FAQ

On 5/24/13 9:53 AM, Andres Freund wrote:
We don't even necessarily need to log the hint bits for all items since
the redo for all_visible could make sure all items are hinted. The only
problem is knowing up to where we can truncate pg_clog...
[all-visible cannot restore hint bits without FPI because of torn pages]
I haven't yet thought about this sufficiently yet. I think we might have
a chance of working around this, let me ponder a bit.

But even if that means needing a full page write via the usual mechanism
for all visible if any hint bits needed to be set we are still out far
ahead of the current state imo.
* cleanup would quite possibly do an FPI shortly after in vacuum
anyway. If we do it for all visible, it possibly does not need to be
done for it.
* freezing would FPI almost guaranteedly since we do it so much
later.
* Not having to rescan the whole heap will be a bigger cost saving...
Would we only set all the hint bits within vacuum? If so I don't think the WAL hit matters at all, because vacuum is almost always a background, throttled process.
--
Jim C. Nasby, Data Architect jim@nasby.net
512.569.9461 (cell) http://jim.nasby.net

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postedMay 23, '13 at 5:51p
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