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New DMF: dm_db_page_info

Pam Lahoud shows off a new Dynamic Management Function in SQL Server 2019:

The primary use case we had in mind when developing this function was troubleshooting page-related waits. Some of the key performance scenarios that require page details to diagnose are tempdb contentionlast page insert contention (also see this blog articlefor code samples) and page-level blocking. All of these scenarios present as either PAGELATCH or lock wait types in sys.dm_exec_query_stats with a page identifier (db_id:file_id:page_id) in the wait_resource column. In order to gain any insight into the problem, you have to know more details about what that page is. Is it a data page, an index page or something else? If it’s a data or index page, which object does it belong to? Prior to SQL Server 2019 CTP 2.0, the only way to gain any of this insight was to use the lightlydocumented command DBCC PAGE. Unfortunately, if you’re using some sort of monitoring tool or script to gather data about waits and blocking, there’s no good way to include this DBCC PAGE call in those scripts. Not to mention it requires sysadmin privileges. Enter sys.dm_db_page_info.

The sys.dm_db_page_info function takes 4 parameters: database ID, file ID, page ID and mode (‘LIMITED’ or ‘DETAILED’). It returns a table with a single row that contains the header information from the page, including things like object_id, index_id and partition_id. As a standalone function, this provides a documented and fully supported way to retrieve this information without the need for a DBCC command. The real benefit however is the ability to JOIN this function with other DMVs such as sys.dm_exec_requests or sys.sysprocesses to gather this information automatically when there is any sort of page-related contention. If you’re familiar with these views you know that the wait_resource column can contain all sorts of different resources, so it’s not in the right format to facilitate a direct join to sys.dm_db_page_info. To support this use case, we had to add another column to these views called page_resource. Whenever the wait_resource column contains a page resource, the page_resource column will be populated with an 8-byte hexadecimal value. The page ID is the first four bytes, followed by 2 bytes for the file ID and then 2 bytes for the database ID.

There are also a couple of supporting functions, so check those out.