MySQL: How can this UNION be improved? - mysql

Question

I have two tables, a for access and p for provider. I then have third table for joining the tables together, standard normalization. However, the provider table is a parent/child table, and the joining table has an option whether the access should be granted for all the provider children or not.
CREATE TABLE p (
p_id int PRIMARY KEY,
name varchar(32),
parent_id int,
FOREIGN KEY (parent_id) REFERENCES p(p_id)
);
CREATE TABLE a (
a_id int PRIMARY KEY,
name varchar(32)
);
CREATE TABLE ap (
a_id int,
p_id int,
sub tinyint,
FOREIGN KEY (a_id) REFERENCES a(a_id),
FOREIGN KEY (p_id) REFERENCES p(p_id)
);
Some sample data, 1 provider with 2 child providers. 2 access users, 1 with no child access and with child access.
INSERT INTO p VALUES(1, 'a', null);
INSERT INTO p VALUES(2, 'a.a', 1);
INSERT INTO p VALUES(3, 'a.b', 1);
INSERT INTO a VALUES(1, 'user 1');
INSERT INTO a VALUES(2, 'user 2');
INSERT INTO ap VALUES(1, 1, 0);
INSERT INTO ap VALUES(2, 1, 1);
The result is that I want to have a list of providers that the user have access to, based on the third table.
Currently I've solved this by joining two queries with UNION. The first query selects possible child providers and the second goes for primary providers.
SELECT p_id, name
FROM p
WHERE parent_id IN(
SELECT p_id FROM ap WHERE a_id = 1 AND sub = 1
)
UNION
SELECT ap.p_id, p.name
FROM ap
LEFT JOIN p ON p.p_id = ap.p_id
WHERE a_id = 1;
I don't like this query, it's ugly and there must be a smarter way :)

Answer 1

If I have the logic correct, then you want all records from p where one of the following is true:
p_id matches a record in ap for the given a_id.
parent_id matches a record in ap for a given a_id.
This suggests using exists for the conditions:
select p.p_id, p.name
from p
where exists (select 1
from ap
where ap.p_id = p.p_id and ap.a_id = 1
) or
exists (select 1
from ap
where ap.p_id = p.parent_id and ap.sub = 1 and ap.a_id = 1
)
With a composite index on ap(p_id, a_id, sub), this should have much better performance than your version of the query.