Select and load polygons intersecting another with PyQGIS (polygons stored in PostGIS)

Question

I'm still a newbie at Python and I'm having issues selecting and loading polygons intersecting others with PyQGIS.

I managed to use PyQGIS to achieve that operation with shapefiles stored in my laptop but I haven't found a solution to proceed the same treatment on shapefiles stored in PostGIS.

This is what I came to so far :

from qgis.core import *
from PyQt4.QtCore import *
from PyQt4.QtCore import QSettings
from PyQt4.QtCore import QSettings
from qgis.core import QgsVectorLayer, QgsDataSourceURI
import processing

uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "name of my database", "my user name", "my password")
uri.setDataSource("database schema", "table name", "geom")
couche = "PM"
layer1 = QgsVectorLayer(uri.uri(), couche, "postgres")
if not layer1.isValid:
    print ('Chargement de la couche: echec !')
else:
    QgsMapLayerRegistry.instance().addMapLayer(layer1)


uri.setDataSource("database schema", "table name2", "geom")
couche = "batiments"
layer2 = QgsVectorLayer(uri.uri(), couche, "postgres" )

subset_layer2 = processing.runalg('qgis:extractbylocation', layer2,layer1,['intersects'],0.0,None)


uri.setDataSource("rbal", "parcelles", "geom")
couche = "parcelles"
layer3 = QgsVectorLayer(uri.uri(), couche, "postgres" )

subset_layer3 = processing.runalg('qgis:extractbylocation', layer3,layer1,['intersects'],0.0,None)

When I run the code I don't get any error but the treatment is loading forever without returning any results...

---

EDIT:

With that script I've managed to select all the polygons contained in layer2 that intersect layer1 but I do have to load layer2 to do so. Is there a way to select and load those intersecting polygons without loading the whole layer in the first place ?

uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "name of my database", "my user name", "my password")
uri.setDataSource("database schema", "table name", "geom")
couche = "PM"
layer1 = QgsVectorLayer(uri.uri(), couche, "postgres")
if not layer1.isValid:
    print ('Chargement de la couche: echec !')
else:
    QgsMapLayerRegistry.instance().addMapLayer(layer1)


uri.setDataSource("database schema", "table name2", "geom")
couche = "batiments"
layer2 = QgsVectorLayer(uri.uri(), couche, "postgres" )
if not layer2.isValid:
    print ('Chargement de la couche: echec !')
else:
    QgsMapLayerRegistry.instance().addMapLayer(layer2)

feats_layer1 = layer1.getFeatures()
for feat in feats_layer1:
    geom_layer1 = feat.geometry()
feats_layer2 = layer2.getFeatures()

for feature in feats_layer2:
    if feature.geometry().intersects(geom_layer1):
        layer2.select(feature.id())

Answer 1

Thanks to J.Monticolo I've found an existing solution on stackoverflow:

PyQGIS - Load a PostgreSQL/PostGIS layer from a SQL query

QgsDataSourceuRI and left outer join

EDIT:

Even though the links above provide (in my example) the best solution, there is a way to do the same thing only with PyQGIS:

from qgis.core import *
from PyQt4.QtCore import *
from PyQt4.QtCore import QSettings
from PyQt4.QtCore import QSettings
from qgis.core import QgsVectorLayer, QgsDataSourceURI
from qgis.gui import QgsMapCanvas
import processing

uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "name of my database", "my user name", "my password")
uri.setDataSource("database schema", "table name", "geom")
couche = "PM"
layer1 = QgsVectorLayer(uri.uri(), couche, "postgres")
if not layer1.isValid:
    print ('Chargement de la couche: echec !')
else:
    QgsMapLayerRegistry.instance().addMapLayer(layer1)


uri.setDataSource("database schema", "table name2", "geom")
couche = "batiments"
layer2 = QgsVectorLayer(uri.uri(), couche, "postgres" )
if not layer2.isValid:
    print ('Chargement de la couche: echec !')
else:
    QgsMapLayerRegistry.instance().addMapLayer(layer2)

inter_feats = []

feats_layer1 = layer1.getFeatures()
for feat in feats_layer1:
    geom_layer1 = feat.geometry()
feats_layer2 = layer2.getFeatures()

for feature in feats_layer2:
    if feature.geometry().intersects(geom_layer1):
        inter_feats.append(feature.id())
        
layer2.select(inter_feats)

intersecting_features = QgsVectorFileWriter.writeAsVectorFormat(layer2, 'C:/Users/Users/Documents/test.shp', "utf-8",  None, "ESRI Shapefile", onlySelected=True)


bati = QgsVectorLayer('C:/Users/Users/Documents/test.shp', "test", "ogr")
if not bat.isValid():
    print("Impossible de charger la couche !")

# add layer to the registry
QgsMapLayerRegistry.instance().addMapLayer(bati)