If you have a scenario where the visibility map incurs a measurable
overhead, let's hear it. I didn't see any in the tests I performed, but
it's certainly possible that if the circumstances are just right it
makes a difference.
Heikki,
The obvious one, i could observe is that it would increase the WAL contention. Am i missing something? All i am suggesting is to reduce the unnecessary work required in those tables, where the visibility map is not required. For example, in data warehouses, people might even have a tables without any indexes. Why do we ask them to incur the overhead of visibility map?
Also since you have made the visibility maps without any page level locking, have you considered whether it would make sure the correct order of inserts into the WAL? i have looked at some random threads, but i couldn't get the complete design of visibility map to be used for index only scans.
Thanks,
Gokul.