Re: optimizing vacuum truncation scans

On Mon, Jun 29, 2015 at 1:54 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> Attached is a patch that implements the vm scan for truncation. It
> introduces a variable to hold the last blkno which was skipped during the
> forward portion. Any blocks after both this blkno and after the last
> inspected nonempty page (which the code is already tracking) must have been
> observed to be empty by the current vacuum. Any other process rendering the
> page nonempty are required to clear the vm bit, and no other process can set
> the bit again during the vacuum's lifetime. So if the bit is still set, the
> page is still empty without needing to inspect it.

Urgh. So if we do this, that forever precludes having HOT pruning set
the all-visible bit. At the least we'd better document that carefully
so that nobody breaks it later. But I wonder if there isn't some
better approach, because I would certainly rather that we didn't
foreclose the possibility of doing something like that in the future.

--
Robert Haas
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