Pulling Top queries returns NULL in query plan and sql text

by Kris Gruttemeyer   Last Updated May 09, 2017 08:06 AM

I'm using the following code to pull our top 20 queries (ordered by CPU):

SELECT TOP 20 qs.sql_handle
    ,qs.total_worker_time AS [Total CPU]
    ,qs.total_worker_time / 1000000 AS [Total CPU in Seconds]
    ,(qs.total_worker_time / 1000000) / qs.execution_count AS [Average CPU in Seconds]
    ,qs.total_elapsed_time / 1000000 AS [Total Elapsed Time in Seconds]
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp
ORDER BY qs.total_worker_time DESC

However, I'm seeing this: enter image description here

Can anyone shed some light as to why the Query Plan and SQL Text are showing up NULL? Are they some sort of system process or external application? We are running SQL 2008 R2.

Thanks, as always, everyone!

Answers 3

There are limitations when using sys.dm_exec_query_plan

5.4 When and How to Use Sys.dm_exec_text_query_plan

The query_plan column returned by sys.dm_exec_query_plan has type XML and suffers from the inherent limitation of the data type that it cannot have nest levels greater than or equal to 128. In SQL Server 2005 RTM and SP1, if the query_plan had a depth of greater than or equal to 128 levels this would prevent the query from returning. The second limitation is that it is not easy to get the query plan of a particular statement in the batch directly from the DMVs without parsing the query plan XML returned from sys.dm_exec_query_plan.

To address both these limitations, in SQL Server 2005 SP2, we introduced sys.dm_exec_text_query_plan. Sys.dm_exec_text_query_plan is a TVF that takes 3 parameters: plan_handle, statement_start_offset and statement_end_offset. It returns the showplan in text format for the batch or for a specific statement within the batch.

Instead of reinventing the wheel, best is to use Brent's - sp_BlitzCache or Glenn's Diagnostic queries. or Aaron's answer here.

From SQL Server Query Performance Analysis using DMVs :

-- Which Queries are taking the most time/cpu to execute
    total_worker_time, total_elapsed_time,
    total_worker_time/execution_count AS avg_cpu_cost, execution_count,
    (SELECT DB_NAME(dbid) + ISNULL('..' + OBJECT_NAME(objectid), '')
        FROM sys.dm_exec_sql_text([sql_handle])) AS query_database,
    (SELECT SUBSTRING(est.[text], statement_start_offset/2 + 1,
        (CASE WHEN statement_end_offset = -1
            THEN LEN(CONVERT(nvarchar(max), est.[text])) * 2
            ELSE statement_end_offset
            END - statement_start_offset) / 2
        FROM sys.dm_exec_sql_text([sql_handle]) AS est) AS query_text,
    total_logical_reads/execution_count AS avg_logical_reads,
    total_logical_writes/execution_count AS avg_logical_writes,
    last_worker_time, min_worker_time, max_worker_time,
    last_elapsed_time, min_elapsed_time, max_elapsed_time,
    plan_generation_num, qp.query_plan
FROM sys.dm_exec_query_stats
    OUTER APPLY sys.dm_exec_query_plan([plan_handle]) AS qp
WHERE [dbid] >= 5 AND DB_NAME(dbid) IS NOT NULL
  AND (total_worker_time/execution_count) > 100
--ORDER BY avg_cpu_cost DESC;
--ORDER BY execution_count DESC;
ORDER BY total_worker_time DESC;
September 10, 2014 14:28 PM

In terms of the Statement Text being NULL, when selecting the text from sys.dm_exec_sql_text, the text value is NULL for encrypted objects.

Check the encrypted value of the record from sys.dm_exec_sql_text.

Mark Sinkinson
Mark Sinkinson
September 10, 2014 14:32 PM

The query_text column for some of the currently executing queries shows NULL. Can we fix this ?


Asmit Adgaonkar
Asmit Adgaonkar
May 09, 2017 08:01 AM

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