Re: preserving forensic information when we freeze

Hi,

On 2013-12-17 16:00:14 -0500, Robert Haas wrote:
> @@ -5874,19 +5858,27 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
> void
> heap_execute_freeze_tuple(HeapTupleHeader tuple, xl_heap_freeze_tuple *frz)
> {
> + tuple->t_infomask = frz->t_infomask;
> + tuple->t_infomask2 = frz->t_infomask2;
> +
> if (frz->frzflags & XLH_FREEZE_XMIN)
> - HeapTupleHeaderSetXmin(tuple, FrozenTransactionId);
> + HeapTupleHeaderSetXminFrozen(tuple);
>
> HeapTupleHeaderSetXmax(tuple, frz->xmax);
>
> if (frz->frzflags & XLH_FREEZE_XVAC)
> + {
> HeapTupleHeaderSetXvac(tuple, FrozenTransactionId);
> + /* If we somehow haven't hinted the tuple previously, do it now. */
> + HeapTupleHeaderSetXminCommitted(tuple);
> + }

What's the reasoning behind adding HeapTupleHeaderSetXminCommitted()
here?

> @@ -823,14 +823,14 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
> * NB: Like with per-tuple hint bits, we can't set the
> * PD_ALL_VISIBLE flag if the inserter committed
> * asynchronously. See SetHintBits for more info. Check
> - * that the HEAP_XMIN_COMMITTED hint bit is set because of
> - * that.
> + * that the tuple is hinted xmin-committed hint bit because
> + * of that.
> */

Looks like there's a "hint bit" too much here.

> @@ -65,6 +65,9 @@ manage to be a conflict it would merely mean that one bit-update would
> be lost and need to be done again later. These four bits are only hints
> (they cache the results of transaction status lookups in pg_clog), so no
> great harm is done if they get reset to zero by conflicting updates.
> +Note, however, that a tuple is frozen by setting both HEAP_XMIN_INVALID
> +and HEAP_XMIN_COMMITTED; this is a critical update and accordingly requires
> +an exclusive buffer lock.

I think it'd be appropriate to mention that this needs to be preserved
via WAL logging, it sounds like it's suficient to set a hint bit without
persistenc guarantees.

(not sure if I already wrote this, but whatever)

Looking good.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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