Absolutely. For a synch standby, you can't tolerate any standby delay
at all. This means that anywhere from 1/4 to 3/4 of queries on the
standby would be cancelled on any high-traffic OLTP server. Hence,
"useless".
Don't agree with your numbers there and you seem to be assuming no
workarounds would be in use. A different discussion, I think.
The only viable workaround would be to delay vacuum on the master, no?
Adding the feedback channel looks trivial to me, once we've got the main
sync rep patch in. I'll handle that.
OK. I thought it would be a major challenge. Ideally, we'd have some
way to turn snapshot publication on or off; for a standby which was not
receiving reads, we wouldn't need it. Maybe make it contingent on the
status of hot_standby GUC? That would make sense.
For this reason, I've removed the "apply" mode from my patch, for now. I
want to get the simplest possible patch agreed and then add things
later.
Um, ok. "apply" mode is still useful for users who are not running
queries against the standby. Which many won't.
--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com
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