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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Re:Driving table
hi
Firstly, can we please keep this online?
My opinion (observation!)..
in RBO the driving table is the rightmost in CBO it need not be so..cos the optimizer decides the order.
can someone confirm the observation regarding CBO? Thanx
PS: sorry If i missed this...in some earlier post.
But can someope please explain to me
how Oracle processes
EXISTS/NOT EXISTS and IN/NOT IN
I have read (been told and also observed)
that EXISTS/NOT EXISTS
performs better than IN/NOT IN, but have always
wondered HOW
Anup,
This can get into a lengthy discussion, but I'll try to keep it short. In general you can use this in both CBO and RBO, since in my experience doing things well for RBO has no ill affect on CBO.
The table order in the 'from' clause should match the order in the 'where' clause. Since Oracle reads a statement backwards, bottom to top, left to right, the last table listed in the 'from' clause should have the most restrictive and therefore the last condition in the 'where' clause. The reason for caring about the driving table is that the rows returned from that table drive the remainder of the query. So if you can return the least number of rows from that table you then have less work to do thereafter.
If you want a more detailed discussion, can we take it off-line.
Dick Goulet
____________________Reply Separator____________________ Author: A.SAXENA_at_ponl.com Date: 4/6/2001 2:45 AM
Hi friends ,
I have got some query regarding , how the query is exactly executed ??
We always take joins and if we see the plan of the query , there is always one main driving table . Some times I get feeling that , in the join the right most table is the driving table . But that is not always true , this is my observation from several plans .
In Oracle documentation alos ita mentioned that ( Server Concepts, 815 or 816 : Optimisation of Joins ), "one driving table is selected" , but at least I cannot makeout which of the several tables will be selected as driving table . What can be the potential criterions of selecting a driving table ?
Please , if you all can throw some light on the same or can redirect to some document , that would be great .
Cheers ,
Anup Saxena
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