Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Tue, Oct 3, 2017 at 9:48 AM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> which shows a HOT-update chain, where the t_xmax are multixacts. Then a
> vacuum freeze comes, and because the multixacts are below the freeze
> horizon for multixacts, we get this:
>
> select lp, lp_flags, t_xmin, t_xmax, t_ctid, to_hex(t_infomask) as infomask,
> to_hex(t_infomask2) as infomask2
> from heap_page_items(get_raw_page('t', 0));
> lp | lp_flags | t_xmin | t_xmax | t_ctid | infomask | infomask2
> ----+----------+--------+--------+--------+----------+-----------
> 1 | 1 | 2 | 0 | (0,1) | 902 | 3
> 2 | 0 | | | | |
> 3 | 1 | 2 | 14662 | (0,4) | 2502 | c003
> 4 | 1 | 2 | 14663 | (0,5) | 2502 | c003
> 5 | 1 | 2 | 14664 | (0,6) | 2502 | c003
> 6 | 1 | 2 | 14665 | (0,7) | 2502 | c003
> 7 | 1 | 2 | 0 | (0,7) | 2902 | 8003
> (7 filas)
>
> where the xmin values have all been frozen, and the xmax values are now
> regular Xids.

I thought that we no longer store FrozenTransactionId (xid 2) as our
"raw" xmin while freezing, and yet that's what we see here. It looks
like pageinspect is looking at raw xmin (it's calling
HeapTupleHeaderGetRawXmin(), not HeapTupleHeaderGetXmin()). What's the
deal with that? Shouldn't the original xmin be preserved for forensic
analysis, following commit 37484ad2?

I guess what you mean is that this is what you see having modified the
code to actually store FrozenTransactionId as xmin once more, in an
effort to fix this?

--
Peter Geoghegan