Non-Unique Index unsuable

RHEL 7, 19c

Hi Gurus, One of the non-unique index went into unusable state causing application errors during transactions. I know that it can't just go into unusable state unless some table maintenance operation is done or explicitly setting index to unusable.

I checked into the audit trails and found no operation on the specific table was done. The default audit policies are enabled and doesn't show any alter for either table or index.

App team needs to know what could have caused the index to go unusable. I already mentioned the above reasons but I can't prove that any such opertaion has been performed.