Re: Disabled features on Hot Standby

On Sat, Jan 14, 2012 at 12:42:02AM -0500, Robert Haas wrote:
> On Fri, Jan 13, 2012 at 8:02 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > Simon spoke to the FPI side of the question. ?For heap pages, the
> > XLOG_HEAP_NEWPAGE consumers are CLUSTER, VACUUM FULL and ALTER TABLE SET
> > TABLESPACE. ?For the last, we will have already logged any PD_ALL_VISIBLE bits
> > through normal channels. ?CLUSTER and VACUUM FULL never set PD_ALL_VISIBLE or
> > visibility map bits. ?When, someday, they do, we might emit a separate WAL
> > record to force the recovery conflict. ?However, CLUSTER/VACUUM FULL already
> > remove tuples still-visible to standby snapshots without provoking a recovery
> > conflict. ?(Again only with hot_standby_feedback=off.)
>
> Is the big about CLUSTER/VACUUM FULL a preexisting bug? If not, why not?

I suspect it goes back to 9.0, yes. I'm on the fence regarding whether to call
it a bug or an unimplemented feature. In any case, +1 for improving it.

> Other than that, it seems like we might be converging on a workable
> solution: if hot_standby_feedback=off, disable index-only scans for
> snapshots taken during recovery; if hot_standby_feedback=on, generate
> recovery conflicts when a snapshot's xmin precedes the youngest xmin
> on a page marked all-visible, but allow the use of index-only scans,
> and allow sequential scans to trust PD_ALL_VISIBLE. Off the top of my
> head, I don't see a hole in that logic...

I wouldn't check hot_standby_feedback. Rather, mirror what we do for
XLOG_HEAP2_CLEAN. Unconditionally add an xid to xl_heap_visible bearing the
youngest xmin on the page (alternately, some convenient upper bound thereof).
Have heap_xlog_visible() call ResolveRecoveryConflictWithSnapshot() on that
xid. Now, unconditionally trust PD_ALL_VISIBLE and permit index-only scans.

The user's settings of hot_standby_feedback, vacuum_defer_cleanup_age,
max_standby_streaming_delay and max_standby_archive_delay will drive the
consequential trade-off: nothing, query cancellation, or recovery delay.

nm