473,856 Members | 1,703 Online
Bytes | Software Development & Data Engineering Community
+ Post

Home Posts Topics Members FAQ

Table Spool\ Lazy Spool

What causes the query optimizer to choose a table spool\lazy spool
action in the execution plan? The explanation of "optimize rewinds"
makes little sense because my query never comes back to that table.

I'm going to have to change the query but it would be helpful if I
knew what I should be trying to avoid.

Jul 20 '05 #1
1 21012
Your question regarding 'why' the optimizer chooses one type of plan over another isn't easily answered, unfortunately. The heuristics and complexities invovled in the query processor are so intricate that it's very hard to tell why it chose the plan it chose. The short answer is that it found its plan to be cheaper than the others it considered.

Spool operators are typically introduced into a query plan to 'cache' the results of complex query elements (large subqueries or remote scans, for instance), or to provide "Halloween protection" for update queries. The operator takes the rows produced by its children (those operators underneath it in the SET STATISTICS PROFILE tree) and stores them off into a table in TEMPDB so that it does not have to rescan or reevaluate the operators underneath it. In the case of a normal spool (usually referred to as an 'eager spool' (ES)), when the parent of the spool asks for a row, then the ES asks for ALL of the rows from the operators underneath it. In the case of a lazy spool (LS), the spool only stores the individual rows that are requested by its parent, not all of them. Many times, this is cheaper than retrieving all of the rows from its child. Unfortunately, while spools can provide a lot of performance benefit in preventing complex query elements from being updated over and over again, they can generate quite a lot of IO while they create the TEMPDB worktable.

Is the IO cost the reason that you are askign your question? Is this a normal SELECT or an UPDATE query? Perhaps some context would help. Can you post the query plan from SET STATISTICS PROFILE ON? Perhaps running your query through the index tuning wizard, or otherwise trying to optimize the index structure underlying your query would eliminate the need for the operator...

Ryan Stonecipher
SQL Server Storage Engine
"david_0" <do*****@yahoo. com> wrote in message news:58******** *************** ***@posting.goo gle.com...
What causes the query optimizer to choose a table spool\lazy spool
action in the execution plan? The explanation of "optimize rewinds"
makes little sense because my query never comes back to that table.

I'm going to have to change the query but it would be helpful if I
knew what I should be trying to avoid.

Jul 20 '05 #2

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

by: Krzys! | last post by:
I'd like to create a table dynamicly. List of column in this table should be taken from select: "select distinct fiel from table " How to do it ? tnx in advance for help K.
by: Thomas R. Hummel | last post by:
Hello, I have a table which has a few million records. It has an IDENTITY column that serves as the primary key. In a part of our application here, a previous record may need to be copied as a new row. Within a stored procedure exists SQL like: INSERT INTO .. (col1, col2, col3, ...) SELECT @var1, col2, col3, ... FROM My_Table
by: Richard Lawrence | last post by:
Hi, Consider the following: #Content { margin:0px 210px 50px 200px; padding:10px; } If I use it with the following HTML:
by: Robert | last post by:
Greetings, In Google I am finding what the deffinition of a table spool operator is but not what it is doing to performance. The execution plan shows the output of a clustered Index scan (54 records) going into the Table Spool operator and having a output of 218,000 records out. Where do the 218,000 records come from??
by: orajit | last post by:
I Have created following PL/sql blok DECLARE vl_n_cnt NUMBER:=0; vl_max_date varchar2(100); CURSOR c1 IS SELECT CREATED_DTM,EVENT_DTM,ACCOUNT_NUM,EVENT_COST_MNY,EVENT_TYPE_ID FROM ist.costedevent_test ; BEGIN
by: akaley | last post by:
HI.. iam using Spool ..for loading the data into file from select statement.. iam facing some problems...Please give me u r valuable suggestion.. This is the sample code used by me.. set echo off; set trimspool on; set verify off; set feedback off;
by: apollock | last post by:
Our application is having an issue where a query in a stored procedure periodically gets a bad plan with a Lazy Spool Operator that changes the query execution time from 5ms to 900ms and kills our CPU. We are running x64 SQL 2005 SP2 on Windows 2003 SP2. We thought it might be parameter sniffing that was causing the bad plan, so we re-wrote the query to disable parameter sniffing. Unfortunately, that made the problem worse as we always...
by: daves1 | last post by:
Hi, I am running an sql script in oracle to spool a csv file. I am using the following set up: Define name='JOE' spool names.txt set linesize 200 set pagesize 50000 set heading off set termout off
by: napstar | last post by:
I have a table in oracle 7.3 and I use SQLPLUS to work with is DB.I would like to know how to spool the contents of a table to a txt file. i tried this spool c:\companies.txt SELECT COMPANY_NO ||','|| COMPANY_NAME FROM companies but i keep getting:ERROR at line 1:ORA-00900: invalid SQL statement
by: marktang | last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, we’ll explore What is ONU, What Is Router, ONU & Router’s main usage, and What is the difference between ONU and Router. Let’s take a closer look ! Part I. Meaning of...
by: Hystou | last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it. First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
by: jinu1996 | last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth. The Art of Business Website Design Your website is...
by: Hystou | last post by:
Overview: Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules. He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms. Adolph will...
by: conductexam | last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one. At the time of converting from word file to html my equations which are in the word document file was convert into image. Globals.ThisAddIn.Application.ActiveDocument.Select();...
by: adsilva | last post by:
A Windows Forms form does not have the event Unload, like VB6. What one acts like?
by: muto222 | last post by:
How can i add a mobile payment intergratation into php mysql website.
by: bsmnconsultancy | last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.