Added support for Pg's hot_standby_feedback option

Merged Yorick Peterse requested to merge hot-standby-feedback into master

See for info about the option.

Enabling this cuts down the number of query conflicts on secondaries to 0:


Here the option was manually enabling a bit before 17:00. After enabling this I did not notice any change in table bloat. For the sake of safety the option is still disabled by default

@marin I added the CHANGELOG entry to 9.0.1. Is this something we're OK with releasing in a patch release, or do we want to wait for 9.1 (or somehow ninja it into 9.0)?