SCOM - Large Table query

SELECT TOP 1000
a2.name AS [tablename], (a1.reserved + ISNULL(a4.reserved,0))* 8 AS reserved,
a1.rows as row_count, a1.data * 8 AS data,
(CASE WHEN (a1.used + ISNULL(a4.used,0)) > a1.data THEN (a1.used + ISNULL(a4.used,0)) - a1.data ELSE 0 END) * 8 AS index_size,
(CASE WHEN (a1.reserved + ISNULL(a4.reserved,0)) > a1.used THEN (a1.reserved + ISNULL(a4.reserved,0)) - a1.used ELSE 0 END) * 8 AS unused,
(row_number() over(order by (a1.reserved + ISNULL(a4.reserved,0)) desc))%2 as l1,
a3.name AS [schemaname]
FROM (SELECT ps.object_id, SUM (CASE WHEN (ps.index_id < 2) THEN row_count ELSE 0 END) AS [rows],
SUM (ps.reserved_page_count) AS reserved,
SUM (CASE WHEN (ps.index_id < 2) THEN (ps.in_row_data_page_count + ps.lob_used_page_count + ps.row_overflow_used_page_count)
ELSE (ps.lob_used_page_count + ps.row_overflow_used_page_count) END ) AS data,
SUM (ps.used_page_count) AS used
FROM sys.dm_db_partition_stats ps
GROUP BY ps.object_id) AS a1
LEFT OUTER JOIN (SELECT it.parent_id,
SUM(ps.reserved_page_count) AS reserved,
SUM(ps.used_page_count) AS used
FROM sys.dm_db_partition_stats ps
INNER JOIN sys.internal_tables it ON (it.object_id = ps.object_id)
WHERE it.internal_type IN (202,204)
GROUP BY it.parent_id) AS a4 ON (a4.parent_id = a1.object_id)
INNER JOIN sys.all_objects a2  ON ( a1.object_id = a2.object_id )
INNER JOIN sys.schemas a3 ON (a2.schema_id = a3.schema_id)
WHERE a2.type <> N'S' and a2.type <> N'IT'