Time Trial sys.dm_exec_query_plan vs sys.dm_exec_sql_text
4November 12, 2012 by Kenneth Fisher
I’ve been working on a way to test performance usage on a server by database. I started out using sys.dm_exec_query_stats since it has every statistic I wanted. Unfortunately it doesn’t contain the database id which makes it rather difficult to split the data out by database. This lead me to cross applying sys.dm_exec_query_plan, which does contain dbid. This worked great! Well, it worked great in test. Imagine my frustration when I pushed everything to production and found that a query that ran in seconds in test, took almost an hour in production. I mean I knew that sys.dm_exec_query_plan was slow, but really. The problem is that my test server only has 4-5000 rows in sys.dm_exec_query_stats, while production has 26000+. This led me to try sys.dm_exec_sql_text. It seemed to work better so I decided to run a time trial to confirm what I was seeing without the rest of my query.
set statistics io on set statistics time on select top 1000 * from sys.dm_exec_query_stats cross apply sys.dm_exec_sql_text(sql_handle) select top 1000 * from sys.dm_exec_query_stats cross apply sys.dm_exec_query_plan(plan_handle)
I used the top 1000 because I don’t really have an hour to wait for one of the queries to complete. I’m old enough as it is. 1000 rows seemed like enough of a sample.
Here are the results from two different servers:
Server 1
sys.dm_exec_sql_text
SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 3 ms. (1000 row(s) affected) SQL Server Execution Times: CPU time = 218 ms, elapsed time = 83024 ms.
Sys.dm_exec_query_plan
SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 2 ms. (1000 row(s) affected) Table 'Worktable'. Scan count 0, logical reads 2545, physical reads 0, read-ahead reads 0, lob logical reads 1282048, lob physical reads 0, lob read-ahead reads 136372. SQL Server Execution Times: CPU time = 70641 ms, elapsed time = 540426 ms.
Server 2
sys.dm_exec_sql_text
SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 0 ms. (1000 row(s) affected) SQL Server Execution Times: CPU time = 47 ms, elapsed time = 543 ms.
Sys.dm_exec_query_plan
SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 26 ms. (1000 row(s) affected) SQL Server Execution Times: CPU time = 8391 ms, elapsed time = 11028 ms.
So that has sys.dm_exec_query_plan at 6-20 times slower. Since the only difference in the output is the sql text vs the query plan and all I want is the dbid I think my solution will have to use sys.dm_exec_sql_text.
Hi,
Very nice article.
You can get the database id from the below query:
select top 1000 *
from sys.dm_exec_query_stats qs
cross apply sys.dm_exec_sql_text(sql_handle)
OUTER APPLY sys.dm_exec_plan_attributes(qs.plan_handle) pa
WHERE pa.attribute = ‘dbid’
Readers can discover a lot more about improving SQL performance via DMVs in this recently published book “SQL Server DMVs in Action” http://www.manning.com/stirk. It contains more than 100 scripts to identify problems, and offers a wide range of solutions. (there are also various test harnesses for doing something similar to what you are doing!)
Chapters 1 and 3 can be downloaded for free. Chapter 1 includes scripts for:
A simple monitor
Finding your slowest queries
Find your missing indexes
Identifying what SQL is running now
Quickly find a cached plan
Thanks
Ian
Absolutely fabulous DMV. I really appreciate you letting me know about it. It’s not only faster but more accurate than what I was looking at. It’s truly amazing how much information is available using them. I’m going to have to talk to my boss about paying for a copy of your book
One question. Is there any reason to use an OUTER APPLY? As best I can tell it looks like there will always be a dbid attribute available let alone any attribute. I’m particularly interested since I wrote an article on outer joins here: Fun with Outer Joins and using a WHERE clause to reference an OUTER joined/applied table tends to stand out to me now.
Thanks again!
Kenneth
Hi Kenneth,
I’m glad you liked the code. And yes it really is amazing how much internals info there is available with DMVs.
You might be right about the OUTER APPLY, I just wanted to ensure I captured everything, even if the dbid is missing.
Enjoy
Ian
Kenneth,
Thanks for posting this.
I thought I was the one going crazy with sub-optimal performance.
Daniel Adeniji