Re: max_standby_delay considered harmful

Greg Stark wrote:
> On Thu, May 6, 2010 at 2:36 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>   
>> One reason I believe this isn't so critical as all that is that it only
>> matters for cases where the operation on the master took an exclusive
>> lock.
>>     
>
> Uhm, or a vacuum ran. Or a HOT page cleanup occurred, or a btree page
> split deleted old tuples.
>   

Right; because there are so many regularly expected causes for query 
cancellation, the proposed boolean setup really hurts the ability of a 
server whose primary goal is high-availability to run queries of any 
useful duration.  For years I've been hearing "my HA standby is idle, 
how can I put it to use?"; that's the back story of the users I thought 
everyone knew were the known audience waiting for this feature.

If the UI for vacuum_defer_cleanup_age that prevented these things was 
good, I would agree that the cases where max_standby_delay does 
something useful are marginal.  That's why I tried to get someone 
working on SR to provide a hook for that purpose months ago.  But since 
the vacuum adjustment we have in completely obtuse xid units, that 
leaves max_standby_delay as the only tunable here that you can even 
think about in terms of human time.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us