If you have ever queried sys.dm_exec_requests or sys.dm_exec_query_stats you may have noticed the query_hash and query_plan_hash columns.  If you’re familiar with a sql_handle and plan_handle the query_hash and query_plan_hash should be easy to understand.  A query_hash is a computed value that points to a query irrespective of literal values.  Similarly a query_plan_hash is a computed […]

On occasion you might query sys.dm_exec_query_plan with a valid plan_handle and it will return NULL for the query_plan column.  This is despite seeing the plan_handle in sys.dm_exec_query_stats, sys.dm_exec_procedure_stats, or sys.dm_exec_cached_plans.  In this short post I’ll explain how to replicate this behavior and why you might see it. Replicating For this example I’ll be using a […]

Tempdb can be a major bottleneck on a busy SQL Server instance.  A common source of this is PFS, GAM, or SGAM page contention.  Lately I’ve seen on more than one SQL 2016 instance where a different page became a major bottleneck (if you are on SQL 2017 this may apply to you as well). […]

Earlier today I read a blog post detailing how to find your busiest database by querying sys.dm_exec_query_stats.  The blog post suggested you use the dbid column from sys.dm_exec_sql_text.  There is a problem with this.  The dbid is NULL for anything that is not a stored procedure. This is what was suggested in the blog post […]

Having the proper indexes in place is important for performance; however, on occasion indexes are created that are never used. Unnecessary indexes incur overhead for any DML. Fortunately this is a simple thing to detect. CREATE TABLE #UnusedIndexes (database_name sysname, table_name sysname, index_id int, object_id int, index_name sysname NULL, user_updates bigint, last_user_update datetime, user_seeks bigint, […]

In a previous post, I detailed how to determine if you have connection leaks. In this post I will show you how to determine which query executions are likely the source of the connection leaks. The approach we will use for finding the connection leaks is to look for sessions that have been in a […]

A common error that pops up in application logs as a result of long executions is: When these start showing up in an application log, it is a good indicator that a performance issue may be occurring in SQL Server.  So far as SQL Server is concerned, it thinks the application dropped or cancelled the […]