SQL Server, Sub-Queries vs. Cross Apply

Hi Guys,

Today for you a basic post!
We do a comparison between a Query with Sub-Queries and with the Apply operator!

So, what are differences between using sub-query and using cross/Outer Apply?
Well, if you don't know you should absolutely read this post!

Introduction

For our comparison we choose these queries:
 
 
A query with two sub-queries (A subquery is simply a query with another query inside it's select)
       
Select
 
  (Select MAX(OrdRig.Qta1) as conta  
   from OrdRig  
   where Ordrig.idordtes = o.Id and OrdRig.IdComrig is not null),
 
  (Select MAX(OrdRig.Qta2) as conta  
   from OrdRig  
   where Ordrig.idordtes = o.Id and OrdRig.IdComrig is not null)
 
From eco..ordtes o
 
A query that use a Cross Apply operator

select t.Qta1, t.Qta2
from eco..ordtes o
CROSS APPLY  
(Select MAX(OrdRig.Qta1) as Qta1, MAX(OrdRig.Qta2) as Qta2 
 from OrdRig  
 where Ordrig.idordtes = o.Id and OrdRig.IdComrig is not null
) as t

 
As you can see, through the Apply operator (cross or outer) you will be able to write a query that return the same results as the Query that use the sub-Query.

Yes, but what are differences between these two ways to write the query?

Surely you will have just noticed that in the first Query it was necessary to use two sub-queries while using the cross apply it is possible to read data from the OrdRig table only once.

Yes actually this is a smart thing!
But let's look at the differences in more detail

Sub-queries vs. Cross Apply

First let's compare how many pages are read executing this T-SQL command:

SET STATISTICS_IO ON 

Now execute both queries:

(123201 righe interessate)
Tabella 'OrdRig'. Conteggio analisi 18, letture logiche 74392, letture fisiche 0, letture read-ahead 30437, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'OrdTes'. Conteggio analisi 9, letture logiche 18112, letture fisiche 3, letture read-ahead 8020, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'Workfile'. Conteggio analisi 0, letture logiche 0, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'Worktable'. Conteggio analisi 0, letture logiche 0, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.

(1 riga interessata)

(123201 righe interessate)
Tabella 'Worktable'. Conteggio analisi 123201, letture logiche 2042619, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'Worktable'. Conteggio analisi 0, letture logiche 0, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'OrdTes'. Conteggio analisi 9, letture logiche 18112, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.
Tabella 'OrdRig'. Conteggio analisi 1, letture logiche 35478, letture fisiche 0, letture read-ahead 0, letture logiche LOB 0, letture fisiche LOB 0, letture read-ahead LOB 0.

(1 riga interessata)
We can see how using a cross apply exactly half of the pages are actually read from the ordrig table. This is the PRO.

But what is the CONS?

The CONS is that, is this case, SQL Server create a temporary table called worktable

Tabella 'Worktable'. Conteggio analisi 123201, letture logiche 2042619, letture fisiche 0,

This table is stored on the tempdb database and will contain these data:

Select MAX(OrdRig.Qta1) as Qta1, MAX(OrdRig.Qta2) as Qta2 
from OrdRig  
where Ordrig.idordtes = o.Id and OrdRig.IdComrig is not null

But, due to the fact that tempdb database is used performances may be affected!

So this proves that using an apply operator is not always better that use two Sub-queries!
 
 

Let's now take a look to the execution plans:

 
Also the execution plan tells the same story.
 
Running both queries togheter shows that the query with two subqueries is faster (39%)
Infact the subtree estimated cost is 67,8.


While using the apply operator the subtree estimated cost is greater (107,7)



Conclusions

Using Apply operator is not always better the using a sub-query.
We proved the a query with inside two sub-queris is faster that the same query using the Apply.
This is due to the fact that Apply operator create a temporary table that is stored into the tempdb.

However, if the number of subqueries were to increase, the result of this test would most likely change in favor of the solution that uses cross apply.

I leave you as homework to check what happens in case the query has four subqueries.
Which of the two solutions would be more efficient?
I look forward to your reply!


To all a very good weekend and see you soon!
If you like these articles, I remind you to subscribe to this blog


Luca Biondi @ SQLServerPerformance blog!

 


 
 
 
 
 
 
 


 
 
A song with the mood of the day: U Can't Touch This



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