[Qgis-user] Cross layer filtering

[Adam Nielsen]

> I've been filtering a layer by adding a virtual column populated with
> is_selected() and then creating a virtual layer from it using the
> filter: WHERE virtualField = 1
> 
> This is working well and allows you to have a copy of the layer that
> only shows the selected records when you are creating a layout.

What's your use case for this?  I just wonder whether there is an
alternative that might be easier?

For example if you're always selecting the same set of features, you
could add a category/classification column and then filter based on
that.

> I now need to bring in an associated layer, but I can't find a
> reliable way to cross layer filter because QGIS just hangs for ages
> and crashes.

I have found that QGIS can be extremely slow sometimes when joining
tables and can often crash, so if there's a solution that can avoid
this you might find it more robust.

> I've searched around and can see a lot of people talk about making
> the database do the work but of course it can't if you are trying to
> use a selection filter.

So there's definitely no way you can classify your data such that you
don't need to use a selection filter?

> What is the most reliable way of setting up a filter based on a join
> because QGIS tells me that I must use a virtual layer and even
> creates it for me; but then it just hangs and crashes.

I suppose you could export the virtual selection layer (or even forget
the layer and just use "export selected features") and put those in a
real database table, which you then link off instead?  It would be a
more laborious process having to do an export each time you wish to
select different features but it might at least allow the workflow to
complete.

Cheers,
Adam.