Re: failover vs. read only queries

Bruce Momjian <bruce@momjian.us> writes:
> Fujii Masao wrote:
>> On Thu, Jun 10, 2010 at 5:06 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> My feeling about it is that if you want fast failover you should not
>>> have your failover target server configured as hot standby at all, let
>>> alone hot standby with a long max_standby_delay.  Such a slave could be
>>> very far behind on applying WAL when the crunch comes, and no amount of
>>> query killing will save you from that.  Put your long-running standby
>>> queries on a different slave instead.
>>> 
>>> We should consider whether we can improve the situation in 9.1, but it
>>> is not a must-fix for 9.0; especially when the correct behavior isn't
>>> immediately obvious.

>> OK. Let's revisit in 9.1.
>> 
>> I attached the proposal patch for 9.1. The patch treats max_standby_delay
>> as zero (i.e., cancels all the conflicting queries immediately), ever since
>> the trigger file is created. So we can cause a recovery to end without
>> waiting for any lock held by queries, and minimize the failover time.
>> OTOH, queries which don't conflict with a recovery survive the failover.

> Should this be added to the first 9.1 commitfest?

Not sure ... it seems like proof of concept for a pretty dubious
concept.  If you want a slave to be ready for fast failover then you
should not be letting it get far behind the master in the first place.
I think there's some missing piece here, but I'm not quite sure what
to propose.
        regards, tom lane