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From Uriah Eisenstein <uriaheisenst...@gmail.com>
Subject Re: Are generated columns stored in the DB or calculated on the fly?
Date Sat, 04 Sep 2010 16:16:41 GMT
OK, filed the generated columns documentation issue in JIRA (Derby-4790).
Possibilities like taking advantage of Comparable java UDTs crossed my mind
too, but I didn't dare suggest it :) I'm playing around with Derby and JDBC
at home currently, and have little knowledge about the actual implementation
of a DB back-end - as well as no access to the SQL standard. I don't know
what these would say about casting UDTs into strings, for instance -
something which should be possible for any Java type using its toString()
Similarly, I don't know if virtual columns have any standard syntax; I only
saw that some DBs define them simply by "AS <expression>" following the
common column definition. Because of that I'd rather not file an improvement
suggestion, unless you think I should despite the missing information.
Uriah Eisenstein

On Fri, Sep 3, 2010 at 11:49 PM, Knut Anders Hatlen

> Uriah Eisenstein <uriaheisenstein@gmail.com> writes:
> > Hi Knut,
> > Thanks for your answer. May I suggest that this be added to the
> documentation
> > of generated columns?
> Yes, you may. :) Please file a JIRA report so that it's not forgotten.
> > I think support for virtual columns, as they're indeed called in some
> other
> > DBs, would really be nice though. For instance, Java Enums fit nicely as
> a
> > UDT, but indexing on them (or even simple comparisons) would require a
> > separate column with the ordinal integer value or the name string; in the
> > latter case, the space requirement is rather superfluous relatively to
> that of
> > the enum value itself. It seems to me this could be used as a partial
> solution
> > at least to expression-based indexes (Derby-455), by requiring indexes on
> > simple columns which may be virtual, or implementing the index by
> defining a
> > hidden virtual column, or something of the kind. Do you know if there's
> any
> > Derby improvement suggestion in that direction? (I havent'found any).
> Not that I'm aware of, but they sound like useful suggestions.
> For the Enum case, one alternative solution may be to improve the UDT
> support to allow comparisons and ordering if the Java type implements
> java.lang.Comparable. Since Enum implements Comparable, that should
> allow creating indexes directly on an Enum-based UDT column.
> --
> Knut Anders

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