Re: Trying to implent (Joe Celko's) Nested Sets, but need more data columns

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From: Joe Celko (jcelko212_at_earthlink.net)
Date: 09/19/04 

Date: Sun, 19 Sep 2004 04:31:26 -0700

>> The lft and rgt columns of the nested set model are not relational,
not verifiable, and not stable. <<

Unh? The (lft,rgt) pairs are easily veriable. They have nothing to do
with a particular physical storage mechanism, unlike a GUID. You look
at the graph of the tree and compute them; they are a kind of logical
"co-ordinate pair" for tree structures.

Explain why they are not stable. Each tree structure has one and only
one nested set representation when you close any gaps in the sequence.
I would understand if you complained that nested sets are TOO stable,
making structural changes expensive.

Tropashko's nested intervals are good when you have a tree structure
that is not stable -- like newgroup threads. But the price is the
overhead of a library of UDFs for rational numbers and some extra
complexity in the basic operations. Always a trade-off.

--CELKO--
Please post DDL, so that people do not have to guess what the keys,
constraints, Declarative Referential Integrity, datatypes, etc. in your
schema are. Sample data is also a good idea, along with clear
specifications.

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