Greg Stark <gsstark@mit.edu> writes:
> You pay in Oracle when you read these records too. If there are pending
> updates you have to do a second read to the rollback segment to get the old
> record. This hits long-running batch queries especially hard since by the time
> they finish a large number of the records they're reading could have been
> updated and require a second read to the rollback segments.
If not third or fourth read, by the time you've found the version you're
supposed to be able to see.
I recall discussing this several years ago with somebody who knew quite
a bit about Oracle innards (though he didn't say how he knew...)
According to him, heavy accesses to the rollback segments have another
problem, which is contention for ownership of locks protecting access
to the rollback segments. I got the impression that it would be like
us needing to take the WALWriteLock anytime we wanted to look at any
not-the-very-latest row version --- there's plenty of write traffic
that needs that lock, and you don't want to load it down with read
traffic too.
regards, tom lane