AWS has a required SSL certificate update for it's RDS instances going out on the 5th. Even though I do not actually use the certificate I went ahead and ran the update so it was done and I wouldn't have any unexpected downtime. Only the SSL Cert should have been updated as I understand it.
Instead my CPU usage went from less than 10% while idle to over 80%. Now I've isolated the cause of this to a query we run every few seconds to retrieve a list of recent transactions. And with some tweaking the CPU usage has returned to normal levels.
But this query has been in place for a few years without issues and it's only after this SSL update that it's caused us any grief. My concern is there is some deeper issue behind the scenes and that changing the query is merely treating a symptom. Before revising the query, I ran all pending updates and rebooted the database with no changes. There was also one other person on the AWS Forums with the same issue but neither of us were able to get any useful responses. Thankfully the rest of the system seems to be behaving itself but I want to know what's going on.
In case it can help identify why a query would suddenly use far more resources here is a (simplified) version of the query prior to my tweak.
SELECT Distinct Top (@NumOfTrx) [trx].* ,[c].*
FROM [dbo].[TRX_Transactions] trx
Inner Join @ProdSelection s on ((trx.Code = s.ID and s.ID != 0) or (s.ID = 0 and (trx.TypeID = 1or StatusID = 4))
or (BatchId > 0 and s.ID in (select b.Code from [dbo].[TRX_Batch] b where b.BatchId = trx.BatchId)))
Join CLI_Details c on trx.UserName = c.UserName
where trx.TransactionDate > DATEADD(Day, -1, GETDATE()) and (trx.Amount >= @Size Or trx.TypeID = 1 or StatusID = 4)
And (@Company = 0 or c.Company = @Company) and (@Agent = '' or [Agent] = @Agent)
order by [trx].[TransactionDate] Desc
Removing the Prod selection join, a filter that is a list of ids that we can operate without for the time being, was what resolved the issue.