Optimizing a Select with Subquery that is loading VERY slow - mysql

Question

I have a SELECT that is a little bit tricky, as I try to display data that has to be calculated on the fly.
The data is logged from a SmartHome system and displayed in the visualization solution Grafana.
So I have to handle all of this in MySQL and can't really edit the data or the frontend to do some of this work.
The diagram should show the average temperature per day for a time range that can be selected in the UI.
The data in MySQL is a table like that:
DEVICE | READING | VALUE | TIMESTAMP
-----------------------------------------------------------------------------
Thermometer | temperature | 20.0 | 2107.10.12 00:12:59
Thermometer | temperature | 20.2 | 2107.10.12 00:24:12
...
The Request first creates a virtual table (that is not in the database) with timestamps for every full hours for about 10 years.
This is running very quick and doesn't seem to be a reason for my slow fetches
After that I strip down the virtual table to values only within the visible time range in my diagram.
On all of these full-hour-timestamps I have to run a sub-select to get the last temperature value that was logged before the full hour.
This values are then grouped by day and the average is calculated.
That way I get the average over 24 values for each full hour from 00:00 to 23:00.
Based on different wether sites, this is how the official average temperature is normally calculated.
Here is the Select Statement:
SELECT
filtered.hour as time,
AVG((SELECT VALUE
FROM history
WHERE READING="temperature" AND DEVICE="Thermometer" AND TIMESTAMP <= filtered.hour
ORDER BY TIMESTAMP DESC
LIMIT 1
)) as value
FROM (
SELECT calculated.hour as hour FROM (
SELECT DATE_ADD(DATE_SUB(DATE($__timeTo()), INTERVAL 10 YEAR), INTERVAL t4.i*10000 + t3.i*1000 + t2.i*100 + t1.i*10 + t0.i HOUR) as hour
FROM (SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
(SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
(SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
(SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
(SELECT 0 as i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t4
) calculated
WHERE calculated.hour >= $__timeFrom() AND calculated.hour <= $__timeTo()
) filtered
GROUP BY DATE(filtered.hour)
For a timespan of a week it already takes about 5-10 seconds for the diagram to show up. For a month you're close to half a minute.
All my other (simple fetches without calculations) diagrams are loading in about or less than a second.
As I'm a completely MySQL noob and just started to build some SELECTs for my smart home, I don't really know how this can be improved.
Any ideas from the pros? :)

Answer 1

Unless I'm overseeing something really obvious and it doesn't really matter on how many results the average per day is calculated you could really simplify your query and get rid of the subquery's. This should also give you a boost in speed.
SELECT DATE(`TIMESTAMP`) AS `date`, AVG(`VALUE`) AS `value` FROM `history` WHERE `READING`='temperature' AND `DEVICE`='Thermometer' AND DATE(`TIMESTAMP`) BETWEEN 'date1' AND 'date2'
Just replace date1 & date2 with the values you want, for example 2017-10-15.