what command can i use to kill these sql quries wich make a big server load
after a big number of visitors who visit my site at the same time (near 800 people)
i am taking hundrends of these queries
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mysql> SELECT post_id FROM wp_postmeta WHERE meta_key = ‘_wp_attached_file’ AND meta_value = ‘medium’ limit 10; Empty set (40.03 sec)
these make my site loads very slowly, even now the number of visitors are very very low (near 300)
i have a powerfull vps with 8 cores and 16 gbyte ram.i use MySQL Version: 5.7.25-0ubuntu0.16.04.2 nad Ubuntu operating system
I repeat the server was working VERY good the problem created nad remains after the big number of visitors
i have the last version of wordpress and i am using the basic plugins

after a big number of visitors who visit my site at the same time (near 800 people)
i am taking hundrends of these queries
mysql> SELECT post_id FROM wp_postmeta WHERE meta_key = ‘_wp_attached_file’ AND meta_value = ‘medium’ limit 10; Empty set (40.03 sec)
these make my site loads very slowly, even now the number of visitors are very very low (near 300)
i have a powerfull vps with 8 cores and 16 gbyte ram.i use MySQL Version: 5.7.25-0ubuntu0.16.04.2 nad Ubuntu operating system
I repeat the server was working VERY good the problem created nad remains after the big number of visitors
i have the last version of wordpress and i am using the basic plugins
Killing the query is the wrong answer and starting with doing this could cause application and database problems that might be unrecoverable. You need to find out why it is taking 40 seconds for the query to run. It is more than likely due to slow disks, unoptimized queries or unindexed queries. If this is supposed to be long running it needs to be run on another server or may need to setup multiple separate database servers to handle your queries.

How large is your database, are you able to store all of the data in RAM to increase query performance so it does not have to hit the disk?

Check your MySQL logs to figure out what your slow queries are for the first step of performance running and check to see if there are other queries running at the same time. You should also look into implementing caching (redis, memcached, etc.) so you can store the results of the queries in memory.

You need to optimize it properly and not just killing it.
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Agreed with all, killing it is not the solution, you need to find the reason and fix it.
Also you can consider using mariadb as server instead of mysql as it perform faster. (make sure you have a backup before doing the server change.)
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[FONT="]after a big number of visitors who visit my site at the same time (near 800 people)[/FONT]
[FONT="]i am taking hundrends of these queries[/FONT]
[FONT="]mysql> SELECT post_id FROM wp_postmeta WHERE meta_key = ‘_wp_attached_file’ AND meta_value = ‘medium’ limit 10; Empty set (40.03 sec)[/FONT]
[FONT="]these make my site loads very slowly, even now the number of visitors are very very low (near 300)[/FONT]
[FONT="]i have a powerfull vps with 8 cores and 16 gbyte ram.i use MySQL Version: 5.7.25-0ubuntu0.16.04.2 nad Ubuntu operating system[/FONT]
[FONT="]I repeat the server was working VERY good the problem created nad remains after the big number of visitors[/FONT]
i have the last version of wordpress and i am using the basic plugins
Like others have said, don't kill the queries. It can corrupt the tables/DB. If you need a temp band-aid use a SQL proxy to either cache these queries or filter them out. That'll buy you some time.

Cheers
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I'd say to implement OPCache, and perhaps convert the wp_postmeta table to InnoDB (ALTER TABLE wp_postmeta ENGINE=InnoDB and re-test after 24 hours.

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