SQL Server, the story of a ROUND function inside a query

Hi Friends,

Today is the day of the light post since tomorrow it’s Saturday: Light but interesting!

Today I would show you a little example in order to see how much the execution plan of a Query may change by adding a simple ROUND function!

We have only a rule: a post and a example!

So, In order to do the simplest example I will use only very simple Queries: one Table and no join at all.

Enjoy the reading!

 

Round & Statistics

For the example we will use the OrdRig table (do you remember..  is one of the tables we usually use for our examples). It contains the n rows of order

So, take a look to this two Queries. 

       
select id from OrdRig where qta1 > 10000000

select id from OrdRig where qta1 > round(10000000,0)
 

What do you expect to happen?  Do they have the same result?

Yes if course the result it is the same but take a look to the execution plan:

If we apply the ROUND function then the execution plan has a decidedly different cost .. even 99 times greater!

 


What's up?

Let's understand what happen looking at the estimated number of lines to read properties:

 

Without the ROUND function the number of pages to be read is just 10 (ten pages of 8KB each)


But if we look at the same property by running the query with the ROUND we see that read pages are many many more! 179K!!!




We can see it clearly there is an incorrect number of pages estimation and this is a the problem. 

REMEMBER: Incorrect estimation of the number of pages is a problem because it can cause SQL Server to choose a suboptimal execution plan.

Now, in order to inspecting the problem in a more deep level we should ask us: how does the optimizer estimate the number of lines to read?

We must remember that the optimizer can take advantage of the statistical informations that it keeps for each index.

We can see statistics associates to an index through the DBCC SHOW_STATISTICS T-SQL command ('nometab', <index name>);

Today we will not explain in detail how statistics works but for our example just looking at the picture below we can see that using the index <xxx> there are only 10 lines to extract where qta is great than 10000000.


But what if I apply the ROUND function?

 

The SQL Server optimizer simply does not use the detail of the statistics but proposes a fixed value regardless of the value specified as a parameter of the round.

Doing so, the value that comes out is not absolutely optimal.

This is a well known behaviour, and in fact with the new versions, from SQL2014 inclusive onwards, this problem has been fixed!

In our example ORDRIG table has 599845 row which divided by 1/3 makes our 179950 value proposed by the execution plan!

What other functions have this behavior besides the ROUND?

Enough…. FLOOR, CEIL, ABS, SQUARE

 Instead who is saved? Fortunately the CAST and the CONVERT!

 

NOTES: 

All of the above is valid with a compatibility level (CL) lower than SQL2014. Starting from SQL2014 inclusive onwards with the new Cardinality estimator this behaviour has been solved.

 

That's all for Today!
Luca











Previous post: Learn SQL Server Window functions and Running Totals in five minutes

 

Comments

Post a Comment

I Post più popolari

SQL Server, execution plan and the lazy spool (clearly explained)

Image
Hi Guys, Welcome back! In one of the last posts we talked about Table Spool . We had explained what Spool are for by telling the somewhat curious story of the halloween problem . Here we specifically talked about a type of spool called Eager Spool   You can look to the execution plan below where the eager Spool is used to avoid the halloween problem in classic query of the "salary increase".. i suggest Well, if the story of the eager spool intrigued you, today instead we will speak about another type of Spool, the Lazy Spool . The Lazy spool Let's take a small step back. So, what is a Spool operation?  As we have already understood a spool operation is simply a temporary storage of data . Data are read from a source table and stored inside a worktable in the Tempdb database . While an Eager Spool is used to prevent the Halloween problem, the Lazy Spool is instead used to store data that will later be needed again when running a query . So, when a Lazy spool is used?  ...
Read more

SQL Server, datetime vs. datetime2

Image
Hi guys, This time, in this post we will speak about datetime and datetime2 datatypes. We will talk about the pros and cons. We will talk about all the differences between the two types of data. Write me in the comments which type of data do you prefer between the two. Enjoy the reading!     Datetime Vs. Datetime2 First of all, it must be said that the datatype datetime is certainly the best known native format for storing a date in SQL Server. However, Microsoft has also been introducing a new type of data for many years now, also used to manage dates. It is in fact from SQL server 2008 that it is present in datetime2 data type. This "new" type of data was born primarily to have higher accuracy . Let's in fact talk about ... precision ! Datetime Certainly the datetime data type is not very precise and certainly there are many cases in which its precision is simply not sufficient. Let's see it with an example. Open the SQL Management studio (SSMS) and write the fo...
Read more

La clausola NOLOCK. Approfondiamo e facciamo chiarezza!

Image
Ehi ragazzi! La domanda è di quelle importanti! Siete pronti per scoprire tutto ma proprio tutto sulla clausola NOLOCK?   Se la riposta è si, allora seguitemi! Oggi ne spiegheremo il funzionamento in maniera approfondita. Faremo una introduzione al concetto di tipi di LOCK Ed infine vedremo come mai una SELECT ne può bloccare un'altra. Non vi nascondo che il motivo di questo articolo è fare chiarezza. Su questo argomento si sentono spesso affermazioni “curiose” ma tra le tante questa è la prima: “ uso la SELECT con l’opzione WITH(LOCK) così non blocco le altre tabelle ”.   Ma, è proprio vero quanto affermato? Beh oggi facciamo il punto!   Buona lettura!     La clausola NOLOCK Diciamo subito per iniziare che cos’è la NOLOCK. NOLOCK è il nome di una clausola che è possibile specificare all’interno di uno statement di tipo SELECT utilizzando la seguente sintassi: SELECT * FROM TABELLA WITH (NOLOCK) Si noti che non possiamo ...
Read more