[QGIS-Developer] QGIS expression based layer filters

[David Signer]

Thanks for the feedback, everyone. Yes, both have advantages and
disadvantages. With provider-side filters, you can do things that are not
possible in QGIS expressions (such as complex PostGIS functions), while
QGIS expressions are more powerful when using variables, etc., and it’s
super nice that they are independent of the provider. I would still avoid
having a combination of both, as it seems fairly fault-prone and hard to
use. Using either one or the other seems right. I would for sure create a
QEP PR before I start.

I’m not sure I understand you, Carlo, correctly. Are you talking about
creating query layers using QGIS expressions? If so, I’d prefer to keep
this aside for now. While it could be interesting, it would introduce a lot
of complexity, and I’d rather focus on filtering only. You can still create
a query layer — or a virtual layer — and then apply an expression filter to
it.

Cheers
Dave

On Wed, Sep 24, 2025 at 11:43 PM Carlo A. Bertelli (Charta s.r.l.) <
carlo.bertelli at gmail.com> wrote:

> Very interesting indeed.
> When I worked with Mapinfo, maybe 40 years ago, I remember using the
> "MapBasic" window to build query-generated layers.
> I think it would be very handy and substitute generating lots of geometric
> views inside the database (what a mess!).
> As Alessandro says, there are drawbacks in any solution, but operating the
> query inside the database (storing the query as a sort of "local view") is
> really much more efficient and does not require duplicating the backend via
> Spatialite with a lot of network traffic and cache size (see what happens
> with joins made at the client level in Properties).
> Meanwhile, the Query Builder is much more convenient and using the same
> syntax all the time is a good idea, but it could be optional, letting the
> user decide what to do.
> Anyway, this means creating "automatic temporary tables"; reducing them to
> filters only means missing some opportunities.
> I think we must make an effort to define use cases in a more rigorous way.
> I see a fuzzy scenario with possible misunderstanding between geodatabase
> parlance and GIS concepts, between application structure and use cases.
> Maybe it's only fog inside my mind, but the long gestation about the Query
> Builder suggests that I am not alone.
> Thanks for keeping reasoning on these issues.
> c
>
>
> On Wed, Sep 24, 2025 at 8:54 PM Nyall Dawson via QGIS-Developer <
> qgis-developer at lists.osgeo.org> wrote:
>
>>
>>
>> On Wed, 24 Sept 2025, 10:56 pm David Signer via QGIS-Developer, <
>> qgis-developer at lists.osgeo.org> wrote:
>>
>>> Hi there
>>>
>>> I've been considering enhancing the Query Builder (accessible via Layer
>>> -> Filter...) by QGIS expression based filters. At least with project and
>>> global scope.
>>> Maybe with tabs to distinguish between "provider side filter" (current
>>> functionality) and "expression filter" (new functionality).
>>>
>>> Any opinions on this? In my view, this could be valuable because it's
>>> available on all kinds of data-layers and allows us to use variables and
>>> functions for filtering as well.
>>>
>>
>> Can you make sure you file a QEP before starting any work? 👍
>>
>> Nyall
>>
>>>
>>> Thanks for any feedback and cheers
>>> Dave
>>>
>>> --
>>>
>>> Dave Signer
>>> Senior Developer & INTERLIS Architect
>>> OPENGIS.ch - Team QGIS & Industry Solutions
>>>
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