Determine memory use by database and object

A Dynamic Management View (DMV) introduced in SQL Server 2005, called sys.dm_os_buffer_descriptors, contains a row for every page that has been cached in the buffer pool. Using this DMV, you can quickly determine which database(s) are utilizing the majority of your buffer pool memory. Once you have identified the databases that are occupying much of the buffer pool, you can drill into them individually. In the following query, I first find out exactly how big the buffer pool currently is (from the DMV sys.dm_os_performance_counters), allowing me to calculate the percentage of the buffer pool being used by each database:

DECLARE @total_buffer INT;

SELECT @total_buffer = cntr_value
   FROM sys.dm_os_performance_counters
   WHERE RTRIM([object_name]) LIKE '%Buffer Manager'
   AND counter_name = 'Total Pages';

;WITH src AS
(
   SELECT
       database_id, db_buffer_pages = COUNT_BIG(*)
       FROM sys.dm_os_buffer_descriptors
       --WHERE database_id BETWEEN 5 AND 32766
       GROUP BY database_id
)
SELECT
   [db_name] = CASE [database_id] WHEN 32767
       THEN 'Resource DB'
       ELSE DB_NAME([database_id]) END,
   db_buffer_pages,
   db_buffer_MB = db_buffer_pages / 128,
   db_buffer_percent = CONVERT(DECIMAL(6,3),
       db_buffer_pages * 100.0 / @total_buffer)
FROM src
ORDER BY db_buffer_MB DESC;

In the above query, I've included the system databases, but you can exclude them by uncommenting the WHERE clause within the CTE. Note that the actual filter may need to change with future versions of SQL Server; for example, in SQL Server 2012, there is a new database for Integration Services called SSISDB. You may want to keep an eye on system databases just to have a complete picture, seeing as there isn't much you can do about their buffer pool usage anyway - unless you are using master or msdb for your own custom objects.

So now I know that I can drill into that database specifically if I want to track down the objects that are taking up most of that memory. You can once again use the sys.dm_os_buffer_descriptors only this time, instead of aggregating the page counts at the database level, we can utilize a set of catalog views to determine the number of pages (and therefore amount of memory) dedicated to each object.

USE ### YOUR DB ###;
GO

;WITH src AS
(
   SELECT
       [Object] = o.name,
       [Type] = o.type_desc,
       [Index] = COALESCE(i.name, ''),
       [Index_Type] = i.type_desc,
       p.[object_id],
       p.index_id,
       au.allocation_unit_id
   FROM
       sys.partitions AS p
   INNER JOIN
       sys.allocation_units AS au
       ON p.hobt_id = au.container_id
   INNER JOIN
       sys.objects AS o
       ON p.[object_id] = o.[object_id]
   INNER JOIN
       sys.indexes AS i
       ON o.[object_id] = i.[object_id]
       AND p.index_id = i.index_id
   WHERE
       au.[type] IN (1,2,3)
       AND o.is_ms_shipped = 0
)
SELECT
   src.[Object],
   src.[Type],
   src.[Index],
   src.Index_Type,
   buffer_pages = COUNT_BIG(b.page_id),
   buffer_mb = COUNT_BIG(b.page_id) / 128
FROM
   src
INNER JOIN
   sys.dm_os_buffer_descriptors AS b
   ON src.allocation_unit_id = b.allocation_unit_id
WHERE
   b.database_id = DB_ID()
GROUP BY
   src.[Object],
   src.[Type],
   src.[Index],
   src.Index_Type
ORDER BY
   buffer_pages DESC;

Please keep in mind that the buffer pool is in constant flux, and that this latter query has explicitly filtered out system objects, so the numbers won't always add up nicely. Still, this should give you a fairly good idea of which objects are using your buffer pool the most.

When investigating the performance of your servers, buffer pool data is only a part of the picture, but it's one that is often overlooked. Including this data will help you to make better and more informed decisions about direction and scale.