Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
thanks,
Joe Weinstein at BEA
7  9341 
On Mon, 15 Nov 2004 08:50:46 -0800, Joe Weinstein wrote: Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
DB2 is smart enough to see that (1=0) and (4>5) will never be true, and
will do neither a table scan nor an index scan for such a simple query.
I don't know the limits of smartness, though (i.e. when predicated become
too tricky to decide on).
--
Greetings from Troels Arvin, Copenhagen, Denmark
The clever approach to answering this question is to do an "explain" on
the query, using a simple table (ie. the sample tables that come with
UDB) and examining the results.
If your predicate stated "where 1 = 1" then an output row would be
generated for every row in the table. A scan would be needed to generate
the correct number of output rows. If the developers of a retrieval
engine are foolish enough to look for this type of code and write
special logic to save the user from his/her own bad coding, then they
deserve the problems it can easily cause. Handling predicates in a
consistant manner, without special case code, is the way to long term
stability, consistancy, and overall performance in the retrieval engine.
Philip Sherman
Joe Weinstein wrote:
Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
thanks,
Joe Weinstein at BEA
Troels Arvin wrote: On Mon, 15 Nov 2004 08:50:46 -0800, Joe Weinstein wrote:
Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
DB2 is smart enough to see that (1=0) and (4>5) will never be true, and
will do neither a table scan nor an index scan for such a simple query.
I don't know the limits of smartness, though (i.e. when predicated become
too tricky to decide on).
Thanks!
Philip Sherman wrote: The clever approach to answering this question is to do an "explain" on
the query, using a simple table (ie. the sample tables that come with
UDB) and examining the results.
Thanks!
As long as the assumption that a DB2 instance is available to the asker is valid.
If your predicate stated "where 1 = 1" then an output row would be
generated for every row in the table. A scan would be needed to generate
the correct number of output rows.
That would be assumed. I'm just interested in the obvious 1 = 0 case.
I hope my assumption, that you are circumlocuting the answer,
"DB2 will not go to an index or to data for such a query" is correct.
If the developers of a retrieval engine are foolish enough to look for this type of code and write
special logic to save the user from his/her own bad coding, then they
deserve the problems it can easily cause. Handling predicates in a
consistant manner, without special case code, is the way to long term
stability, consistancy, and overall performance in the retrieval engine.
Thanks again. I'm not sure who you're referring to, but I've seen some
applications do that (adding a where 1 = 0 clause) even onto queries they
didn't generate themselves, in order to get the metadata about the query,
which will be sent to the client, even for a zero-row return. Not me, but
I get paid to deal with other's problems, even self-inflicted...
Philip Sherman
Joe Weinstein wrote:
Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
thanks,
Joe Weinstein at BEA
Joe Weinstein wrote:
Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
thanks,
Joe Weinstein at BEA
Yes.
On Mon, 15 Nov 2004 15:58:53 -0500, Serge Rielau wrote: Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
[...] Yes.
Is there any good documentation on what kinds of semantic query
optimizations like this DB2 tries to perform?
--
Greetings from Troels Arvin, Copenhagen, Denmark
Troels Arvin wrote: On Mon, 15 Nov 2004 15:58:53 -0500, Serge Rielau wrote:
Hi. Some DBMSes are clever enough not to go to data pages if a
knowably constant search criterion is false. Is DB2 among them?
[...]
Yes.
Is there any good documentation on what kinds of semantic query
optimizations like this DB2 tries to perform?
Not that I'm aware of. DB2 UDB for LUW knows some 100 semantic query
rewrite rules. Some of which have patented algorithms which you can look
up, but in general this is all part of the secret mix ;-)
Here is a link to the "theorem prover" (note that it's not fully
exploited in rewrite to keep the compiler snappy :) It was originally
added in DB2 V5.2 for typed view hierarchy optimization. http://patft.uspto.gov/netacgi/nph-P...&RS=PN/6728952
Cheers
Serge This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
by: Marcus |
last post by:
Hi
I have a very complex sql query and a explain plan. I found there is a
full table scan in ID=9
9 8 TABLE ACCESS (FULL) OF 'F_LOTTXNHIST'
(Cost=84573 Card=185892 Bytes=7063896)
How can I correlate which part of the SQL statement is running on full
table scan. Please see below for the code and explain plan
SQL Code
|
by: Botao |
last post by:
Hi, Every Guru,
I'd like to put a button on a page. When clicking the button, the
table below it gets selected so the user can do Ctrl C to copy the
entire table without using the mouse to select the table which can be
big. How do I do it using javascript? I tried:
<INPUT TYPE=Button NAME='Select' SIZE='10' VALUE='SelctTable'
onClick="document.MyTable.select();">
|
by: robert |
last post by:
just got out of a "class" on EXPLAIN, from a 390/v6 maven. was told
that ACCESSTYPE = 'R' in the plan_table meant a TableSpace scan.
hmmmm. next step up (or down, depending on your outlook) wasn't a
Table Scan.
thought this might be a MainFrame thing; but a look at DB2Info on my
UDB 7.2 server showed the same thing.
this is truly puzzling. is there an explanation of EXPLAIN which
describes how to get a Table Scan???? or are we...
|
by: Mike L. Bell |
last post by:
Query:
update table1 t1
set end_time = (
select end_time
from table2 t2
where t2.key1 = t1.key1
and t2.key2 = t1.key2
)
where exists
|
by: news.swissonline.ch |
last post by:
Hi,
DB2 7 FP 13
Windows XP
I have a table in which on of the columns has the data type DATE. The date
column is indexed (asc non-unique index). Every SQL I create which tries to
filter by this date column produces a table scan, even a simple
select DATECOL from MYTABLE where DATECOL=date('2004-09-16')
| | |
by: selma |
last post by:
I am puzzled by visual explain for a simple query ( select * from
mytable ) chooses an index scan. That index has only a single column
and the table has 30 columns not covered by the index. When running
explain for another database (same schema structure), it showed table
scan. These are all production databases with >10 GB of real data, and
stats are all up to date. Reorgchk showed nothing bad.
Per chance visual explain GUI was acting...
|
by: Dia |
last post by:
At the company I work job applicants are required to do a little test.
The human resource manager recently had a candidate who claimed one of
the questions was ambiguous.
Dependent upon the version of DB2 one or the other answer of a multiple
choice question could be right, the candidate said.
The HRM asked me to look into the matter, but since I am no expert in
the workings of DB2 I submit the question to you hoping any of you can
|
by: BD |
last post by:
Hi, all.
My background is more Oracle than db2. My skills at SQL tuning are
quite limited.
I'm running 8.2 on Windows.
I'm tasked with some SQL optimization, and am doing some explain plans
on various queries.
|
by: Arun Srinivasan |
last post by:
Hi
I was using a query previously, that was efficient
select * from table where pred1 and pred2 and pred3;
Later I was asked to introduce new ones, but they were not based on
table columns but variables declared in SP.
select * from table where pred1 and pred2 and pred3 and variable1
='number1 and variable2 =number2;
|
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: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in...
| | |
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: tracyyun |
last post by:
Dear forum friends,
With the development of smart home technology, a variety of wireless communication protocols have appeared on the market, such as Zigbee, Z-Wave, Wi-Fi, Bluetooth, etc. Each protocol has its own unique characteristics and advantages, but as a user who is planning to build a smart home system, I am a bit confused by the choice of these technologies. I'm particularly interested in Zigbee because I've heard it does some...
|
by: agi2029 |
last post by:
Let's talk about the concept of autonomous AI software engineers and no-code agents. These AIs are designed to manage the entire lifecycle of a software development project—planning, coding, testing, and deployment—without human intervention. Imagine an AI that can take a project description, break it down, write the code, debug it, and then launch it, all on its own....
Now, this would greatly impact the work of software developers. The idea...
|
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: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
| |
