify an exact percentage of each table to scan. For every table. So if you specify 3%, then 100 row tables would…. yeah.
Neither of these options are good options.
You could use a maintenance plan to kick off a third party script that’s smarter about this, but don’t use a maintenance plan with an Update Statistics task. I found cases where that was set up using the default fullscan and it was taking many hours to run against a tiny database.
Can Statistics be Updated Using Parallelism?
If you update statistics with FULLSCAN, SQL Server may choose a parallel plan since SQL Server 2005.
If you create or update statistics with sampling, including the default sampling used in automatic statistics creation, SQL Server may choose a parallel plan as of SQL Server 2016.
When Should You Update Statistics with FULLSCAN?
As we just covered, if you update statistics on an entire table with FULLSCAN:
- For any column statistic where there is not an index leading on that column, SQL Server will scan the whole table
- For any column statistic where there IS an index leading on that column, or for an index statistic, SQL Server will scan the index
As tables grow, updating with FULLSCAN starts taking longer and longer. It uses IO and resources on the instance. You started updating statistics because you were concerned about performance, and now you’re impacting performance.
Only update statistics with FULLSCAN if you are solving a specific problem, and you don’t have a better way to handle it. If you need to do this:
- Identify the column or index statistic that needs the special treatment, and only update that one with FULLSCAN
- Run the update as rarely as possible. If you start having to run this throughout the day against multiple statistics, it becomes increasingly hard for you to diagnose performance regressions and manage the instance
Manual statistics update with FULLSCAN can sometimes be used as a temporary measure to contain the pain of a problem while you research what query changes may stabilize the execution plans of the problem query, but this solution rarely satisfies users and business owners. Those folks are usually happier if something changes that guarantees the problem query will be consistently fast, even if a bunch of data changes. This usually means:
- A query rewrite
- A table or query hint
- A mechanism to “freeze” a specific query plan, using a dreaded Plan Guide or the new Query Store feature in SQL Server 2016
The essence of the problem with using manual statistics updates for a performance problem is that this is a reactive measure, and almost never prevents the problem from occurring entirely.
Should I Update Statistics with sp_updatestats?
The built in sp_updatestats procedure is smarter than the “Update Statistics Task” in maintenance plans. It rolls through every table in the database, and is smart enough to skip tables if nothing has changed. It’s smart enough to use the default sampling.
It’s a smaller sledgehammer than the maintenance plan task, but arguably still wastes resources, particularly when it comes to statistics associated with indexes. Consider this scenario:
- sp_updatestats is run
- Nightly maintenance runs a script to rebuild all indexes which are more than 70% fragmented, and reorganize all indexes which are between 40% and 70% fragmented
The ALTER INDEX REBUILD command creates a beautiful new index structure. To do this, it has to scan every row in one big operation. And while it’s doing it, it updates statistics with FULLSCAN, meaning based on every row.
So our maintenance did a bunch of work updating stats with the default sampling. Then redid the same work for every index that got rebuilt.
Erin Stellato is not a big fan of sp_updatestats. Read why in her excellent article here.
If you’re going to the trouble to set this up, it’s a better use of your resources to use an indexing script that can handle that statistics update inline for columns and for indexes where REORGANIZE is done, and just skip it for REBUILD.
Why Doesn’t Updating Statistics Help if Queries Use a Linked Server?
Linked servers are special. Especially frustrating.
Prior to SQL Server 2012 SP1, any query using a linked server would not have permission to use statistics on the remote (target) database unless it used an account with db_owner or db_ddladmin permissions, or higher. In other words, read-only permissions meant that SQL Server couldn’t use statistics to generate a good query plan.
Stated differently: prior to SQL Server 2012 SP1, you must choose between better security and better performance. You can’t have both.
Great reason to upgrade! Read more about this issue in Thomas LaRock’s article on linked server performance.
How do Duplicate Statistics Get Created?
Let’s revisit our example table. Two column statistics were created on this table, one on FirstNameId, and one on Gender:
Let’s say that later on we create an index on Gender named ix_Gender. It will have index statistics created for it! I now have a column statistic on Gender, and an index statistic on Gender.
Someone could also manually create another column statistic on Gender using the CREATE STATISTICS statement. That rarely happens, but never say never. These statistics are technically ‘duplicates’.
Do I Need to Drop Duplicate Statistics?
I’ve never found a case where dropping duplicate statistics improved performance in a measurable way. It is true that you don’t need them, but statistics are so small and lightweight that I wouldn’t bother writing and testing the code to clean them up unless I had a super good reason.
What are Multi-Column Statistics?
Multi-Column statistics are only sort of what the name sounds like. These can get created in a few different ways:
- When you create an index with multiple columns in the key, the associated statistic is a multi-column statistic (based on the keys)
- You can create a multi-column statistic with the CREATE STATISTICS statement
- You can run the Database Tuning Advisor, which you can tell to apply a bunch of those CREATE STATISTICS statements. It seems to love them. A lot.
But multi-column statistics don’t contain complete information for all the columns. Instead, they contain some general information about the selectivity of the columns combined in what’s called the “Density Vector” of the index.
And then they contain a more detailed estimation of distribution of data for the first column in the key. Just like a single column statistic.
What Can Go Wrong if I Create Multi-Column Statistics?
It’s quite possible that nothing will change. Multi-column statistics don’t always change optimization.
It’s also possible that the query you’re trying to tune could slow down. The information in the density vector doesn’t guarantee better optimization.
You might even make a schema change fail later on, because someone tries to modify a column and you created a statistic on it manually. (This is true for any user created statistic, whether or not it’s multi-column.)
It’s pretty rare to make a query faster simply by creating a multi-column statistic, and there’s a very simple reason: if a multi-column statistic is critically important, an index probably is even more critical to speed up execution. So usually the big win comes in creating or changing an index. (And yes, that also creates the multi-column statistic.)
What are Filtered Statistics, and Do I Need Them?
Filtered statistics are statistics with a “where” clause. They allow you to create a very granular statistic. They can be powerful because the first column in the statistic can be different from the column or columns used in the filter.
Filtered statistics are automatically created when you create a filtered index. That’s most commonly where you’ll find a filtered statistic: helping a filtered index.
It’s rare to need to create filtered statistics manually, and beware: if your queries use parameters, the optimizer may not trust the statistics when it optimizes plans — because the plan might be reused for parameters that go outside of the filtered. So you could potentially create a bunch of filtered statistics for nothing.
For loads of details on filtered stats, watch Kimberly Tripp’s free video, Skewed Data, Poor Cardinality Estimates, and Plans Gone Bad.
How Can I Tell How Many Times a Statistic Has Been Used to Compile a Query?
There is no dynamic management view that lists this. You can’t look at an instance and know which statistics are useful and which aren’t.
For an individual query, there are some undocumented trace flags you can use against a test system to see which statistics it’s considering. Read how to do it in Paul White’s blog here (see the comments for a note on the flag to use with the new cardinality estimator).
Can I Fake Out/ Change the Content of Statistics without Changing the Data?
You can, but don’t tell anyone I told you, OK?
Read how to do this undocumented / bad idea / remarkably interesting hack on Thomas Kejser’s blog.
What are Temporary Statistics, and When Would I Need Them?
Temporary statistics are an improvement added in SQL Server 2012 for read-only databases. When a database is read-only, queries can’t create statistics in the database– because those require writes. As of SQL Server 2012, temporary statistics can be created in tempdb to help optimization of queries.
This is incredibly useful for:
- Readable secondaries in an AlwaysOn Availability Group
- Readable logshipping secondary databases
- A point in time database snapshot which is queried (whether against a database mirror or live database)
- Any other read-only database that has queries run against it
Prior to SQL Server 2012, if you use logshipping for reporting and the same workload does not run against the logshipping publisher, consider manually creating column level statistics. (Or upgrading SQL Server.)
Disambiguation: temporary statistics are unrelated to statistics on temporary tables. Words are confusing!
Do I Need to Update Statistics on Temporary Tables?
Table variables don’t have statistics, but temporary tables do. This is useful, but it turns out to be incredibly weird… because statistics on temporary tables can actually be reused by subsequent executions of the same code by different sessions.
Yeah, I know, it sounds like I’m delusional. How could different sessions use the same statistics on a (non-global) temporary table? Start by reading Paul White’s bug, “UPDATE STATISTICS Does Not Cause Recompilation for a Cached Temporary Table,” then read his post, “Temporary Tables in Stored Procedures.”
How Can I Tell if Statistics are Making a Query Slow?
If you run into problems with slow queries, you can test the slow query and see if updating statistics makes the query run faster by running UPDATE STATISTICS against a column or index statistic involved in the query.
This is tricker than it sounds. Don’t just update the statistics first, because you can’t go back!
- First, get the execution plan for the ‘slow’ query and save it off. Make sure that if they query was run with parameters, you test each run of the query with the same parameters used to compile it the first time. Otherwise you may just prove that recompiling the query with different parameters makes it faster.
- Do not run the UPDATE STATISTICS command against the entire table. The table likely has many statistics, and you want to narrow down which statistic is central to the estimation problem.
In many cases, queries are slow because they are re-using an execution plan that was compiled for different parameter values. Read more on this in Erland Sommarskog’s excellent whitepaper, Slow in the Application, Fast in SSMS – start in this section on “Getting information to solve parameter sniffing problems.”