speed up sqlalchemy orm dynamic relationship slicing with negative indicies

Question

I have the following SQLA models and relationships. I am logging a measurement for each channel every second, so there are lots of measurements in the DB.

class Channel( Model ) :
    __tablename__   = 'channel'
    id              = Column( Integer, primary_key=True )
    #! --- Relationships ---
    measurements    = relationship( 'Measurement', back_populates='channel', lazy='dynamic' )

class Measurement( Model ) :
    __tablename__   = 'measurement'
    id              = Column( Integer, primary_key=True )
    timestamp       = Column( DateTime, nullable=False )
    value           = Column( Float, nullable=False )
    #! --- Relationships ---
    channel         = relationship( 'Channel', back_populates='measurements', uselist=False )

If I want to get the latest measurement I can get it via ORM and slicing with a negative index.

channel.measurements[-1]

However, it is very very slow !!

I can do another filter the relationship query further with .filter() and .order_by() etc, to get what I want, but I like using the ORM (why have it otherwise?)

I noticed that if I slice with a positive index that it is fast (similar to explicit SQLA queries mentioned above).

channel.measurements[0]

I changed the relationship to keep measurements in reverse order, and that seems work in conjunction with using a zero index.

    measurements    = relationship( 'Measurement', back_populates='channel', lazy='dynamic', order_by='Measurement.id.desc()' )

So, why is negative index slicing so slow ??

Is it a bug in SQLAlchemy? I would have thought it would be smart enough to do the correct SQL to get only the latest item from the DB?

Is there something else I need to do to have the measurements sorted in natural order and use negative index slicing and get the same speed as the other methods ??

Answer 1

It seems the answer is that efficient slicing or realtionship collections with negative indicies is not supported in SQLA. In fact there seems to be some kludgy attempt in the code but is going to be removed from SQLA as it wasn't carefully thought out.

https://github.com/sqlalchemy/sqlalchemy/issues/5605

I have solved my issue by implementing a hybrid property that returns me the latest measurement, rather than slicing the relationship collection directly.

    @hybrid_property
    def latest_measurement( self ) -> float :
        """
        Hybrid property that returns the latest measurement for the channel.
        """
        measurement = self.measurements.order_by( Measurement.id.desc() ).first()
        return measurement

Answer 2

You haven't given any ordering, so it has to load all of the objects into a list, and then get the last one.

If you add the echo=True parameter, you can see the difference in the queries:

For measurements[0], it selects just one (LIMIT 1) of the measurements matching the channel:

SELECT measurement.id AS measurement_id, measurement.ts AS measurement_ts,
  measurement.value AS measurement_value,
  measurement.channel_id AS measurement_channel_id
FROM measurement
WHERE %(param_1)s = measurement.channel_id
 LIMIT %(param_2)s
{'param_1': 6, 'param_2': 1}

For measurements[-1], it selects all of the measurements matching the channel. You haven't ordered it, so it has to ask the database to return the rows in whatever order it decides (maybe the primary key on measurement, but no guarantee):

SELECT measurement.id AS measurement_id, measurement.ts AS measurement_ts,  
  measurement.value AS measurement_value,
  measurement.channel_id AS measurement_channel_id
FROM measurement
WHERE %(param_1)s = measurement.channel_id
{'param_1': 6}

If you want just the latest Measurement, select it and order by timestamp field; you probably want indices on channel_id and your timestamp field:

db.session.query(Measurement)\
    .filter(Measurement.channel_id == channel_id)\
    .order_by(Measurement.ts.desc())\
    .limit(1)\
    .first()