[Digikam-devel] extragear/graphics/digikam/libs/database

[Andi Clemens]

Ok here is some link, not the one I thought of, but it also says that LEFT 
JOIN can restrict ordering of the tables and therefore restrict optimization:

http://www.mail-archive.com/sqlite-users@sqlite.org/msg39455.html

The

a JOIN b ON a.x = b.x

should be the same as

a,b WHERE a.x = b.x

in our case.

Andi

On Thursday 28 May 2009 20:58:02 Andi Clemens wrote:
> I found only a quote now:
>
> "So when you use LEFT JOIN, that forces a particular ordering
> of tables, and greatly restricts the query engines opportunities to
> optimize.  So you should avoid using LEFT JOIN if you don't really
> need it."
>
> Seems that LEFT JOIN will not be optimized like the normal table connection
> by WHERE.
>
> Andi
>
> On Thursday 28 May 2009 20:54:24 Andi Clemens wrote:
> > All I know is that LEFT JOIN seems to generate more internal SQL code,
> > which might be a little bit slower.
> > There is also some statement to see what SQL is doing internally, but I
> > forgot it.
> >
> > I think I have a bookmark on this topic, I will attach it to this thread
> > when I find it.
> >
> > I learned it this way, never used LEFT JOIN. But I don't think that it
> > will make any difference in our case.
> >
> > Andi
> >
> > On Thursday 28 May 2009 20:47:08 Marcel Wiesweg wrote:
> > > Thanks Andi. I am out of exercise with the database code ;-)
> > > The query their is not old but just left over, I will remove it.
> > > Btw I would have coded the statement like this:
> > >
> > > SELECT count() FROM Images
> > > LEFT JOIN ImageInformation ON imageid == id
> > > WHERE Images.status== 1  AND ImageInformation.format='JPG';
> > >
> > > Do you know pros and cons of either variant?
> > >
> > > Marcel
> >
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>
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