[QGIS-Developer] QGIS expression based layer filters

[Andreas Neumann]

Hi David and colleagues,

This whole topic of introducing QGIS expression support into provider side
filtering is related to (or almost the same) as the SQL compiler that
Matthias worked on: making sure that as much as possible, QGIS expressions
are converted/translated into server/provider side SQL commands where
possible (e.g. PostgreSQL, SQL Server, SqLite) or OGR filters (a limited
subset of filters).

As an end user I would like to get a visual feedback whether the QGIS
expression in the filter can be translated/run on provider side or if the
data has to be loaded in full and needs to be filtered in QGIS as a client.
Such a visual feedback would give the end user an idea of how efficiently
the filtering can be expected.

Just some thoughts around that. I am also interested in seeing a QEP on
this topic. We discussed this very same issue a few years ago, and back
them it was deemed to complicated / too much effort. But things have
changed since then ...

Greetings,
Andreas



On Thu, 25 Sept 2025 at 08:44, David Signer via QGIS-Developer <
qgis-developer at lists.osgeo.org> wrote:

> 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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-- 

--
Andreas Neumann
QGIS.ORG board member (treasurer)
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