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From Manish Khettry <manish.khet...@gmail.com>
Subject Re: jira question
Date Wed, 03 Aug 2005 17:51:03 GMT
I think using arbitrarily complex expressions to create indexes may be
overkill. Dan's idea of restricting expression indexes on function
calls on a single base column (actually one _or more_ base columns
shouldn't be that much harder) is probably better.

Isn't storing the expression text for a generated column also
problematic? I can think of a few other things
-- The expression will also have to be compiled and evaluated for DDL
like alter table (drop column)  possibly others.
-- We'll need a good way to check if two expressions are the same.
Obviously a textual comparison may not work. Making sure that two
expr's are the same is not trivial if we want to do it right-- say
(x+y) > 10 is the same as 10 < (y+x).


On 8/3/05, Rick Hillegas <Richard.Hillegas@sun.com> wrote:
> Thanks, Jeff. I agree. Recompiling these expressions can occur when we
> compile the triggering SELECT/UPDATE/INSERT statements. Compiling a
> SELECT/UPDATE/INSERT is a relatively heavyweight operation and I don't
> think anyone will notice a little extra time spent recompiling these
> expressions.
> Cheers,
> -Rick
> Jeffrey Lichtman wrote:
> >
> >> The expression used for the generated column must be stored in the
> >> data dictionary. Does derby serialize expressions? I was looking
> >> around in the classes in impl/sq/compile (ValueNode and such) and
> >> these classes don't seem to implement Formatable.
> >
> >
> > It would be better to store the expressions as text, and to parse and
> > bind them when needed. I learned the hard way while working on the
> > internals of other database systems that storing internal data
> > structures in the data dictionary causes trouble when the structures
> > change. Also, it's useful for users to be able to look at the text of
> > their DDL statements.
> >
> >
> >                        -        Jeff Lichtman
> >                                 swazoo@rcn.com
> >                                 Check out Swazoo Koolak's Web Jukebox at
> >                                 http://swazoo.com/

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