On 10/12/2007, Cesar D. Rodas <[EMAIL PROTECTED]> wrote:
>
> What Kess Nuyt is asking is if you want to replace a column with a data,
> which is the data, will SQLite update it or SQLite is smart enough for
> avoid write the same thing?, very important for keep executing time,
> because write something to HDD is very expensive.
>
> I hope I understand well your question Kees, unfortunately I couldn't
> answer your question, because I don't know, I think Dr. Hipp will be able to
> answer your question :-)
It was Clodo question, sorry :-)
On 10/12/2007, Kees Nuyt <[EMAIL PROTECTED]> wrote:
> >
> > On Mon, 10 Dec 2007 12:22:53 +0100, Clodo <[EMAIL PROTECTED]>
> > wrote:
> >
> > >In this case:
> > >
> > >-------------------------
> > >CREATE TABLE test (
> > > Field01 text PRIMARY KEY NOT NULL,
> > > Field02 text
> > >);
> > >
> > >insert into test values ('alpha','beta');
> > >
> > >update test set Field01='alpha', Field02='gamma';
> > >-------------------------
> > >
> > >In the "update" statement, i re-set the primary field "Field01" to a
> > >value that field already have.
> >
> > >Sqlite detect this situation and don't update the primary index,
> >
> > SQLite will update the primary key, verifying all constraints
> > (NOT NULL, UNIQUE). The performance impact isn't very big,
> > because the relevant pages will be loaded in the cache anyway.
> >
> > EXPLAIN UPDATE test SET Field01='alpha', Field02='gamma';
> > tells the whole story.
> >
> > >or suppose that developers optimize situation like that?
> >
> > The statement does not do what you seem to need.
> > Usually you want to do:
> > UPDATE test SET Field02='gamma' WHERE Field01='alpha';
> >
> > Without the WHERE clause ALL rows will be updated, which will
> > fail because column Field01 will not be unique anymore.
> >
> > Extend your test set with:
> > insert into test values ('delta','kappa');
> > and rerun your test to see what happens.
> >
> > >P.s. in our program, the "update" statement are generated from a
> > >database-layer, and optimize the statement generation is a big work..
> > >for that i'm trying to understand if will be a biggest optimization or
> > >not..
> > >thanks for feedback!
> >
> > HTH
> > --
> > ( Kees Nuyt
> > )
> > c[_]
> >
> >
> > -----------------------------------------------------------------------------
> > To unsubscribe, send email to [EMAIL PROTECTED]
> >
> > -----------------------------------------------------------------------------
> >
> >
>
>
> --
> Best Regards
>
> Cesar D. Rodas
> http://www.cesarodas.com
> http://www.thyphp.com
> http://www.phpajax.org
> Phone: +595-961-974165
--
Best Regards
Cesar D. Rodas
http://www.cesarodas.com
http://www.thyphp.com
http://www.phpajax.org
Phone: +595-961-974165
