Re: Logical decoding on standby

On 23 November 2016 at 03:55, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Nov 22, 2016 at 1:49 AM, Craig Ringer <craig@2ndquadrant.com> wrote:
>> On 22 November 2016 at 10:20, Craig Ringer <craig@2ndquadrant.com> wrote:
>>> I'm currently looking at making detection of replay conflict with a
>>> slot work by separating the current catalog_xmin into two effective
>>> parts - the catalog_xmin currently needed by any known slots
>>> (ProcArray->replication_slot_catalog_xmin, as now), and the oldest
>>> actually valid catalog_xmin where we know we haven't removed anything
>>> yet.
>>
>> OK, more detailed plan.
>>
>> The last checkpoint's oldestXid, and ShmemVariableCache's oldestXid,
>> are already held down by ProcArray's catalog_xmin. But that doesn't
>> mean we haven't removed newer tuples from specific relations and
>> logged that in xl_heap_clean, etc, including catalogs or user
>> catalogs, it only means the clog still exists for those XIDs.
>
> Really?

(Note the double negative above).

Yes, necessarily so. You can't look up xids older than the clog
truncation threshold at oldestXid, per our discussion on txid_status()
and traceable commit. But the tuples from that xact aren't guaranteed
to exist in any given relation; vacuum uses vacuum_set_xid_limits(...)
which calls GetOldestXmin(...); that in turn scans ProcArray to find
the oldest xid any running xact cares about. It might bump it down
further if there's a replication slot requirement or based on
vacuum_defer_cleanup_age, but it doesn't care in the slightest about
oldestXmin.

oldestXmin cannot advance until vacuum has removed all tuples for that
xid and advanced the database's datfrozenxid. But a given oldestXmin
says nothing about which tuples, catalog or otherwise, still exist and
are acessible.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services