sys.dm_os_buffer_descriptors - Returns information about all the data pages that are currently in the SQL Server buffer pool
sys.dm_db_index_usage_stats - Returns counts of different types of index operations and the time each type of operation was last performed in SQL Server.
These query for search index usage in buffer pool
SELECT count(*) AS cached_pages_count, obj.name, index_id, i.name AS IndexName
FROM sys.dm_os_buffer_descriptors AS bd
INNER JOIN
(
SELECT object_id, 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_id, 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
LEFT JOIN sysindexes i ON obj.object_id = i.id AND obj.index_id = i.indid
WHERE database_id = db_id()
GROUP BY obj.name, index_id, i.name
--HAVING obj.name = 'INVENTSERIAL'
ORDER BY cached_pages_count DESC;
