>>>>> "Andrey" == Andrey Zhidenkov <andrey.zhidenkov@xxxxxxxxx> writes: Andrey> We have a few database clusters (1 master, 2 hot standbys) with Andrey> hot_standby_feedback setting on. But I am constantly seeing Andrey> query conflicts on standbys because dead rows on the masters Andrey> are deleted by VACUUM. Usually I notice it shortly after VACUUM Andrey> ends, because the replication lag on replicas is starting to Andrey> increase. Feedback can only try and avoid one of the approximately five possible causes of conflicts (albeit the most common one). Next most likely is a pin conflict, especially if you have any tables involved which are both small and with a relatively high update frequency. (Unfortunately with a max delay of 0 the standby queries won't be getting cancelled which in turn means that there are no statistics about the causes of conflicts.) The most likely scenario for a pin conflict is if you have queries which are (a) long-running, and (b) contain a sequential scan of a small (one or a few blocks) table that is _not_ underneath a Hash node or similar; for example, if the Seq Scan appears as the outer path of a Nestloop join at or near the top of the plan. In this case, vacuum of the small table may block because the query is holding pin for an extended period of time on the block that vacuum wants to clean up. (It can happen with index scans too, but is less likely.) -- Andrew (irc:RhodiumToad)
