Re: Allow "snapshot too old" error, to prevent bloat

>> But max_standby_streaming_delay, max_standby_archive_delay and
>> hot_standby_feedback are among the most frequent triggers for
>> questions and complaints that I/we see.
>>
> Agreed.
> And a really bad one used to be vacuum_defer_cleanup_age, because
> of confusing units amongst other things. Which in terms seems
> fairly close to Kevins suggestions, unfortunately.

Particularly my initial suggestion, which was to base snapshot
"age" it on the number of transaction IDs assigned.  Does this look
any better to you if it is something that can be set to '20min' or
'1h'?  Just to restate, that would not automatically cancel the
snapshots past that age; it would allow vacuum of any tuples which
became "dead" that long ago, and would cause a "snapshot too old"
message for any read of a page modified more than that long ago
using a snapshot which was older than that.