Precise type for max_ aggregate function

[KAction]

Say we have following table:

create table foo (id bigserial primary key, value bigint NOT NULL, name text NOT NULL);

In this query, max is nullable because table may not have specific name, and application must handle it:

select max(value) from foo where name = 'banana'

but in this query, return value of max is never null:

select name, max(value) from foo group by name

Beam seems to always assume that result of max aggregate is nullable with following type:

max_ :: QExpr be s a -> QAgg be s (Maybe a)

Is there any way to express this distinction and get precise type, or I have no choice but to use fromJust ?

[LaurentRDC]

My understanding was that max_ must return a Maybe in case there isn't any data in the aggregation?

As per the Postgres documentation, for example:

It should be noted that except for count, [aggregate functions] return a null value when no rows are selected

[tathougies]

In the SQL standard, I believe all aggregates can return a null in certain situations (except for COUNT). The correct way (mentioned in the beam docs as well - https://haskell-beam.github.io/beam/user-guide/queries/aggregates/ -- see the note on returning null and use of coalesce_) is to use coalesce. One of the advantages / disadvantages of the type checker is that your queries must be NULL-correct!