Occasional MySql issue, hits 100% CPU

[LukeD]

We run a fairly large network of XF sites, running on AWS against Aurora MySql (our 100+ XF sites are partitioned across several writer/read replica clusters). It runs well for quite awhile (weeks, sometimes months) but with no obvious trigger the DB writer in a cluster pegs 100% CPU and requests start backing up and all sites partitioned to that DB cluster obviously stop responding with 5xx errors and timeouts.

Normally even under a decent load our writer CPU doesn't get past 30-40%...

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