Re: Quick idea for reducing VACUUM contention

ITAGAKI Takahiro wrote:
>
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:
>
> > > I think we might need additional "freezing-xmax" operations to avoid
> > > XID-wraparound in the first path of vacuum, though it hardly occurs.
> >
> > I'm not sure I follow. Can you elaborate? Do you mean storing a
> > separate relfrozenxmax for each table or something like that?
>
> We need to work around wraparound of xmax in dead tuples. If we miss to
> vacuum them and XID is wrapped, we cannot remove them until the next
> XID-wraparound, because we treat them to be deleted in the *future*.

Oh, but this should not be a problem, because a tuple is either frozen
or removed completely -- xmax cannot precede xmin.

> > > We just add XID of the vacuum to dead tuples we see in the
> > > first path. When backends find a dead tuple and see the transaction
> > > identified by XID in it has commited, they can freely reuse the area of
> > > the dead tuple because we can assume index entries pointing the tuple
> > > have been removed by the vacuum.
> >
> > I would be worried about leftover index entries being later used by new
> > tuples in the heap. Then when you visit the index, find that entry, go
> > to the heap and find the new tuple and return it, which could be bogus.
>
> Avoiding dangling index entries, I'm thinking about reusing dead tuples
> only if we see the VACUUM transaction have committed successfully.
> That means the VACUUM transaction removed all index entries corresponding
> those dead tuples; They are now Heap-Only-Tuples, so that we can recycle
> them in the same manner as HOT updated tuples.

Hmm. OK, I admit I have no idea how HOT works.

--
Alvaro Herrera http://www.CommandPrompt.com/
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