Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the following as you can (you can cut and paste this into your topic):
Nextcloud version (eg, 10.0.2): 12.0.2
Operating system and version (eg, Ubuntu 16.04): Ubuntu 16.04.3 LTS
Apache or nginx version (eg, Apache 2.4.25): 2.4.18
PHP version (eg, 5.6): 7.0.22
Is this the first time you’ve seen this error?:
Can you reliably replicate it? (If so, please outline steps):
Yes, the issue always comes back after a few hours of resetting mysql service
The issue you are facing:
After a few hours of the mysql service being running it starts to hug a lot of cpu time (it’s uses 100% of 1 thread) and will stay this way indefinitely until I restart the service which fixes the issue for a few hours and then it comes back.