How to update a value using string_agg without using a function? - postgresql

Question

I solved this problem with a function and I like my solution, but I want to know if there is a way to solve this problem without using functions. Here is the thing:
There are four tables that are relevant to this:
entities: entities of the system (tenants)
members: members of an entity
member_sets: sets of members
members_and_sets: table to join members and sets (many to many)
The member_sets table has a column named bits which is a binary representation of the set, so, for example, if an entity has 5 members and one specific set has the third member, the value of the bits column is 00100, the entity has three special kinds of sets: universe, empty and unit, their binary repesentation is: 11111, 00000 and 10000 respectively, assuming the unit set has the first member.
The challenge is keep this binary representation of the set up to date; Whenever one member is added to the entity, all binary representations must be updated. This is easy to do with a trigger and a function, my solution is this:
CREATE FUNCTION setbits(INTEGER) RETURNS VARBIT AS
$$SELECT STRING_AGG(belongs, '')::VARBIT AS setbits FROM (
SELECT LEAST(COALESCE(members_and_sets.set_id, 0), 1)::text AS belongs
FROM members LEFT JOIN members_and_sets
ON members.id=members_and_sets.member_id
AND members_and_sets.set_id=$1
GROUP BY members.id,members_and_sets.set_id
ORDER BY members.id)
AS bitsring;$$
LANGUAGE SQL
RETURNS NULL ON NULL INPUT;
-- calling this function in a trigger after inserting a new member:
UPDATE member_sets ms
SET bits=setbits(ms.id)
WHERE ms.entity_id=NEW.entity_id;
Now my question is: Can I do this without using a function? I tried with CTE but apparently I'm to noob to accomplish this; I couldn't pass the set_id to the must inner query so my solution was to wrap the query in a function and pass the set_id as an argument to the function. Again, this solution works perfectly, I just want to know if there is no way I can do this without a function call.

Answer 1

As your function body is simply a SELECT, you should be able to replace the function call with a subquery:
UPDATE member_sets ms
SET bits= (
SELECT STRING_AGG(belongs, '')::VARBIT AS setbits FROM (
SELECT LEAST(COALESCE(members_and_sets.set_id, 0), 1)::text AS belongs
FROM members LEFT JOIN members_and_sets
ON members.id=members_and_sets.member_id
AND members_and_sets.set_id=ms.id
GROUP BY members.id,members_and_sets.set_id
ORDER BY members.id)
AS bitsring
)
WHERE ms.entity_id=NEW.entity_id;