JOOQ: multisetAgg or toSet filtering out NULL - jooq

Question

Quite often, the new feature of multisetAgg is used along with LEFT JOINs.
Let's say, I have a user as dimension table and fact table paid_subscriptions. I want to query a specific user with all of his paid subscriptions and for each subscription do some processing (like sending an email or whatever).
I would write some JOOQ like this:
ctx
.select(row(
USER.ID,
USER.USERNAME,
multisetAgg(PAIDSUBSCRIPTIONS.SUBNAME).as("subscr").convertFrom(r -> r.intoSet(Record1::value1))
).mapping(MyUserWithSubscriptionPOJO::new)
)
.from(USER)
.leftJoin(PAIDSUBSCRIPTIONS).onKey()
.where(someCondition)
.groupBy(USER)
.fetch(Record1::value1));
The problem here is: the multisetAgg produces a Set which can contain null as element.
I either heve to filter out the null subscriptions I don't care about after JOOQ select, or I have to rewrite my query with something like this:
multisetAgg(PAIDSUBSCRIPTIONS.SUBNAME).as("subscr").convertFrom(r -> {
final Set<String> res = r.intoSet(Record1::value1);
res.remove(null); // remove possible nulls
return res;
})
Both don't look too nice in code.
I wonder if there is a better approach to write this with less code or even an automatic filtering of null values or some other kind of syntactic sugar avilable in JOOQ? After all, I think it is quite a common usecase especially considering that often enough, I end up with some java8 style stream processing of my left joined collection and first step is to filter out null which is something I forget often :)

Answer 1

You're asking for a few things here:
SET instead of MULTISET (will be addressed with #12033)
Adding NULL filtering (is already possible with FILTER)
The implied idea that such NULL values could be removed automatically (might be addressed with #13776)
SET instead of MULTISET
The SQL standard has some notions of a SET as opposed to MULTISET or ARRAY. For example:
#13795
It isn't as powerful as MULTISET, and it doesn't have to be, because usually, just by adding DISTINCT you can turn any MULTISET into a SET. Nevertheless, Informix (possibly the most powerful ORDBMS) does have SET data types and constructors:
LIST (ARRAY)
MULTISET
SET
So, we might add support for this in the future, perhaps. I'm not sure yet of its utility, as opposed to using DISTINCT with MULTISET (already possible) or MULTISET_AGG (possible soon):
#12033
Adding NULL filtering
You already have the FILTER clause to do this directly in SQL. It's a SQL standard and supported by jOOQ natively, or via CASE emulations. A native SQL example, as supported by e.g. PostgreSQL:
select
t.a,
json_agg(u.c),
json_agg(u.c) filter (where u.b is not null)
from (values (1), (2)) t (a)
left join (values (2, 'a'),(2, 'b'),(3, 'c'),(3, 'd')) u (b, c) on t.a = u.b
group by t.a
Producing:
|a |json_agg |json_agg |
|---|----------|----------|
|1 |[null] | |
|2 |["a", "b"]|["a", "b"]|
So, just write:
multisetAgg(PAIDSUBSCRIPTIONS.SUBNAME).filter(PAIDSUBSCRIPTIONS.ID.isNotNull())
The implied idea that such NULL values could be removed automatically
Note, I understand that you'd probably like this to be done automatically. There's a thorough discussion on that subject here: #13776. As always, it's a desirable thing that is far from easy to implement consistently.
I'm positive that this will be done eventually, but it's a very big change.