Re: VACUUMs and WAL

On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote:
> Looking at a VACUUM's WAL records makes me think twice about the way we
> issue a VACUUM.
>
> 1. First we scan the heap, issuing a HEAP2 clean record for every block
> that needs cleaning.

IIRC the first heap pass just collects info and does nothing else.
Is this just an empty/do-nothing WAL record ?

> 2. Then we scan the index, issuing WAL records as appropriate.
>
> 3. Then we rescan the heap, issuing a HEAP2 clean record for every
> block.
>
> I don't see a reason why we would issue 2 WAL records per block for a
> VACUUM, nor why we would prune and remove in two steps, dirtying the
> block each time.

The first pass should just be collecting info and not dirtying anything.
Could it be side effect of setting some transaction visibility bits on
first visit ?

In that case It would be good, if we could disable doing that that for
vacuum.

> Seems like we could write approximately half the amount
> of data that we do.
>
> Surely we can come up with a better plan than that one?

-------------------
Hannu Krosing

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