On Thu, Apr 14, 2011 at 10:51 AM, Tim Schaub <[email protected]> wrote:
> On 4/14/11 12:29 AM, Andrea Aime wrote:
> > On Thu, Apr 14, 2011 at 2:20 AM, Tim Schaub<[email protected]> wrote:
> >> WFS 2.0 describes both paging with and without transactional
> >> consistency. It's not a requirement to be consistent there even
> >> (PagingIsTransactionSafe can be true or false), right?
> >>
> >>> Also it seems that WMS GetMap already supports paging... and this is
> >>> more or less following suite what it is doing.
> >>>
> >>>
> >>> Also curious about sorting... I remember something like sorting by
> >>> default
> >>> on the feature id while paging, but my memories of it are foggy,
> we
> >>> discussed
> >>> this with Gabriel a loong time ago (when the paging machinery was
> >>> added to
> >>> GeoTools).
> >>>
> >>>
> >>> As I understand things how they are implemented now using startIndex
> >>> requires the underlying datastore to be able to do sorting. And when
> the
> >>> client does not specify an explicit attribute to sort on this means
> >>> doing a natural sort (feature id). If you look at
> >>> ContentFeatureSource.getReader(Query) you will see a check there.
> >>>
> >>> Which more or less means that paging can only be used with jdbc
> >>> datastores. Which personally I think is fine since they are the only
> >>> ones that can really do it efficiently. Thoughts on that?
> >>
> >> Would it be possible (later) to do sorting for all stores? I understand
> >> it would be inefficient, but it's a bigger drawback (in my opinion) to
> >> have a feature be store specific. As the client has no way to know
> >> about these distinctions.
> >
>
> Sorry, I meant to ask about *paging* on all stores.
>
Right, but to do paging consistently implicitly requires sorting since you
can't rely 100% of the time on a data source returning features in
a consistent ordering. Or am I missing something?
>
> > A trivial way to implement sorting is to do it in memory. The day someone
> > decides to sort on that 2GB shapefile boom!
> > We'd need a secondary storage sort approach (mergesort), writing one
> would
> > require quite a bit of time and the assurance everything in the feature
> is
> > serializable.
> > Another approach would be to have a hsql db, dump everything in it, and
> > make it do the sort.
> >
> > In both cases it is to be noted that we first have to sort the large
> collection
> > and then extract the page of data we need, so in the case of the large
> shapefile
> > you'd have to first sort everything in order to get your 50 records page.
> > Not efficient... (a db could do that efficiently only if it has a
> > index on the sorting
> > column, something that is normally available for the primary key)
> >
> > Cheers
> > Andrea
> >
>
>
> --
> Tim Schaub
> OpenGeo - http://opengeo.org
> Expert service straight from the developers.
>
>
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--
Justin Deoliveira
OpenGeo - http://opengeo.org
Enterprise support for open source geospatial.
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