Re: Hot standby and b-tree killed items

On Fri, 2008-12-19 at 11:54 -0600, Kevin Grittner wrote:
> >>> Simon Riggs <simon@2ndQuadrant.com> wrote: 
>  
> > If I was going to add anything to the btree page header, it would be
> > latestRemovedLSN, only set during recovery. That way we don't have
> to
> > explicitly kill queries, we can do the a wait on OldestXmin then let
> > them ERROR out when they find a page that has been modified.
> > 
> > I have a suspicion that we may need some modification of that
> solution
> > for all data blocks, so we don't kill too many queries.
>  
> If the failure is caused by the timing of various database
> transactions, and the query is likely to run successfully after a
> delay and a retry, please use SQLSTATE of '40001'.  Some software
> (ours, for one) will recognize this and retry the query automatically,
> so that the user impact is essentially the same as blocking.

I understand the need, but we won't be using SQLSTATE = 40001.

That corresponds to ERRCODE_T_R_SERIALIZATION_FAILURE, which that error
would not be.

The error message ought to be "snapshot too old", which could raise a
chuckle, so I called it something else.

The point you raise is a good one and I think we should publish a list
of retryable error messages. I contemplated once proposing a special log
level for a retryable error, but not quite a good idea.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support