Re: Refactoring

Manfred Koizar wrote:
> On Wed, 19 Jan 2005 18:57:48 +0100, I wrote:
> > My first vacuum.c
> >refactoring patch, rev 1.281 2004-06-08, added these comments in
> >repair_frag():
> >
> >/*
> > * VACUUM FULL has an exclusive lock on the relation. So
> > * normally no other transaction can have pending INSERTs or
> > * DELETEs in this relation. A tuple is either
> > * (a) a tuple in a system catalog, inserted or deleted by
> > * a not yet committed transaction or
> > * (b) dead (XMIN_INVALID or XMAX_COMMITTED) or
> > * (c) inserted by a committed xact (XMIN_COMMITTED) or
> > * (d) moved by the currently running VACUUM.
> > * In case (a) we wouldn't be in repair_frag() at all.
> > * In case (b) we cannot be here, because scan_heap() has
> > * already marked the item as unused, see continue above.
> > * Case (c) is what normally is to be expected.
> > * Case (d) is only possible, if a whole tuple chain has been
> > * moved while processing this or a higher numbered block.
> > */
>
> It turns out that this comment is not quite correct. It is incomplete.
> Case (b) should be: known dead (XMIN_INVALID, or XMAX_COMMITTED and xmax
> is visible to all active transactions).
>
> And there is a fifth possibility: (e) deleted (XMAX_COMMITTED) but at
> least one active transaction does not see the deleting transaction.
>
> The patch seems to imply that case (e) is a subcase of (b), but
> effectively tuples in this state are treated more like (c).

OK, comment updated to:

/* ---
* VACUUM FULL has an exclusive lock on the relation. So
* normally no other transaction can have pending INSERTs or
* DELETEs in this relation. A tuple is either:
* (a) a tuple in a system catalog, inserted or deleted
* by a not yet committed transaction
* (b) known dead (XMIN_INVALID, or XMAX_COMMITTED and xmax
* is visible to all active transactions)
* (c) inserted by a committed xact (XMIN_COMMITTED)
* (d) moved by the currently running VACUUM.
* (e) deleted (XMAX_COMMITTED) but at least one active
* transaction does not see the deleting transaction
* In case (a) we wouldn't be in repair_frag() at all.
* In case (b) we cannot be here, because scan_heap() has
* already marked the item as unused, see continue above. Case
* (c) is what normally is to be expected. Case (d) is only
* possible, if a whole tuple chain has been moved while
* processing this or a higher numbered block.
* ---
*/

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