MySQL LEFT JOIN only last row painfully slow - mysql

Question

I have three tables, which looks like this
forts
|id|lat|lon|
fort_sightings
|id|fort_id|team|
fort_raids
|id|fort_id|raid_level|
I need a query that fetches all the rows from forts and then select the latest information from fort_sightings and fort_raids, if any. There might be several rows where fort_id has the same value, so I need the latest information.
Currently, I have this, which might not be the prettiest
SELECT
*
FROM
forts c
LEFT JOIN fort_sightings o ON o.id = (
SELECT
id
FROM
fort_sightings
WHERE
fort_id = c.id
ORDER BY
id DESC
LIMIT 1
)
LEFT JOIN fort_raids r ON r.id = (
SELECT
id
FROM
fort_raids
WHERE
fort_id = c.id
ORDER BY
id DESC
LIMIT 1
)
But it's painfully slow, the query takes over 10 seconds. There's only ~350 rows in forts, so it really shouldn't take this long. I believe it's from all the SELECT queries in the JOIN, but I don't know any alternative.
EXPLAIN

Answer 1

This is your query:
SELECT *
FROM forts c LEFT JOIN
fort_sightings o
ON o.id = (SELECT fs2.id
FROM fort_sightings fs2
WHERE fs.fort_id = c.id
ORDER BY fs2.id DESC
LIMIT 1
) LEFT JOIN
fort_raids r
ON r.id = (SELECT fr2.id
FROM fort_raids fr2
WHERE fr2.fort_id = c.id
ORDER BY fr2.id DESC
LIMIT 1
);
I find it strangely structured. But, see if this works:
fort_sightings(fort_id, id)
fort_raids(fort_id, id)
It is important that the fort_id be the first key in the indexes.
If this doesn't work, then you might need to modify the query.

Answer 2

Aside from the index recommendation from Gordon, you are doing a correlated subquery which means that for every record in the forts table, you are re-running the query to the sightings and raids. I would change slightly to pull all max() per fort from each THEN join... like
SELECT
c.id,
c.lat,
c.lon,
fs2.team,
fr2.raid_level
FROM
forts c
LEFT JOIN
( select fs.fort_id, max( fs.id ) as MaxSightID
from fort_sightings fs
group by fs.fort_id ) S
on c.id = s.fort_id
LEFT JOIN fort_sightings fs2
on s.MaxSightID = fs2.id
LEFT JOIN
( select fr.fort_id, max( fr.id ) as MaxRaidID
from fort_raids fs
group by fr.fort_id ) R
on c.ID = r.fort_id
LEFT JOIN fort_raids fr2
on r.MaxRaidID = fr2.id
The second part of the left-joins goes back to the original raid and sighting tables to pull the corresponding team and raid level in the final results if any such are found.

Answer 3

If you need the most efficient query, you should add columns latest_fort_sighting_id latest_fort_raid_id to table forts. MySQL does not have powerful features such as Materialized Views or Hash Joins like PostgreSQL, we need to handle them manually. Do not forget using transaction for updates.
If you limit range of forts, alternatively, you can run optimized query only using LEFT JOIN.
select - SQL join: selecting the last records in a one-to-many relationship - Stack Overflow
SELECT forts.*, fs1.team, fr1.raid_level FROM forts
LEFT JOIN fort_sightings fs1 ON fs1.fort_id = forts.id
LEFT JOIN fort_sightings fs2 ON fs2.fort_id = forts.id AND fs1.id < fs2.id
LEFT JOIN fort_raids fr1 ON fr1.fort_id = forts.id
LEFT JOIN fort_raids fr2 ON fr2.fort_id = forts.id AND fr1.id < fr2.id
WHERE fs2.id IS NULL AND fr2.id IS NULL AND forts.id > 5 ORDER BY forts.id LIMIT 5;