Enhance Postgresql query (many subqueries) - sql

Question

I have a postgrsql query which takes a lot of time to execute (5 min) because of sub queries I think. I would like to find a way to enhance this query:
select v.id, v.pos, v.time, v.status, vi.name,vi.type,
(select c.fullname
from company c
where vi.registered_owner_code = c.owcode ) AS registered_owner
,(select c.fullname
from company c
where vi.group_beneficial_owner_code=c.owcode) AS group_beneficial_owner
,(select c.fullname
from company c
where vi.operator_code = c.owcode ) AS operator
,(select c.fullname
from company c
where vi.manager_code = c.owcode ) AS manager
from (car_pos v left join cars vi on v.id = vi.id)
where age(now(), v.time::time with time zone) < '1 days'::interval

Answer 1

because of subqueries I think
This is not really a guessing game. You can get query execution plan explanation in pgadmin or just under console
http://www.pgadmin.org/docs/1.4/query.html
http://www.postgresql.org/docs/current/static/sql-explain.html
then you can see what's going on and what takes that much time.
After analysis you can add indexes or change something else but at least you will know what needs to be changed.

Answer 2

The WHERE condition can't use an index, you have to change that one. v.time should not be in a volatile function, age() in this case.

Answer 3

3 key ingredients:
Do away with the correlated subqueries, use JOIN instead - like
other answers mentioned already.
In the WHERE clause, don't use an expression on your column, which cannot utilize an index. #Frank already mentions it. Only the most basic, stable expressions can be rewritten by the query planner to use an index. See how I rewrote it.
Create suitable indexes.
SELECT v.id, v.pos, v.time, v.status, c.name, c.type
,r.fullname AS registered_owner
,g.fullname AS group_beneficial_owner
,o.fullname AS operator
,m.fullname AS manager
FROM car_pos v
LEFT JOIN cars c ON USING (id)
LEFT JOIN company r ON r.owcode = c.registered_owner_code
LEFT JOIN company g ON g.owcode = c.group_beneficial_owner_code
LEFT JOIN company o ON o.owcode = c.operator_code
LEFT JOIN company m ON m.owcode = c.manager_code
WHERE v.time > (now() - interval '1 day');
You need unique indexes on cars.id and company.owcode (primary keys do the job, too).
And you need an index on car_pos.time like:
CREATE INDEX car_pos_time_idx ON car_pos (time DESC);
Works without descending order, too. If you have lots of rows (-> big table, big index), you might want to create a partial index that covers only recent history and recreate it on a daily or weekly basis at off hours:
CREATE INDEX car_pos_time_idx ON car_pos (time DESC);
WHERE time > $mydate
Where $mydate is the result of (now() - interval '1 day'). This matches the condition of your query logically at any time. Effectiveness slowly deteriorates over time.
Aside: don't name a column of type timestamp "time", that's misleading from a documentation point of view. Actually, rather don't use time as column name at all. It's a reserved word in every SQL standard and a type name in PostgreSQL.

Answer 4

select v.id, v.pos, v.time, v.status, vi.name,vi.type,
c1.fullname as Registered_owner,
c2.fullname as group_beneficial_owner,
c3.fullname AS operator,
c4.fullname AS manager
from car_pos v
left outer join cars vi on v.id = vi.id
left outerjoin company c1 on vi.registered_owner_code=c1.owcode
left outerjoin company c2 on vi.group_beneficial_owner_code=c2.owcode
left outerjoin company c3 on vi.operator_code=c3.owcode
left outerjoin company c4 on vi.manager_code=c4.owcode
where age(now(), v.time::time with time zone) < '1 days'::interval

Answer 5

One trivial solution would be to convert it to joins
select v.id, v.pos, v.time, v.status, vi.name,vi.type,
reg_owner.fullname AS registered_owner,
gr_ben_owner.fullname AS group_beneficial_owner,
op.fullname AS operator,
man.fullname AS manager
from
car_pos v
left join cars vi on v.id = vi.id
left join company reg_owner on vi.registered_owner_code = reg_owner.owcode
left join company gr_ben_owner on vi.group_beneficial_owner_code = gr_ben_owner.owcode
left join company op on vi.operator_code = op.owcode
left join company man on vi.manager_code = man.owcode
where age(now(), v.time::time with time zone) < '1 days'::interval
I suspect however, that it might be possible by doing only one join of the table Company... I'm not 100% sure about the exact syntax to, and I have doubts that this will enhance performance (because of all the CASE-WHEN, GROUP by, etc) compared to the four time join solution, but I think this should work too. (I assumed, that cars-car_pos is a one-to-one relationship)
select v.id, MAX(v.pos) as pos, MAX(v.time) as vtime, MAX(v.status) as status, MAX(vi.name) as name,MAX(vi.type) as type,
MAX(CASE WHEN c.owcode = vi.registered_owner_code THEN c.fullname END) AS registered_owner,
MAX(CASE WHEN c.owcode = vi.group_beneficial_owner_code THEN c.fullname END) AS group_beneficial_owner,
MAX(CASE WHEN c.owcode = vi.operator_code THEN op.fullname END) AS operator,
MAX(CASE WHEN c.owcode = vi.manager_code THEN man.fullname END) AS manager
from
car_pos v
left join cars vi on v.id = vi.id
left join company c on c.owcode IN (vi.registered_owner_code, vi.group_beneficial_owner_code, vi.operator_code, vi.manager_code)
group by v.id
having age(now(), vtime::time with time zone) < '1 days'::interval
If you could put the table creation DDL scripts, and some inserts into the question, it would be easy to try in SQL fiddle...