Optimizer Timeouts with XQuery

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xraygogsI was looking at performance of a database and I noticed a few of the plans were very large and timing out in the optimizer. This made me wonder, just how many of them were timing out?

This sounds like a job for XQuery!

There’s really nothing to it. Once you start plucking stuff out of the execution plans using XQuery, it’s kind of hard to stop. So here’s my little bit of code.

[sourcecode language=”sql”]WITH XMLNAMESPACES(DEFAULT N’http://schemas.microsoft.com/sqlserver/2004/07/showplan’),  QueryPlans
AS  ( 
SELECT  RelOp.pln.value(N’@StatementOptmEarlyAbortReason’, N’varchar(50)’) AS TerminationReason,
        RelOp.pln.value(N’@StatementOptmLevel’, N’varchar(50)’) AS OptimizationLevel,
        –dest.text,
        SUBSTRING(dest.text, (deqs.statement_start_offset / 2) + 1,
                  (deqs.statement_end_offset – deqs.statement_start_offset)
                  / 2 + 1) AS StatementText,
        deqp.query_plan,
        deqp.dbid,
        deqs.execution_count,
        deqs.total_elapsed_time,
        deqs.total_logical_reads,
        deqs.total_logical_writes
FROM    sys.dm_exec_query_stats AS deqs
        CROSS APPLY sys.dm_exec_sql_text(deqs.sql_handle) AS dest
        CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle) AS deqp
        CROSS APPLY deqp.query_plan.nodes(N’//StmtSimple’) RelOp (pln)
WHERE   deqs.statement_end_offset > -1        
)   
SELECT  DB_NAME(qp.dbid),
        *
FROM    QueryPlans AS qp
WHERE   (qp.dbid = 13 OR qp.dbid IS NULL)
        AND qp.optimizationlevel = ‘Full’
ORDER BY qp.execution_count DESC ;
[/sourcecode]

The mandatory warning now, XQuery like this can be somewhat processor intensive. I wouldn’t suggest running this on a production system unless you were to put more filters in place to trim the data down a bit.

7 Comments

  • John Johnson

    Your presentation at the Richmond SQL Server Users Group meeting was excellent. You demonstrated this sql there which I want to try on our systems. However, I get errors when I parse it. Please help.

  • What errors are you getting? Oh, and I didn’t mention this but it is looking at one database in particular, which you can see as qp.dbid=13. That’s one issue that you might need to address. Other than that, without the error message, I don’t know what to tell you.

  • Looks like there’s a formatting issue on the first line. Not sure why it’s doing that, it’s not that way in code. It should read this way:

    WITH XMLNAMESPACES(DEFAULT N’http://schemas.microsoft.com/sqlserver/2004/07/showplan’), QueryPlans
    AS (

  • In SQL Server, what is the best way to identify the most frequently executed queries that do not utilize an index? Can this be done with a query instead of an external tool?…

    It is possible to use DMOs to query the plans in cache. You can use sys.dm_exec_query_stats to see the most frequentnnly called queries because this DMO shows an aggregation of performance stats of the queries in cache. You can combine this with sys.dm…

OK, fine, but what do you think?