Allow more Expression scopes in virtual Layer SQL

[cxcandid]

I figured out, that I can use global and project variables in virtual layers:
SELECT var('qgis_version') as qgis,var('project_basename') as project

It's also documented that we can speed up our queries when using the hidden column
_search_frame_.

But what I really would like to do is: SELECT ... WHERE _search_frame_ = var(map_extent) which is not possible yet, because - I guess - the variable scope "MapSetting" is not supported.
Maybe that's an oversight, because inside layer labels I can use i.e. geom_to_wkt( @map_extent ) .

It's possible to write a custom Expression function as workaround:

@qgsfunction(args='auto', group='Custom')
def currentExtent(feature, parent):
    return QgsGeometry.fromRect(iface.mapCanvas().extent())

SELECT ... WHERE _search_frame_ = currentExtent()

but this only uses the map canvas extents and not the extents of a layout map window.

[cxcandid]

Maybe @mhugo could add MapSettingsScope to the available Expression scopes:

src/providers/virtual/qgsvirtuallayersqlitemodule.cpp

900  // initialize the expression context
901  qgisFunctionExpressionContext << QgsExpressionContextUtils::globalScope();
902  qgisFunctionExpressionContext << QgsExpressionContextUtils::projectScope( QgsProject::instance() );
>>903  qgisFunctionExpressionContext << QgsExpressionContextUtils::mapSettingsScope( QgsMapSettings::instance() );

[cxcandid]

Ok, I guess it's not that easy. It has to do with mapSettingsScope, but honestly I don't know how.
If we can use i.e. var('map_extent') in QGIS label strings, showing the right values in canvas and layout map windows, it should be possible to use it in virtual Layer SQL as well... I'm only thinking loud.

[mhugo]

@cxcandid Thanks for your proposal. That would be indeed a nice addition to SQL layers.

Ok, I guess it's not that easy. It has to do with mapSettingsScope, but honestly I don't know how.

Did you try that modification you proposed ?

[cxcandid]

@mhugo No unfortunately not, and I guess it doesn't work, because IMHO mapSettingsScope needs the settings of the layout map. And btw. I have absolutely no C++ knowledge.

Maybe you find the answer in the code for parsing Expression functions in label strings?

[nyalldawson]

It won't be possible to add the map settings scope for virtual layers -- virtual layers are used everywhere, and not just in situations associated with any particular map. The proposed patch mentions a QgsMapSettings::instance() object which just doesn't exist (and cannot POSSIBLY exist!)

[cxcandid]

@nyalldawson Thanks for clarification. Totally forgot about the processing tool.