I’ve been trying to tackle this problem myself for weeks now and I finally found what caused it for me. It was the woocommerce plugin, so you might want to try and disable it to see if this also is the case for you.
I’ve already opened a support ticket with woocommerce:
https://wordpress.org/support/topic/major-database-issues-server-chrash/
Edit:
I must give woocommerce a little more credit, as (in my case) it’s the combination of woocommerce with a server setup (apache & mysql) that’s totally outdated.
Furthermore, the notice seems to be a know issue that hasn’t been resolved yet, see: https://github.com/woocommerce/woocommerce/issues/21756