Re: Re: SPGiST versus hot standby - question about conflict resolution rules

Noah Misch <noah(at)leadboat(dot)com> writes:
> On Thu, Aug 02, 2012 at 02:49:45PM -0400, Tom Lane wrote:
>> * In spgRedoVacuumRedirect, call ResolveRecoveryConflictWithSnapshot
>> with the newest-redirect XID.

> There's an obsolete comment in spg_redo().

[ squint ... ] Comparing that to btree_redo, I realize that there's a
bug in what I did yesterday: the ResolveRecoveryConflictWithSnapshot
call has to happen before we call RestoreBkpBlocks, else the new state
of the index page could be exposed to processes that need the old one.

Will fix. I think the code in btree_redo could use a better (or any)
comment about this point, too.

>> But we still have to enforce the interlock against hot standby xacts.

> We achieve that division of labor for XLOG_BTREE_DELETE by examining the old
> contents before RestoreBkpBlocks(). This is safe, I think, because we only
> examine the page when the system has running hot standby backends, and we only
> allow hot standby connections when recovery has proceeded far enough to fix
> all torn and ahead-of-EndRecPtr pages.

Egad. That's seriously underdocumented as well. And I think it needs
an explicit test that the page is actually older than the current WAL
record, because it would otherwise be doing the wrong thing.

regards, tom lane