Update another table through a trigger where new value is the result of a SELECT query - sql

Question

I have these tables:
Users
Skills (name - string, count - integer)
Has_skills (skill_id - skills.id, user_id users.id)
Has_skills is a many to many table between the first two through these FK:
user_id (users.id) and skill_id (skills.id).
What I want to do is update the count column inside skills when a new row is inserted into has_skills. I want to do this through an update trigger on table has_skills. The new value for count I will get through a select query:
SELECT COUNT(*) AS cnt FROM skills
JOIN has_skills hs ON skills.id = hs.skill_id
WHERE hs.skill_id = 1;
The ID above is hardcoded (1), but it works.
I also tested this code in isolation, and it works (although hardcoded, as well):
UPDATE skills
SET count = subquery.cnt
FROM (
SELECT COUNT(*) AS cnt FROM skills
JOIN has_skills hs ON skills.id = hs.skill_id
WHERE hs.skill_id = 1
) AS subquery
WHERE skills.id = 1;
RETURN NEW;
Alright, so here's probably where the problem is. Below is the trigger function and also the trigger itself.
Function:
CREATE OR REPLACE FUNCTION update_skill_count() RETURNS trigger AS
$func$
BEGIN
UPDATE skills
SET count = subquery.cnt
FROM (
SELECT COUNT(*) AS cnt FROM skills
JOIN has_skills hs ON skills.id = hs.skill_id
WHERE hs.skill_id = NEW.skill_id
) AS subquery
WHERE skills.id = NEW.skill_id;
RETURN NEW;
END;
$func$ LANGUAGE plpgsql;
Trigger:
CREATE TRIGGER on_has_skills_insert
AFTER INSERT ON has_skills
FOR EACH ROW
EXECUTE PROCEDURE update_skill_count();
I successfully create the function and trigger, but when I insert new data into has_skills, it doesn't change the count column inside skills. What could be the problem?

Answer 1

There's no need for a select in the trigger function at all. The key for the skill table is directly available in new.skill_id so just use it directly:
-- trigger function and trigger
create or replace function update_skill_count()
returns trigger
as $func$
begin
update skills sk
set count = count+1
where sk.skill_id = new.skill_id;
return new;
end;
$func$ language plpgsql;
create trigger on_has_skills_insert
after insert on has_skills
for each row
execute procedure update_skill_count();

Answer 2

I'm not much familiar with postgresql, but having understanding of Oracle and SQL Server, this looks to be a mutating trigger problem, which is: Trying to read from or write into the same table within a row level trigger on which the trigger is placed.
One of the ways to get rid of mutating trigger/table problem can be changing the row level trigger to a statement level trigger and changing the function accordingly. Here is a psudo code you can try out (not providing the exact tested code as I do not have Postgresql installed):
Function:
CREATE OR REPLACE FUNCTION update_skill_count() RETURNS trigger AS
$func$
BEGIN
UPDATE skills
SET count = subquery.cnt
FROM (
SELECT hs.skill_id, COUNT(*) AS cnt
FROM new_table hs
GROUP BY hs.skill_id
) AS subquery
WHERE skills.id = subquery.skill_id;
RETURN NULL;
END;
$func$ LANGUAGE plpgsql;
Trigger:
CREATE TRIGGER on_has_skills_insert
AFTER INSERT ON has_skills
REFERENCING NEW TABLE AS new_table
FOR EACH STATEMENT EXECUTE PROCEDURE update_skill_count();