Is it possible to use the DB Manager in QGIS (I am using version 3.4) to run a SQL query between a project layer (e.g. shapefile) and a PostGIS layer?
For example, I have a polygon shapefile in my QGIS project in the layers panel and I want to select all point features (saved in a PostGIS database but not loaded in my layers panel) that are within the shapefile polygon.