gistfile1.txt

SELECT COALESCE(hmr.datetime_heat_meter, hmr.timestamp_utc) AS timestamp_utc,
            hm.pod_id,
            hmr.energy,
            hmr.flow,
            hmr.power,
            hmr.forward_temperature,
            hmr.return_temperature,
            hmr.pressure,
            COALESCE(hmr.volume, 0.0::double precision) AS volume,
            NULL::real AS pressure2
           FROM heat_meter_reading hmr
             JOIN heat_meter hm ON hm.serial_number = hmr.serial_from_message
          WHERE COALESCE(hmr.datetime_heat_meter, hmr.timestamp_utc) IS NOT NULL
        UNION ALL
         SELECT modbus_reading.timestamp_utc,
            modbus_reading.pod_id,
            modbus_reading.energy,
            modbus_reading.flow,
            modbus_reading.power,
            modbus_reading.flow_temperature AS forward_temperature,
            modbus_reading.return_temperature,
            modbus_reading.flow_pressure AS pressure,
            0.0::double precision AS volume,
            modbus_reading.return_pressure AS pressure2
           FROM modbus_reading
        )
 SELECT readings.timestamp_utc,
    readings.energy,
    readings.flow,
    readings.power,
    readings.forward_temperature,
    readings.return_temperature,
    readings.pressure,
    readings.pod_id,
    readings.volume,
    readings.pressure2
   FROM readings