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Re: pointers on representing tree in db?

From: Philip Lijnzaad <lijnzaad_at_ebi.ac.uk>
Date: 23 Apr 2001 11:00:20 +0100
Message-ID: <u7vgnwq6kr.fsf@sol6.ebi.ac.uk>

Lennart> Each row in the Datatable represent a treenode. parent_id is a
Lennart> reference to another row which acts as a parent for the current
Lennart> Lennart> one. 

>>
>> why ? this information is already in the Ancestor table. So what is the
>> difference between Data.parent_id and Ancestor.ancestor_id ? Is this a
>> deliberate denormalization or a typo ?
>>

Lennart> Another aproach would be to remove the id - parentid row from Lennart> Ancestor, thus keeping all ancestors but parent there.

Ah ... now I'm starting to get it; does the ancestor table contain, for each node, _all_ ancestors (not just the direct parent) up to the top? (Ancestors is the transitive closure of Data, in other words)? That is interesting, if expensive, much too expensive too my liking ... I think I would only keep an explicit transitive closure for complicated general graphs, not for trees.

Sorry for not getting this earlier ...

Lennart> This would however make queries like "look for all Data that Lennart> fulfills condition x in my subtree" more complicated.

yes, certainly.

Lennart> Major drawback is that the ancestor table grows quite fast for large
Lennart> trees.  I grows big since adding a node at level x means adding
Lennart> (x-1) rows in the ancestor table.

yes, now I see.

Lennart> Whether one can minimize it or not is an open issue.

well, not so open: Celko's nesting set representation does really minimize this. The trick is that you use the ordering of the natural numbers to describe subtrees implicitly, rather than explicitly using a complete explicit description using the ancestors tables.

To me it is now clear that the solution you presented is really very wrong (sorry to be so adamant ...). Celko's nested set representation (with the addition of direct parent_id, which in a sense is a denormalization) gives you this and more (ordering of the direct children, easy detection of leaf nodes) for far less.

Cheers,

                                                                      Philip
-- 
Why don't Alice and Bob simply get married?
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Philip Lijnzaad, lijnzaad_at_ebi.ac.uk \ European Bioinformatics Institute,rm A2-08
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Received on Mon Apr 23 2001 - 05:00:20 CDT

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