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sys.dm_os_buffer_descriptors (Transact-SQL)

Returns information about all the data pages that are currently in the SQL Server buffer pool. The output of this view can be used to determine the distribution of database pages in the buffer pool according to database, object, or type.

When a data page is read from disk, the page is copied into the SQL Server buffer pool and cached for reuse. Each cached data page has one buffer descriptor. Buffer descriptors uniquely identify each data page that is currently cached in an instance of SQL Server. sys.dm_os_buffer_descriptors returns cached pages for all user and system databases. This includes pages that are associated with the Resource database.

Column name

Data type

Description

database_id

int

ID of database associated with the page in the buffer pool. Is nullable.

file_id

int

ID of the file that stores the persisted image of the page. Is nullable.

page_id

int

ID of the page within the file. Is nullable.

page_level

int

Index level of the page. Is nullable.

allocation_unit_id

bigint

ID of the allocation unit of the page. This value can be used to join sys.allocation_units. Is nullable.

Note   sys.dm_os_buffer_descriptors might show nonexistent values in allocation_unit_id for clustered indexes that are created in versions of SQL Server earlier than SQL Server 2005.

page_type

nvarchar(60)

Type of the page, such as: Data page or Index page. Is nullable.

row_count

int

Number of rows on the page. Is nullable.

free_space_in_bytes

int

Amount of available free space, in bytes, on the page. Is nullable.

is_modified

bit

1 = Page has been modified after it was read from the disk. Is nullable.

numa_node

int

Nonuniform Memory Access node for the buffer. Is nullable.

read_microsec

bigint

The actual time (in microseconds) required to read the page into the buffer. This number is reset when the buffer is reused. Is nullable.

Requires VIEW SERVER STATE permission on the server.

sys.dm_os_buffer_descriptors returns pages that are being used by the Resource database. sys.dm_os_buffer_descriptors does not return information about free or stolen pages, or about pages that had errors when they were read.

From

To

On

Relationship

sys.dm_os_buffer_descriptors

sys.databases

database_id

many-to-one

sys.dm_os_buffer_descriptors

<userdb>.sys.allocation_units

allocation_unit_id

many-to-one

sys.dm_os_buffer_descriptors

<userdb>.sys.database_files

file_id

many-to-one

A. Returning cached page count for each database

The following example returns the count of pages loaded for each database.

SELECT COUNT(*)AS cached_pages_count
    ,CASE database_id 
        WHEN 32767 THEN 'ResourceDb' 
        ELSE db_name(database_id) 
        END AS database_name
FROM sys.dm_os_buffer_descriptors
GROUP BY db_name(database_id) ,database_id
ORDER BY cached_pages_count DESC;

B. Returning cached page count for each object in the current database

The following example returns the count of pages loaded for each object in the current database.

SELECT COUNT(*)AS cached_pages_count 
    ,name ,index_id 
FROM sys.dm_os_buffer_descriptors AS bd 
    INNER JOIN 
    (
        SELECT object_name(object_id) AS name 
            ,index_id ,allocation_unit_id
        FROM sys.allocation_units AS au
            INNER JOIN sys.partitions AS p 
                ON au.container_id = p.hobt_id 
                    AND (au.type = 1 OR au.type = 3)
        UNION ALL
        SELECT object_name(object_id) AS name   
            ,index_id, allocation_unit_id
        FROM sys.allocation_units AS au
            INNER JOIN sys.partitions AS p 
                ON au.container_id = p.partition_id 
                    AND au.type = 2
    ) AS obj 
        ON bd.allocation_unit_id = obj.allocation_unit_id
WHERE database_id = db_id()
GROUP BY name, index_id 
ORDER BY cached_pages_count DESC;

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