SQL Clone
SQLServerCentral is supported by Redgate
 
Log in  ::  Register  ::  Not logged in
 
 
 

Use Filtered Statistics to Improve Performance on Very Large Tables

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?

 

You can disable auto-update statistics, and replace it with your own job that runs daily and update statistics with full scan. This will definitely improve accuracy, but since the table is very large, it’s a very heavy operation that you can’t afford to run on your mission-critical 24×7 database. And we already agreed that you can’t use partitioning or the wonderful incremental statistics feature (introduced in SQL Server 2014). But there are good news – you can use filtered statistics.

 

You can create a daily job that creates statistics only for the last day. You’ll need to use dynamic SQL for that, but it’s not a big deal. Since it’s just the last day, you can create it with full scan, and it will be super-fast and super-accurate. Don’t forget to drop the statistics from the previous day in each job execution. Now, you also need to rewrite your queries to use dynamic SQL (or OPTION(RECOMPILE)), so that the optimizer can be sure that the filtered statistics covers your query. This part can be a pain, I know. It is much easier in newer versions of SQL Server (2016 and above), so if you needed more motivation to upgrade, there you go…

 

As soon as you have that mechanism in place, you’ll start to see good execution plans with very accurate row estimations. And the users will be happy again. Try it and see for yourself…

The post Use Filtered Statistics to Improve Performance on Very Large Tables appeared first on Madeira Data Solutions.

Guy Glantser

Guy Glantser, Data Platform MVP, is the leader of the Israeli PASS chapter and also the CEO and founder of Madeira Data Solutions. His career has been focused on the Microsoft Data Platform for the past 20 years, performing various database roles as either an on-site DBA, an external consultant or a speaker. Guy is involved in many activities in the Microsoft Data Platform community. He occasionally speaks at community events, such as PASS Summit, SQLBits, SQL Saturdays and user groups around the world. He also co-hosts the SQL Server Radio podcast.

Comments

Leave a comment on the original post [www.madeiradata.com, opens in a new window]

Loading comments...