SQL Server Database Settings Performance Checklist
When auto_create_statistics is turned on (which it is by default), statistics are automatically created on all columns used in the WHERE clause of a query. This occurs when a query is optimized by the Query Optimizer for the first time, assuming the column doesn’t already have statistics created for it. The addition of column statistics can greatly aid the Query Optimizer so that it can create an optimum execution plan for the query.
If this option is turned off, then missing column statistics are not automatically created, when can mean that the Query Optimizer may not be able to produce the optimum execution plan for the query, and the query’s performance may suffer. You can still manually create column statistics if you like, even when this option is turned off.
There is really no down-side to using this option. The very first time that column statistics are created, there will be a short delay as they are created before the query runs for the first time, causing the query to potentially take a little longer to run. But once the column statistics have been created, each time the same query runs, it should now run more efficiently than if the statistics did not exist in the first place.
As part of your audit, if you find this option turned off, you will need to research why it was turned off. If you can’t find the reason, or if the reason is poor, turn this option on.
In order for the Query Optimizer to make smart query optimization decisions, the column and index statistics need to be up-to-date. The best way to ensure this is to leave the auto_update_statistics database option on (the default setting). This helps to ensure that the optimizer statistics are valid, helping to ensure that queries are properly optimized when they are run.
But this option is not a panacea. When a SQL Server database is under very heavy load, sometimes the auto_update_statistics feature can update the statistics on large tables at inappropriate times, such as the busiest time of the day.
If you find that the auto_update_statistics feature is running at inappropriate times, you may want to turn it off, and then manually update the statistics (using UPDATE STATISTICS) when the database is under a less heavy load.
But again, consider what will happen if you do turn off the auto_update_statistics feature. While turning this feature off may reduce some stress on your server by not running at inappropriate times of the day, it could also cause some of your queries not to be properly optimized, which could also put extra stress on your server during busy times.
Like many optimization issues, you will probably need to experiment to see if turning this option on or off is more effective for your environment. But as a rule of thumb, if your server is not maxed out, then leaving this option on is probably the best decision.
Some databases need to be shrunk periodically in order to free up disk space as older data is deleted from the database. But don’t be tempted to use the auto_shrink database option, as it can waste SQL Server resources unnecessarily.
By default, the auto_shrink option is turned off, which means that the only way to free up empty space in a database is to do so manually. If you turn this option on, SQL Server will then check every 30 minutes to see if it needs to shrink the database. Not only does this use up resources that could better be used elsewhere, it also can cause unexpected bottlenecks in your database when the auto_shrink process kicks in and does its work at the worst possible time.
If you need to shrink databases periodically, perform this step manually using the DBCC SHRINKDATABASE or DBCC SHRINKFILE commands, or you can use the SQL Server Agent or create a Database Maintenance Plan to schedule regular file shrinking during less busy times.
As part of your audit, if you find this option turned on, you will need to research why it was turned off. If you can’t find the reason, or if the reason is poor, turn this option off.
If a database will be used for read-only purposes only, such as being used for reporting, consider setting the read_only setting on (the default setting is off). This will eliminate the overhead of locking, and in turn, potentially boost the performance of queries that are being run against it. If you need to modify the database on rare occasions, you can also turn the setting off, make your change, then turn it back on.
Because data pages in SQL Server (8K) and NT Server or Windows Server (512 bytes) are different sizes, it is possible during power failures, or if you are have disk driver or physical disk problems, for your database to become corrupted.
Here’s why. Every time the operating system writes an 8K SQL Server data page to disk, it must break up the data into multiple 512 byte pages. After the first 512 byte of data is written, SQL Server assumes that the entire 8K has been written to disk successfully. So if the power should go out before all of the 512 byte pages that make up the 8K SQL Server page are written, then SQL Server does not know what has happened. This is known as a torn page.
As you can imagine, this corrupts the data page, and in effect makes your entire database corrupt. There is no way to fix a database made corrupt due to a torn page, except by restoring a known good backup. One of the best ways to prevent this problem is to ensure your server has battery backup. But this does not prevent all problems, because a defective disk driver can also cause similar problems (I have seen this.)
If you are worried about getting torn pages in your SQL Server databases, you can have SQL Server tell you if they occur (although it can’t prevent them or fix them after they have occurred). There is a database option called “torn page detection” that can be turned on and off at the database level. If this option has been turned on, and if a torn page is discovered, the database is marked as corrupt and you have little choice but to restore your database with your latest backup.
In SQL Server 7.0, this option is turned off by default, and you must turn it on for every database you want it on for. In SQL Server 2000, this option is turned on by default for all databases.
So what’s the big deal, why not just turn it on and be safe? The problem is that turning this feature on hurts SQL Server’s performance. Not much mind you, but if you already have a SQL Server that is maxed out, then it might make a noticeable difference, and you may want to keep this option turned off. As a DBA, you must weight the pros and cons of using this option, and make the best decision for your particular situation.