RE: Index rebuilding

Regarding specifically the sentence:

: So if you're looking at the diagram with
a whole
: pile of 25% full or 50% full buckets toward the left and all full or
nearly
: full buckets toward the right, I'd call that unbalanced.

You shouldn't.

When talking about "Balanced B-trees", you need to honour the technical meaning of word "Balanced" in the context that is being used, otherwise you confuse the issue.

It would be an interesting experiment to see if there is any sort of general visual image that people use in response to the word "unbalanced". I suspect that many people would think of a set of old-style scales with one side hanging down further than the other. Conversely, I don't think many people would have an image of (something like) a plank tipping because a fulcrum was off-centre.

Apply these images to 'unbalanced indexes', and I think you'll agree that the more (likely to be) popular image is the wrong

Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/seminar.html Optimising Oracle Seminar - schedule updated Sept 19th

:
: From: "Mark W. Farnham" <mwf_at_rsiz.com>
: Subject: RE: Index rebuilding
: Date: Fri, 12 Nov 2004 14:41:22 -0500
:
: Depends what you mean by "unbalanced."
:
: If you mean that number of levels from the root to the leaves varies, I
: think Richard is pretty clear about why that can never happen, since in
: Oracle's implementation indexes grow in height by splitting the root.
: (Richard represents the tree as upside down, metaphorically, from trees
you
: see growing, so perhaps they grow in depth. I guess that makes the leaves
: all blades of grass.) For Richard, at least in the paper in question, that
: is certainly what he is talking about.
:
: Now, on the other hand, if by unbalanced (versus balanced) you mean there
: are more leaves to the right than to the left, Richard illustrates nicely
: when and why that happens. So if you're looking at the diagram with a
whole
: pile of 25% full or 50% full buckets toward the left and all full or
nearly
: full buckets toward the right, I'd call that unbalanced.

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