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Allan Simonsen
Allan Simonsen

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SQL-Quick tip #11 - Most intensive queries

Sql Server tips and tricks

This is part of a series of quick tips and tricks I have accumulated over the year, that I think can be useful for others.
If you have similar short tips and tricks please leave a comment.

Most intensive queries

Most of us working with databases will at some point get task where we need to find out why the database is not performing as we expect and when running a big system that has intensive database usage it can be very tricky to find out what the cause is.
The query below is using the System dynamic Management View dm_exec_query_stats to list the performance statistics for the cached query plans in the Sql Server.
The scripts will provide you with some hints to which queries are the most CPU intensive.

SELECT SUBSTRING(qt.TEXT, (qs.statement_start_offset/2)+1,
                           ((CASE qs.statement_end_offset
                                        WHEN -1 THEN DATALENGTH(qt.TEXT)
                                        ELSE qs.statement_end_offset
                           END
                           - qs.statement_start_offset)/2)+1)
                           as [Text],
       qs.execution_count,
       qs.total_logical_reads, qs.last_logical_reads,
       qs.total_logical_writes, qs.last_logical_writes,
       qs.total_worker_time,
       qs.last_worker_time,
       -- converting microseconds to seconds
       qs.total_elapsed_time/1000000 total_elapsed_time_in_S,
       qs.last_elapsed_time/1000000 last_elapsed_time_in_S,
       qs.last_execution_time,
       qp.query_plan
  FROM sys.dm_exec_query_stats qs
        -- Retrieve the query text
        CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
        -- Retrieve the query plan
        CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
 ORDER BY qs.total_worker_time DESC -- CPU time

SELECT TOP 50
       [Avg. MultiCore/CPU time(sec)] = qs.total_worker_time / 1000000 / qs.execution_count,
       [Total MultiCore/CPU time(sec)] = qs.total_worker_time / 1000000,
       [Avg. Elapsed Time(sec)] = qs.total_elapsed_time / 1000000 / qs.execution_count,
       [Total Elapsed Time(sec)] = qs.total_elapsed_time / 1000000,
       qs.execution_count,
       [Avg. I/O] = (total_logical_reads + total_logical_writes) / qs.execution_count,
       [Total I/O] = total_logical_reads + total_logical_writes,
       Query = SUBSTRING(qt.[text], (qs.statement_start_offset / 2) + 1,
             (
                    (
                           CASE qs.statement_end_offset
                                 WHEN -1 THEN DATALENGTH(qt.[text])
                                 ELSE qs.statement_end_offset
                           END - qs.statement_start_offset
                    ) / 2
             ) + 1
       ),
       Batch = qt.[text],
       [DB] = DB_NAME(qt.[dbid]),
       qs.last_execution_time,
       qp.query_plan
  FROM sys.dm_exec_query_stats AS qs
 CROSS APPLY sys.dm_exec_sql_text(qs.[sql_handle]) AS qt
 CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp
 WHERE qs.execution_count > 5      --more than 5 occurences
 ORDER BY [Total MultiCore/CPU time(sec)] DESC
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