Mysql only keep the highest value per id per value - mysql

Question

I have a database that looks like this:
post metrics minutes(There is only data for post id 1 in this example)
| post id | date updated local | reach |
|1 | 2018-01-01 01:00:00 | 10 |
|1 | 2018-01-01 01:05:00 | 20 |
|1 | 2018-01-01 01:15:00 | 22 |
|1 | 2018-01-01 16:05:00 | 100 |
|1 | 2018-01-02 03:00:00 | 121 |
|1 | 2018-01-02 21:00:00 | 140 |
|1 | 2018-01-04 01:00:00 | 147 |
My system is designed to fetch data for all posts every 5 minutes and put the results in the above table if the reach is not the same as the last time it was stored for that post (this to prevent getting a shitload of data that is exactly the same).
Now there are thousands of posts and the table start to grow out of control making my website lot slower when loading the data from this table.
So I decided that I can decrease the data by only keeping the last row per post per day, so I want to delete all rows that are not the max date updated local for that post. The result would be:
| post id | date updated local | reach |
|1 | 2018-01-01 16:05:00 | 100 |
|1 | 2018-01-02 21:00:00 | 140 |
|1 | 2018-01-04 01:00:00 | 147 |
I came up with:
DELETE FROM `post metrics minutes`
WHERE EXISTS (
SELECT *
FROM `post metrics minutes` pmmtemp
WHERE pmmtemp.`post id` = `post metrics minutes`.`post id`
AND pmmtemp.`date updated local` > `post metrics minutes`.`date updated local`
AND DATE(pmmtemp.`date updated local`) = DATE(`post metrics minutes`.`date updated local`)
);
But this gives me the following error:
Error Code: 1093. Table 'post metrics minutes' is specified twice, both as a target for 'DELETE' and as a separate source for data
Hope anyone can help me out!

Answer 1

One cannot delete or update on the same table as a subquery one.
One could create a temp table of post_ids to delete.
But marking the records first does it too. This way both queries do not interfere with each other.
For the nested table, instead of FROM tablename I do FROM (SELECT * FROM tablename) for the temp table.
Here I abused the column reach.
UPDATE `post metrics minutes` p
SET p.reach = -1
WHERE EXISTS (
SELECT *
FROM (SELECT * FROM `post metrics minutes`) pmmtemp
WHERE pmmtemp.`post id` = p.`post id`
AND pmmtemp.`date updated local` > p.`date updated local`
AND DATE(pmmtemp.`date updated local`) = DATE(p.`date updated local`)
);
DELETE FROM `post metrics minutes`
WHERE reach = -1;

Answer 2

As per my comment, it's often quicker to create a new table with the desired dates, then delete the old table and replace it with the newer one.
My column/table names may be very slightly different from yours, but something like...
CREATE TABLE my_new_table AS
SELECT x.*
FROM my_old_table x
JOIN
( SELECT post_id,MAX(dt) dt FROM my_old_table GROUP BY post_id,DATE(dt)) y ON y.post_id = x.post_id
AND y.dt = x.dt;