Fast SQLAlchemy counting (avoid query.count() subquery)

gistfile1.py

def get_count(q):
    count_q = q.statement.with_only_columns([func.count()]).order_by(None)
    count = q.session.execute(count_q).scalar()
    return count

q = session.query(TestModel).filter(...).order_by(...)

# Slow: SELECT COUNT(*) FROM (SELECT ... FROM TestModel WHERE ...) ...
print q.count()

# Fast: SELECT COUNT(*) FROM TestModel WHERE ...
print get_count(q)

Nice function! FYI it does not consider the query limit.

So i extended the function for my usage.

count = q.session.execute(count_q).scalar()
return min(q._limit, count) if q._limit else count

THIS METHOD DOES NOT APPLY TO QUERIES WITH LIMIT AND OFFSET

What if I want to know the total count? Normally in an API's pagination, (page, page_size, total_count) is needed.

Say

SELECT  count(*) AS total_count
FROM user
WHERE user.is_deleted = false
 LIMIT 20 OFFSET 5;

If you execute this in a DB, it'll return nothing, which is None in Python. In fact, the count_q cannot include limit and offset:

Although you can work around like this, it still isn't ideal due to overheads.

Solution

To avoid using limit and offset in the query.

Hi,
somebody an idea how that looks like in sqlalchemy 1.4?
I am getting:

sqlalchemy.exc.ArgumentError: Query has only expression-based entities - can't find property named "template". where template is something from my db I guess

⚠️ YOU DO NOT NEED THIS ⚠️

https://dba.stackexchange.com/questions/168022/performance-of-count-in-subquery

⚠️ IF YOUR QUERY IS SLOW, YOU SHOULD REMOVE ORDER BY FIRST ⚠️

count_stmt = select(func.count()).select_from(statement.order_by(None).subquery())
await session.scalar(count_stmt)

This worked perfectly for me after I upgraded from sa 1.3 to 1.4 and a count query went from 80ms to 800ms.

by switching form the built in .count() to the suggested first gist the query went back to 80ms. I believe the problem was .count() loading all columns into python which is not required and very slow for thousands or rows, using with_only_columns and removing the sub query took it back to 80ms. No idea what broke it in 1.4.

    count_q = q.statement.with_only_columns([func.count()]).order_by(None)
    count = q.session.execute(count_q).scalar()

I'm not sure why people are putting warnings on this gist or writing very long replies expecting help. If you are stuck ask on Stackoverflow!

My thanks to @hest

This worked perfectly for me after I upgraded from sa 1.3 to 1.4 and a count query went from 80ms to 800ms.

by switching form the built in .count() to the suggested first gist the query went back to 80ms. I believe the problem was .count() loading all columns into python which is not required and very slow for thousands or rows, using with_only_columns and removing the sub query took it back to 80ms. No idea what broke it in 1.4.

    count_q = q.statement.with_only_columns([func.count()]).order_by(None)
    count = q.session.execute(count_q).scalar()

I'm not sure why people are putting warnings on this gist or writing very long replies expecting help. If you are stuck ask on Stackoverflow!

My thanks to @hest

This thread is generally a kind of misinformation. If you check the stackoverflow link above, these sqls are exactly the same:

-- on postgres
EXPLAIN ANALYZE SELECT COUNT(*) FROM some_big_table WHERE some_col = 'some_val'

EXPLAIN ANALYZE SELECT COUNT(*) FROM ( SELECT col1, col2, col3, col4 FROM some_big_table WHERE some_col = 'some_val' )

If you find your query executing slow, the first is to try removing order by.

If you find your query executing slow, the first is to try removing order by.

I tried this, and the methods outlined in the stackoverflow link, and it did not work. Please don't say I'm providing misinformation by outlining what worked for me it is rude.

@davidjb99

Sorry for letting you misunderstand. I meant the gist (which was posted in 2015) is likely misinformation now, not your comment.

This is the method, I'm using

def get_count(self, model_fields, filter_clause):
        """ Note: filter_clause should not be 'None' or 'Null' for this method to work """
        query = self.session.query().with_entities(*model_fields)
        query = query.filter(filter_clause)
        count_query = query.statement \
                .with_only_columns([func.count()]) \
                .order_by(None)
        result = query.session.execute(count_query).scalar()
        return result