Get first AND last element with SQLAlchemy

Question

In my Python (Flask) code, I need to get the first element and the last one sorted by a given variable from a SQLAlchemy query.

I first wrote the following code :

first_valuation = Valuation.query.filter_by(..).order_by(sqlalchemy.desc(Valuation.date)).first()
# Do some things
last_valuation = Valuation.query.filter_by(..).order_by(sqlalchemy.asc(Valuation.date)).first()
# Do other things

As these queries can be heavy for the PostgreSQL database, and as I am duplicating my code, I think it will be better to use only one request, but I don't know SQLAlchemy enough to do it... (When queries are effectively triggered, for example ?)

What is the best solution to this problem ?

Answer 1

1) How to get First and Last record from a sql query? this is about how to get first and last records in one query.

2) Here are docs on sqlalchemy query. Specifically pay attention to union_all (to implement answers from above). It also has info on when queries are triggered (basically, queries are triggered when you use methods, that returns results, like first() or all() . That means, Valuation.query.filter_by(..).order_by(sqlalchemy.desc(Valuation.date)) will not emit query to database).

Also, if memory is not a problem, I'd say get all() objects from your first query and just get first and last result via python:

results = Valuation.query.filter_by(..).order_by(sqlalchemy.desc(Valuation.date)).all()
first_valuation = results[0]
last_valuation = results[-1]

It will be faster than performing two (even unified) queries, but will potentially eat a lot of memory, if your database is large enough.

Answer 2

No need to complicate the process so much.

first_valuation = Valuation.query.filter_by(..).order_by(sqlalchemy.desc(Valuation.date)).first()
# Do some things
last_valuation = Valuation.query.filter_by(..).order_by(sqlalchemy.asc(Valuation.date)).first()

This is what you've and it's good enough. It's not heavy for any database. If you think that it's becoming too heavy, then you can always use some index.

Don't try to get all the results using all() and retrieving from it in list style. When you do, all() it loads everything into the memory which is extremely and extremely bad if you have a lot of results. It's a lot better to execute just two queries to get those items.