Re: consistent gets

freCho wrote:
>
> Mentre cercavo di capire la differenza tra una select e un drop database,
> Jonathan Lewis ha scritto:
>
> >
> > Immediately after you deleted the huge amount of
> > data, there were lots of empty leaf blocks at the
> > left-hand edge of the index.
> >
> > Your query for min() would have to start at the
> > left hand edge, and walk through all those empty
> > blocks (building read-consistent versions if necessary)
> > to find the first row that still existed. This is why you
> > get so many CR gets.
>
> [CUT]
>
> > The empty blocks will eventually be re-used or
> > taken out of the index structure, but for very
> > large deletes, the time-delay for a proper cleanout
> > can be significant.
>
> [CUT]
>
> Thank you very much Jonathan a very clear explanation.
> But this means that the index *is not* always balanced,
> because in such situations the leaf blocks at the left hand
> are empty.
> The dml I've used was "delete from TABLE where cdate < data parallel ...."
> so I think that *all* the blocks starting from the left side up to
> a block in the middle (which contains data > data_used_on_dml)
> should be deleted. If this is right....than I don't understand how can
> the index stay balanced in such kind of situations.
>
> regards

Its 'balanced' in the sense that every leaf block is the same 'distance' from the root of the index. But the distribution of index entries across leaf blocks may not be

hth
connor

-- 
Connor McDonald
Co-author: "Mastering Oracle PL/SQL - Practical Solutions"
Co-author: "Oracle Insight - Tales of the OakTable"

web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: connor_mcdonald_at_yahoo.com


"GIVE a man a fish and he will eat for a day. But TEACH him how to fish,
and...he will sit in a boat and drink beer all day"

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