in addition, we never see in logs an error about index damage,
As I said, it's not going to be corruption. (and one does not depend on the error log anyway to identify corruption in a database)
we will try scheduling statics,
btw, how often is recomended statics update?
You mean 'statistics updates'? Often enough that the queries using the statistics get accurate enough information. Yup, that's about all that can be said.
If you can afford to update stats daily, then do so.
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: Discussions on DB performance with occasional diversions into recoverability
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