Comments on: SQL SERVER – Execution Plan – Estimated I/O Cost – Estimated CPU Cost – No Unit

By: Doug

Doug — Wed, 23 Feb 2011 16:21:43 +0000

Please see pauls whites blog on this —

The I/O cost of 0.003125 is exactly 1/320 – reflecting the model’s assumption that the disk subsystem can perform 320 random I/O operations per second

http://sqlblog.com/blogs/paul_white/archive/2010/09/01/inside-the-optimizer-plan-costing.aspx

By: Manjunath

Manjunath — Mon, 11 Oct 2010 16:52:05 +0000

Hi Darshan,

Thank you. But can you please let me know how other process can cause my current query execution Plan.

Regards,
Manjunath

By: Darshan Shah

Darshan Shah — Mon, 11 Oct 2010 15:10:53 +0000

Manjunath,
That is because of some other processes are going on your production server. That why execution plan is diifer then dev server.

Thanks
Darshan

By: keshav

keshav — Fri, 01 Oct 2010 20:52:10 +0000

Hi Pinal
I was looking your notes on performance tuning. I was very impressed after looking those.I am very new to SQL DBA. I have one question for you I just want to knowhow we can tune the stored procedures in sql server2005.
I was trying to tune the sp’s but I am able to change the indexes on the table. i can still see the higher i/o and CPU values. I can tune the sp’s upto some value but not more than that. I will be very helpful if you help me in this.

Thanks,
Keshav

By: Manjunath

Manjunath — Thu, 30 Sep 2010 19:14:53 +0000

Hi Pinal,

There is a Query.
If i run it on Production environment it takes around 6 mins if same i try to run on my DEV Server its taking more than an hour. can you please help me out. And also is there any way to compare two execution plans?

Thanks
Manjunath

By: steve

steve — Wed, 21 Jul 2010 22:15:05 +0000

Dear Pinaldave,

I came across the phrase: the total I/O was two logical reads in interpreting an actual execution plan. Could you please explain a bit about it?

Thank you in advance.

Steve

By: pinaldave

pinaldave — Tue, 25 May 2010 14:55:59 +0000

Sometime they will be and sometime they may not.

Usual case is they are same in most of the cases.

By: Lee

Lee — Tue, 25 May 2010 14:24:43 +0000

Pinal

I understand that “unit” has no predefined value. It may be 1 second, 1.5 seconds, or 4 years.

You said that the unit is used to compare different possible execution plans, and then chooses the plan with the lowest overall cost. For this to happen, obviously the unit, whatever it may be, has to be consistent between the different execution plans.

My question is that are these units consistent between entirely different statements? For example if I submit one query, which is very I/O intensive, and then 5 minutes later, on a different database connection, submit another query which is very CPU intensive, will the units for CPU and I/O be the same for both the queries (of course their numbers will be different)?

Thanks,
Lee

By: Marko Parkkola

Marko Parkkola — Sat, 27 Mar 2010 18:34:42 +0000

I can second that!

I’m working in a project where we have two almost identical databases, but not quite. One is used in development and one in testing. Dev-db is 100x larger than the test-db and dev-server is 10x more powerfull (more cores and memory).

Now if you go and just take execution plan from dev-db and then compare with execution plan from test-db you can get wildly different plans and wildly different estimations. Only way to be sure if optimizations works as planned is to set STATISTICS TIME ON and run the same query on both dbs.

By: Pinal Dave

Pinal Dave — Sat, 27 Mar 2010 16:05:21 +0000

Hello Juan,

As I write in article that these estimates should be used by us in the same way by which the SQL Server uses it – The estimate should be used to compare different queries with each other.

Regards,
Pinal Dave

By: Juan Carlos

Juan Carlos — Fri, 26 Mar 2010 15:53:37 +0000

All right, let’s no look for a unit or any other measurement. But how do I know if a given number is low or high. For example:
I/O Estimated Cost = 0.89874 is high or low?

By: Pinal Dave

Pinal Dave — Tue, 23 Feb 2010 16:15:22 +0000

Hi Abhijit,

The set statistics io result should not differ than the actual execution plan’s io cost. The estimated cost is just an estimation of SQL optimizer on the basis of data statistics and that may differ considerably if data statistics is not updated.

Regards,
Pinal Dave

By: Abhijit

Abhijit — Tue, 23 Feb 2010 07:29:33 +0000

Hi Pinal,
is it possible that
set statistics io will differ with the actual /estimated execution plan’s estimated cost?

By: ravikolli

ravikolli — Tue, 02 Feb 2010 22:34:11 +0000

Use can use

SET STATISTICS TIME ON

before the query and then

SET STATISTICS TIME OFF

and execute to know the exact time a query is taking to execute…

some times this helps with the comparition.

By: ROHAN

ROHAN — Sat, 30 Jan 2010 10:31:08 +0000

WHY WE USE COMPUTE

By: pinaldave

pinaldave — Sat, 23 Jan 2010 05:20:30 +0000

Marko,

Another great comment!

By: paresh prajapati

paresh prajapati — Sat, 23 Jan 2010 04:37:27 +0000

Hi Pinal,

What should be the minimum value of this number for the best execution plan?

By: Marko Parkkola

Marko Parkkola — Fri, 22 Jan 2010 20:27:10 +0000

I use this tool almost daily and it is great! Here’s a short example how I use execution plans…

What I do is. First I try to find where the problem lies. If user interface is slow on some page/view/whatever I go through every SQL operation on that page. I run them “by hand” using SSMS. This way I can quickly isolate the queries that are causing problems.

Next I check their execution plans. Different operations shows how much they cost relative to the whole query. I concentrate on finding operations which cost the most. Usually I find that my tables are lacking indexes.

So I go ahead and open another SMSS instance. I create index based on the previous execution plan and check the execution plan again. This way I’m able to compare execution plans side-by-side and quickly see if my new indexes are working properly.

Now why would I want to see the other, not-so-good, execution plans? Those are irrelevant. Only thing that matters is what the Query Optimizer chooses to use.

Sometimes I must say it chooses something that doesn’t make sense to me but I trust that Microsoft’s programmers knows better :)

By: Wojtek

Wojtek — Fri, 22 Jan 2010 18:44:21 +0000

I believe that if the SQL Query optimizer already chooses the best execution plan it can find, and that the estimated CPU and IO costs are still too high for your needs, then it’s a good indication that your query or database needs a structure change.

By: Ramdas

Ramdas — Fri, 22 Jan 2010 14:45:38 +0000

Hi,
These execution plans are very useful, what does estimated operator cost 32% mean?

By: Sumit

Sumit — Fri, 22 Jan 2010 07:53:10 +0000

@paresh
Yes that is true SQL server Query Optimizer creates several alternative plans to get expecyed results and we not able to see all the execution plans because this is by design that Query optimizer choose the best plan to get the expected results. So we are able to see only the best execution plan by Query Optimizer and i think that is what the name suggests SQL Server Query Optimizer.
This is my share of knowledge.
if Pinal sir can spot more light on this thing that if we able to see all the plans produced by Query Optimizer as per my knowledge it is not possible.

By: pinaldave

pinaldave — Fri, 22 Jan 2010 06:42:31 +0000

Hi Sumit,

As mentioned it is just combined value. There is no unit like ms or threads or anything. That is the point of this post. There is no unit. You can use this number for comparison with other executions counts.

Kind Regards,
Pinal

By: Paresh Prajapati

Paresh Prajapati — Fri, 22 Jan 2010 06:27:40 +0000

“When any query is executed, the SQL Server Query Optimizer prepares several alternative execution plans to execute the query”

As per above your statement how can we see all the several alternative execution plans?
Why we can able to see only one execution plan?

If all execution plan have different estimated I/O cost and estimated CPU cost then SQL server display and use execution plan with minimum value with estimated I/O cost and estimated CPU cost , is it right?

Can you please more specific?

By: Sumit

Sumit — Fri, 22 Jan 2010 06:24:09 +0000

Hi sir,
but that is the thing we want to know that what does 0.003125 means. Is that seconds, worker threads, milli seconds or what. means it is ok we know if the value is large it is bad but what does this value denotes to like i am jus concerned about unit.
Hope you will spot some light on this.