Fast query slows down when within a subquery - mysql

Question

I have this query:
SELECT timestmp
FROM devicelog
WHERE device_id = 5
ORDER BY id DESC LIMIT 1
which takes less than 0.001 seconds to fetch, but once I put it in a subquery, it slows down to about 3.05 seconds. Any reason to why it does this, or how I can remedy it?
Here is the second query (which is the one I want to optimize):
SELECT device.id,
(SELECT timestmp
FROM devicelog
WHERE device_id = device.id
ORDER BY id DESC LIMIT 1) as timestmp
FROM device
Table "device" only has like 10-15 records in it (devicelog has several million), so I would assume it goes 1 by 1 through each record and then executes the subquery, but obviously it's doing something else. The PK of devicelog is the id, and the PK in device is its id as well. There is an index on devicelog for timestmp (which is a datetime) and device_id which is also a FK back to devicelog. There are other indices as well, but they are irrelevant (things like names, descriptions, etc).
I just need it to loop through devices, then display the last timestamp record.
If I list each device in PHP, then perform the first query separately, it will be perform extraordinary well, but I want to do this in one entire query. Like, I could do something like (pseudocode):
foreach($row in <devicelog>)
query('<first query> where id = $row[id]')
Doing an entire join would be too expensive on devicelog just because the high count.

Answer 1

Your question and query do not match what you are looking for per the comment to the first answer offered.
What you can do is a pre-aggregate on a per-device basis to get the max ID log, then join that to your master list of devices...
SELECT
d.name,
d.id,
DeviceMax.lastTime
from
device d
LEFT JOIN ( select dl.device_id,
max( dl.timestamp ) lastTime
from
devicelog dl
group by
dl.device_id ) as DeviceMax
ON d.id = DeviceMax.device_id
Now, if you needed other stuff from the device log for that specific entry, we could just add on to that...
LEFT JOIN devicelog dl2
on DeviceMax.Device_id = dl2.Device_id
AND DeviceMax.lastTime = dl2.timestamp
then you can get any other columns from the "dl2" alias added to your query.
Also, for your device log table, I would have a covering index on (device_id, timestamp)
COMMENT FROM FEEDBACK
Then I would offer this as a suggestion for you, which is something also common in the world of web development when someone needs "highest count", or "most" of something, or "most recent" etc.
Denormalize your Device table with only one respect... add a column for the LastDeviceLogID. Then, whenever your DeviceLog has an entry added to it, you just use an after insert trigger that does...
update Device
set LastDeviceLogID = newRecord.DeviceLogID
where ID = newRecord.Device_ID
Columns may not be exact, but the principle is there. This way, you never need to do a LIMIT 1, MAX(), etc and go through your millions of records, you can get as simply as doing
SELECT
d.name,
d.id,
dl.timestamp,
dl.othercolumns
from
device d
LEFT JOIN devicelog dl
on d.LastDeviceLogID = dl.DeviceLogID

Answer 2

try this with inner join
SELECT d.id, dl.timestamp
FROM device d
INNER JOIN devicelog dl
ON dl.device_id = d.id
ORDER BY d.id DESC LIMIT 1
to list all devices you may consider the GROUP BY
SELECT d.id, dl.timestamp
FROM device d
INNER JOIN devicelog dl
ON dl.device_id = d.id
GROUP BY d.id
ORDER BY d.id DESC
Edit:
if you have many indices , thn better to index your column id
try this
ALTER TABLE `device` ADD INDEX `id` (`id`)