postgresql - Load PostGIS layer/table to QGIS canvas using Python

I want to load a PostGIS layer to QGIS canvas. I am using the following code but it is not loading the layer in canvas.

sql = "(SELECT * FROM table WHERE id = 20)"
uri = QgsDataSourceURI()
uri.setConnection("host", "5432", "databasename", "user", "password")

uri.setDataSource("public","table", "the_geom", sql,"gid")
print uri.uri()
vlayer = QgsVectorLayer(uri.uri(), "test", "postgres")
QgsMapLayerRegistry.instance().addMapLayer(vlayer)
canvas.refresh()

But the layer is not loaded. I don't know what I am doing wrong here.