[QGIS-Developer] Processing: questions on temporary files

[Matthias Kuhn]

On 12/8/19 11:35 PM, Nyall Dawson wrote:
> On Fri, 6 Dec 2019 at 23:19, Matthias Kuhn <matthias at opengis.ch> wrote:
>> Other proposals are very welcome as well. I don't insist on GeoPackage.
>> All I do is being a bit skeptic that rolling our own format will
>> magically solve problems that one hundred other formats did not solve :-)
>>
> The issue is that the memory provider, by design, MUST be a lossless
> reflection of all the capabilities offered in QGIS vector layers. That
> means support for every geometry type, and field type handled by QGIS.
> Geopackage might be capable of that now... but it won't always be, due
> to the inherent limitations of that format. (No multi-geometry
> support, no mixed CRS geometry support, no support for advanced field
> types like range/interval fields, etc).

Having our own binary format makes sense from a full QGIS data type 
support perspective.

As mentioned before, I don't insist on GeoPackage (but now that you say 
it, does it not have multi-geometry support? According to the specs it 
should. And just like we can binary dump all our field formats into our 
own binary file, we could dump them into any file, like e.g. GeoPackage).

> I don't think it's useful to even think of rolling our own data format
> -- that's NOT what's required here. All that we need is a way of
> paging the currently in-memory storage of features used by the memory
> provider to disk when required. This disk based paging would be
> totally temporary (always deleted when the layer is removed), have no
> requirement for a stable binary format (we could change the paging
> binary structure version by version if needed), and absolutely no need
> for ANY other programs to be able to parse or interpret. It's not a
> data format at all -- it's just a disk based reflection of what we
> currently store in RAM, along with some smart indexing to optimize
> access.

Implementing indexing and paged loading of requested data resolved by an 
index sound like fun, do you think the effort (or the risks involved) is 
smaller than tacking support for the missing bits to another format?

I am counting the days until someone wants to recover from one of these 
files ;)

Matthias