Specifying layer ID as source for QGIS Virtual Layer

Question

When creating a virtual layer in QGIS, we can use the layer name of any existing layer in a query, like:

SELECT layer1.*, layer2.*
FROM layer1
JOIN layer2 ON layer1.id = layer2.id

However, layer names aren't necessarily unique. The docs say The query can use the name (or id) of existing vector layers. I assumed this would be the Identifier we can set in a layer's metadata options, but using these in a query results in a Referenced table layer_identifier in query not found! message. Nor is it possible to use the identifier in the Source definition of an embedded layer:

Query execution error on DROP TABLE IF EXISTS "table1"; CREATE VIRTUAL TABLE "table1" USING gsVLayer('spatialite','layer_identifier',UTF-8): 1 - Provider error`

We could use embedded layers, but then my problem is that we cannot use relative paths, as far as I can see (the project should be usable on different computers and OSes).

Is there a more robust mechanism than using layer names for creating a virtual layer?

Answer 1

You can get the layer ID from the layer properties / variables / layerID

Then the virtual layer can use it like

select * from lpr_000a16a_e20180319075213250

enter image description here

• arjan: Ah yes. Upon seeing the metadata identifier field description popup it's clear that is meant for some external id. Too bad the layer_id variable isn't editable/overridable, it would be nice to get rid of the long generated string.