Outdated database statistics decrease SharePoint Server performance, cause time-outs, and generate run-time errors

About database statistics
Statistics for query optimization are objects that contain statistical information about the distribution of values in one or more columns of a table or indexed view. The query optimizer uses these statistics to estimate the cardinality, or number of rows, in the query result. These cardinality estimates enable the query optimizer to create a high-quality query plan.

For example, the query optimizer can improve query performance by using cardinality estimates to select the index seek operator instead of the more resource-intensive index scan operator. Otherwise, outdated statistics may decrease query performance by using inefficient query plans.

Large enterprise SharePoint deployments must have database maintenance plans to update database statistics on content databases that reside in Microsoft SQL Server. Customers should not rely only on SharePoint-based database maintenance jobs to perform these tasks. For more information, see Best practices for SQL Server in a SharePoint Server farm.
Symptoms
When database statistics become outdated, SharePoint Server installations may experience one or more of the following symptoms:
  • Slow load times and decreased performance that may generate an HTTP 500 error when you open a site page
  • Slower performance that generates error messages such as the following:

    Service unavailable

    Unknown SQL Exception 53

    Server Error in '/' Application Runtime Error
  • Performing search crawls causes unexpected SQL Server performance, database locking, and blocking
  • Long-running timer jobs such as the "Microsoft SharePoint Foundation Usage Data Processing" jobs that take progressively longer to finish during each iteration
  • The inability to open a SharePoint site, and an error message that resembles the following:

    Unexpected System.Web.HttpException: Request timed out
  • Site rendering time-outs when you load navigation, and the following error message:

    PortalSiteMapProvider was unable to fetch children for node
  • High CPU usage on the server that's running SQL Server when it processes SharePoint queries
Cause
These issues may be caused by outdated database statistics. SharePoint runs a timer job daily to update database statistics by using the proc_updatestatistics SQL procedure. However, for various reasons, this timer job may not be completed or may not update all tables consistently. For example, if a backup is running against the content database from SQL Server concurrently with the SharePoint timer job, the job won't continue.

When the SharePoint timer job that updates statistics is completed, the followings events may be written to the ULS logs:
  • e9bf "An error occurred while updating statistics in the database, {0}"
  • cm1y "Updating statistics in the database, {0}"
  • dbl2 "Skipping statistics update of the database {0} because it's status is {1}"
  • cm1x "Updating statistics in all databases on {0}"
If these conditions aren't monitored closely and if corrective actions aren't taken, database statistics become outdated, and SharePoint performance issues eventually occur.
Resolution
To prevent these symptoms and potential service outages, SQL Server maintenance plans should be implemented to keep SharePoint content database statistics updated by using the FULLSCAN option. For more information, see Index statistics.

When implementing the SQL Server maintenance plan to update the statistics on your SharePoint databases, it is not required to disable the job from SharePoint. However, because these maintenance tasks perform similar functions from both locations, it is permissible to disable the timer job from the SharePoint farm. For more information about how to manage the index update job from SharePoint Server, see Databases used by SharePoint have outdated index statistics (SharePoint 2013).
More information
Updating the statistics of SharePoint content databases, using the FULLSCAN option, on a daily basis from the SQL Server is a recommend best practice. For more information, see Best practices for SQL Server in a SharePoint Server farm and Database maintenance for SharePoint Foundation 2010.

However, if your SharePoint farm is currently experiencing performance issues because of outdated STATS, the following information can be used as a one-time mitigation step to alleviate this issue.

To display database statistics information from a specific database, run the following query:
-- Checking the DB Statsselect a.id as 'ObjectID', isnull(a.name,'Heap') as 'IndexName', b.name as 'TableName', stats_date (id,indid) as stats_last_updated_time from sys.sysindexes as a inner join sys.objects as b on a.id = b.object_id where b.type = 'U'
For more information about database statistics review, see DBCC SHOW_STATISTICS.

To update database statistics on a single database that uses the FULLSCAN option, run the following query:
-- Update DB StatsEXEC sp_MSforeachtable 'UPDATE STATISTICS ? WITH FULLSCAN'
Important note The “sp_MSforeachtable” option is an undocumented procedure that's provided “as is” and should only be used to mitigate the immediate issue. We do not recommended that you use this procedure as part of a regular maintenance plan. Instead, see our UPDATE STATISTICS (Transact-SQL) documentation about how to implement a plan to UPDATE STATISTICS, using the FULLSCAN option.

Depending on how outdated the database statistics have become, you may have to clear the query plan cache by running the DBCC FREEPROCCACHE command after you update the database statistics. You'll find the syntax and arguments for this procedure in DBCC FREEPROCCACHE (Transact-SQL). Doing this makes sure that new queries use the optimal execution plan after the database statistics are updated. For example, see the following query:
-- Remove all elements from the plan cacheDBCC FREEPROCCACHE
Important note Running the DBCC FREEPROCCACHE command clears the cache for all query plans in in the SQL instance. This command should be well understood before you execute it during production hours.

If the DBCC FREEPROCCACHE command was not executed after updating the outdated database statistics, queries with inefficient execution plans may still reside in cache and be used. If this is the case, force a recompile on the specified stored procedure by using the stored procedure (see sp_recompile (Transact-SQL)). For example, see the following query:
USE SP2013_Content_DBGOsp_recompile proc_getwebnavstruct
Running the sp_recompile command together with procedure, function, or table parameters targets a single element in the cache for removal without affecting the instance.
Vlastnosti

ID článku: 3103194 - Poslední kontrola: 10/10/2015 00:30:00 - Revize: 2.0

Microsoft SharePoint Server 2013, Microsoft SharePoint Server 2010, Microsoft Office SharePoint Server 2007

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