RE: Performance off "count(*)"

OK, Rub it in, Yeah I am Old !  

;-)  

Nope, Niall I wasn't referring to the "count(1)" myth.  

Let's rephrase the question.  

Given that all statistics are in order and up to dat and given the basics Jonathan pointed out and given there is and index ( and not containing a NULL value in its column), then while doing a "select count(*) on "tab", it should use the index or not...?  

I am on 11gR1
The "table" is 40 Gb big and contains roundabout 7 million records  

(Yep, the stupid, not so relational, XML stuff ;-)          

Van: Niall Litchfield [mailto:niall.litchfield_at_gmail.com] Verzonden: vr 18-7-2008 20:29
Aan: Marco Gralike
CC: jonathan_at_jlcomp.demon.co.uk; oracle-l_at_freelists.org Onderwerp: Re: Performance off "count(*)"

I suspect that what you are remembering is a rebuttal of an old myth (well it *might* have been true once, but I doubt it) that using select count(1) from <tab>;
was more efficient than
select count(*) from <tab>;
in determining rowcounts. This hasn't been true, if it ever was, for at least a decade. For normal tables there's no difference - obviously those who indulge in a bit of xml may experience odd results :)  

Niall

On Fri, Jul 18, 2008 at 6:28 PM, Marco Gralike <Marco.Gralike_at_amis.nl> wrote:

	Am I correct in my "small understanding" of Oracle that "count(*)" has
	been optimized.
	
	I think I once heard or read this from Tom Kyte, but then again I am
	also becoming old and could be mistaken (hearing not good and all)
	
	;-)
	
	
	The "issue" for me is that if "count(*)" is optimized, then I have found
	a small feature that the XMLDB Development Team can use in its
	advantage.
	
	Regards
	
	
	Marco
	





	-----Original Message-----

	From: Jonathan Lewis [mailto:jonathan_at_jlcomp.demon.co.uk]
	Sent: vrijdag 18 juli 2008 16:20
	To: oracle-l_at_freelists.org
	Subject: Re: Performance off "count(*)"
	
	
	In principle the optimizer knows that count(*) means
	"how many rows in the table" and will choose the
	smallest object that could return the result to do the
	scan.
	
	This means it may choose to do a fast-full scan of an
	 index if at least one column of the index is declared
	as not null.
	
	
	Depending on  version, the choice between indexes of
	identical cost may simply be based on alphabetical ordering
	of the names - although number of distinct keys is supposed
	to have an effect as well in recent versions
	
	Regards
	
	Jonathan Lewis
	http://jonathanlewis.wordpress.com <http://jonathanlewis.wordpress.com/> 
	
	Author: Cost Based Oracle: Fundamentals
	http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
	
	The Co-operative Oracle Users' FAQ
	http://www.jlcomp.demon.co.uk/faq/ind_faq.html
	
	


	----- Original Message -----

	From: "Marco Gralike" <Marco.Gralike_at_AMIS.nl>
	To: "Gints Plivna" <gints.plivna_at_gmail.com>
	Cc: <oracle-l_at_freelists.org>
	Sent: Friday, July 18, 2008 2:11 PM
	Subject: RE: Performance off "count(*)"
	
	
	
	That's not really what I ment.
	
	
	While using "count(*)", Oracle will performance optimize (as far as I
	know) the "count(*)" so it will do a smarter job then "only" a full
	table scan.
	
	I am interested in the how and what regarding the mechanics / methods
	behind it. In my "not so relational XMLDB" environment, I see a full
	table scan were I would have hoped for the smarter way to go for an
	index. The count via the index will last for 3 minutes. The count via
	the full table scan will take more than 1 day (17 Gb of XML data,
	approx. 7 milion records).
	
	
	
	I have an small example of this smarter behavior below (I thought it was
	somehow hardcoded):
	
	
	
	
	Also see the differences (I have to force it to go via the PK unique
	index via a hint)
	


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-- 
Niall Litchfield
Oracle DBA
http://www.orawin.info <http://www.orawin.info/>  

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