id (integer), rid (integer), fdate (date), rast(raster), layer(integer)

# -*- coding: utf-8 -*-

from datetime import timedelta, date
from qgis.analysis import QgsRasterCalculator, QgsRasterCalculatorEntry

config = {
    "dbname"  : "model",
    "user"    : "postgres",
    "host"    : "localhost",
    "password": "******"
}

start_date = date(2009, 1, 1)
end_date = date(2017, 1, 1)
delta = timedelta(1)

initial_date = start_date
layer_entry = []
num = 1
while start_date < end_date:
    connString_ = "PG: dbname=%(dbname)s host=%(host)s user=%(user)s password=%(password)s port=5432 mode=2 column=rast table=soil_moist where=%(where)s" %
                      {"dbname": config["dbname"],
                       "host": config["host"],
                       "user": config["user"],
                       "password": config["password"],
                       "where": 'fdate='%s' AND layer=1' % start_date
                       }
    layer = QgsRasterLayer(connString_, "%s_%s" % (str(start_date), str(num)))
    raster_entry = QgsRasterCalculatorEntry()
    raster_entry.ref = 'layer@%s' % num
    raster_entry.raster = layer
    raster_entry.bandNumber = 1
    layer_entry.append(raster_entry)
    num+=1
    if ((start_date + delta).month > start_date.month) or ((start_date + delta).year > initial_date.year):
        output_path = "D:/output/%s.tif" % start_date
        expression = "("
        for idx, layer_ in enumerate(layer_entry):
            expression += "layer@%s*" % str(idx+1)
        expression += "1)"
        expression += "/%s" % str(len(layer_entry))
        calc = QgsRasterCalculator(expression, output_path, "GTiff", layer.extent(), layer.width(), layer.height(), layer_entry)
        calc.processCalculation()
        # Reset Values
        num = 1
        layer_entry = []
    start_date+=delta