We are seeing an issue on RDS 9.4.5 where replica lag gets really high.
This lag seems to be correlated to a saturation of disk read throughput,
and vacuums of one of our larger and more frequently updated tables.
I've heard that there's a bug where the replica will scan indexes
unnecessarily when doing the WAL replay when the vacuums happen. Can
anyone confirm this and maybe point me towards the commit where it was
fixed? Has there been a backport to the 9.4 series?
Any other ideas for investigation or mitigation?
