Just about every mainstream DB has a feature to calculate checksums per page, per sector, or per record.

Now for a DB that does full recover after any crash, like PostgreSQL, is a checksum even useful? There will be no data loss as long as the xlog is ok, no matter what kind of corruption happened to the data itself, as the redo log is replayed every committed transaction will be restored. So checksums are useless on restore.

Doesn't the filesystem or disk keep checksums anyway to detect corruption? So unless the checksum is per record, all it does is tell you there is corruption - which the OS should be yelling at you the minute you try to read it - so useless in operation?

I can't imagine how a checksum can be helpful in any sane database - but since they all use them - I'd say that's just failure of imagination on my part. So how is it useful?