Make sure you update the statistics

You might know that statistics can have a high impact on performance. As you add, remove and modify data, the statistics will be more and more outdated.

Sure, there’s the database option that updates statistics automatically, but it is a bit … rough. For a table with 10,000,000 rows, you have to modify 2,000,000 rows prior to 2016 (20%) and 100,000 rows as of 2016 with db compat level 2016 (SQRT(1000*@rows)). Also, when auto-update stats kicks in, it will sample the underlying data, in order for it not to take too long etc.

Many of us use Ola Hallengren’s excellent maintenance solution. I’m sure that some of us have our own favorite modifications we do to the jobs that the installation scrips creates for us. My point here is that the installation script do not update statistics by default. Here’s how job step in the “IndexOptimize – USER_DATABASES” job look like on my SQL Server 2017 instance. All by default:

EXECUTE [dbo].[IndexOptimize]
@Databases = 'USER_DATABASES',
@LogToTable = 'Y'

The default value for the @UpdateStatistics parameter is NULL which means “Do not perform statistics maintenance” See this page for reference.

So, by using the defaults, you end up defragmenting your indexes (which might not give you that much nowadays, considering that there isn’t that much of a difference between random and sequential I/O on modern disk subsystems). For some of the indexes that you defragment, you will get new statistics as a by-product. Those are the indexes that you rebuild – rebuild is internally creating a new index and the dropping the old one. But you likely have many indexes that don’t reach the 30% fragmentation threshold. And consider indexes over a key which is steadily increasing or decreasing. Inserting new rows will not cause fragmentation, but the statistics will become out-of-date.

What to do? Easy, just add a new job in which you call the IndexOptimize procedure with options to update all statistics where at least one row has been modified. Ola even has such example on his web-site, Example D. Here it is, I just added a parameter to log the commands to the CommandLog table:

EXECUTE dbo.IndexOptimize
@Databases = 'USER_DATABASES',
@FragmentationLow = NULL,
@FragmentationMedium = NULL,
@FragmentationHigh = NULL,
@UpdateStatistics = 'ALL',
@OnlyModifiedStatistics = 'Y',
@LogToTable = 'Y'

Schedule above as you wish. I prefer to do it every early morning if I can. But your circumstances like database size, how the data is modified etc will influence the frequency.

Another option is toe modify the existing “IndexOptimize – USER_DATABASES” job and just add below. This will both defrag your indexes and also update statistics.

@UpdateStatistics = 'ALL'

What about Maintenance Plans, you might think? Well, we all know that they aren’t that … smart – which is why we use scripts like Ola’s in the first place. Regarding statistics updates, they will update all stats, regardless of whether any rows at all has been modified since last time. It is a complete waste of resources to update stats if nothing has changed. Even sp_updatestats is smarter in this regard.

Note: This is in no way a criticism of the maintenance solution that Ola provides. He has no way of knowing our requirements in the job he creates. If the installation script could read our minds, I’m sure that it would schedule the jobs for us as well. OTOH, if Ola could create a T-SQL installation script that could read our minds, then he probably would do something else in the first place. 🙂

2 Replies to “Make sure you update the statistics”

Leave a Reply

Your email address will not be published.