SQL SERVER查询效率最差的SQL语句

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT TOP 20
CAST(qs.total_elapsed_time / 1000000.0 AS DECIMAL(28, 2))
AS [Total Elapsed Duration(s)],
  qs.execution_count,
  SUBSTRING (qt.text, (qs.statement_start_offset/2) + 1,
  ((CASE WHEN qs.statement_end_offset = -1
THEN LEN(CONVERT(NVARCHAR(MAX), qt.text)) * 2
ELSE
qs.statement_end_offset
END-
qs.statement_start_offset)/2) + 1) AS [Individual Query]
, qt.text AS [Parent Query]
, DB_NAME(qt.dbid) AS [DatabaseName]
, qp.query_plan
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp

ORDER BY total_elapsed_time DESC