Re: [BUG] pg9.4.10 Logical decoding did not get the correct oldtuplelen

On Sun, Jan 1, 2017 at 9:06 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> OK, I can see the problem. And I can see as well that things have been
> tackled in this area with 9.5 thanks to commit 2c03216d...

(Adding Andres in CC for some input on the matter)
Looking at that... As HeapTupleData->t_len is uint32, it is really an
oversight in the 9.4 WAL logging machinery that xl_heap_header_len
uses uint16. If xl_heap_update->flags still had a bit free, we could
have for example use XLOG_HEAP_CONTAINS_OLD_TUPLE to track the
existing short version of xl_heap_header_len and add a new bit for a
"long" version where a version of xl_heap_header_len including uint32
as t_len is read. This would preserve replay from doing stupid things
for existing 9.4 deployments and allow new records to work fully. But
there are no bits free here. As well as there are no bits for
XLOG_HEAP[1|2]. Andres, perhaps you have an idea?

Looking at this code, It is worth noting that
XLOG_HEAP_CONTAINS_OLD_TUPLE and XLOG_HEAP_CONTAINS_OLD_KEY are always
used just for XLOG_HEAP_CONTAINS_OLD, so we could merge them and have
a single flag to do the whole thing, the parent's replica identity
value being here to track if a full version of the old tuple is logged
or not.
--
Michael