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Re: A DBMS implementation Question

From: owen <member25756_at_dbforums.com>
Date: Sat, 01 Mar 2003 14:23:57 +0000
Message-ID: <2591944.1046528637@dbforums.com>

Originally posted by Larry Coon
> owen wrote:
>
> > Some small rational dbms use B tree for indexing.
>
> I wouldn't limit it to "some small."
>
> > I found a problem in
> > the condition that the key for the table is a multiple key.
>
> Actually, as I read through the rest of your post, your problem
> doesn't really pertain to composite keys or even keys in general.
> Nor are they specific to any implementation of indexing, such as
> B-tree variants.
>
> > For example
> > , there is a table which is denoted as emp(no, age, name), and
> (no +
> > age) is the key for indexing.
>
> I'd normally question whether (no, age) is even a candidate key,
> but I'll leave it alone for your example.
>
> > In the B Tree, the values of "no" and "age"
> > are transfered to String and merge together,
>
> That'd be an implementation issue. And it might be a poor way
> to do it, since any lookup would incur a lot of overhead in
> string conversion.
>
> > such as the key value of
> > (1, 20, "abc") is "00010020",and the the B Tree of the table
> is
> > established by the sequence of the key.
> > Now , I have a problem , if I want to query all the people
> whose
> > age are more than 20 , which is described as "select * from
> emp
> > where age > 20", i can't use b tree for indexing,
>
> No, for that query, then THAT PARTICULAR index is of little use.
>
> > if i use , the
> > number of records that i must scan may be very large, how
> to sovle
> > the problem.
> > The problem can be expressed as "how to deal with scaning
> the table
> > using non-primary key or non-key field".
>
> The problem is really, "what column(s) are good candidates for
> indexing?" The primary key is a given, since a PK must be unique
> and we need an efficient way of enforcing uniqueness. But nothing
> prevents you from adding more indexes. If you perform a lot of
> queries such as the one above, then the age column might be a good
> candidate for its own index (with a possible argument against being
> the amount of selectivity that index would provide -- if the table
> contained something like voter registration where the vast majority
> of rows had age > 20, then the index might be ignored in favor of a
> table scan in any event).
>
>
> Larry Coon
> University of California
> larry_at_assist.org
> and lmcoon_at_home.com

thank you for replying, now i am in the study of database techology, my class design is to implement a small rational db, I have read some papers, and I decide to use b tree for indexing, and I want to express key by string, which is the means I described above, but the problem I expressed has anonyed me until now , so do you have some advise for me?i am waiting for you , thanks

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Received on Sat Mar 01 2003 - 08:23:57 CST

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