On Wed, Sep 23, 2015 at 12:26 PM, Peter Eisentraut <pete...@gmx.net> wrote:
> On 9/23/15 10:44 AM, Robert Haas wrote:
>> On Tue, Sep 22, 2015 at 3:35 PM, Peter Eisentraut <pete...@gmx.net> wrote:
>>> On 9/16/15 5:52 PM, Simon Riggs wrote:
>>>> IMHO the default is the best one at the current time.
>>>> See recovery_min_apply_delay.
>>> The applications of recovery_min_apply_delay are likely to be varied and
>>> specific, so there might not be a general answer to this, but wouldn't
>>> you want hot_standby_feedback on with it?  Because the longer you wait
>>> on the standby, the more likely it is that the primary will clean stuff
>>> away.
>> If min_recovery_apply_delay was set to 1 hour, and if the standby had
>> hot standby feedback turned on, wouldn't that mean that the master had
>> to not do any HOT pruning or vacuuming of tuples until they had been
>> dead for at least an hour?  That seems like it would be bad.
> I suppose that's what would happen, and it might be bad, but the
> alternative is that the primary might vacuum away everything and you
> won't be able to make much use of the delayed standby.
> I'm not clear on the intended usage scenarios for
> recovery_min_apply_delay, but the ramifications don't appear to be well
> explained anywhere.

Well, the alternative to enabling hot standby feedback is that the
query might get cancelled.  But it might also NOT get cancelled.  I
mean, if recovery_min_apply_delay is set to an hour, and the query
runs for a minute, you're only going to get a cancel if some data that
is needed got pruned during the correponding minute an hour earlier on
the master.  And even then you can avoid a cancel by setting
max.*standby_delay to at least 61 seconds, which is more likely to be
acceptable for a standby that intentionally lags the master.  But even
if you don't do that, it's not as if every query you issue is going to
get cancelled.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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