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Re: Do we need so many hint bits?

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org, Merlin Moncure <mmoncure(at)gmail(dot)com>
Subject: Re: Do we need so many hint bits?
Date: 2012-11-16 15:09:42
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Lists: pgsql-hackers
On 2012-11-15 16:42:57 -0800, Jeff Davis wrote:
> Related to discussion here:
> It occurred to me recently that many of the hint bits aren't terribly
> important (at least it's not obvious to me). HEAP_XMIN_COMMITTED clearly
> has a purpose, and we'd expect it to be used many times following the
> initial CLOG lookup.

> But the other tuple hint bits seem to be there just for symmetry,
> because they shouldn't last long. If HEAP_XMIN_INVALID or
> HEAP_XMAX_COMMITTED is set, then it's (hopefully) going to be vacuumed
> soon, and gone completely. And if HEAP_XMAX_INVALID is set, then it
> should just be changed to InvalidTransactionId.

It can take an *awfully* long time till autovacuum crosses the
thresholds the next time for a big table.
I also think we cannot dismiss the case of longrunning transactions
because vacuum won't be able to cleanup those rows in that case.

Wrt HEAP_(XMIN|XMAX)_INVALID: yes, if we are in need of new flag bits
those sound like a good target to me.

> Also, I am wondering about PD_ALL_VISIBLE. It was originally introduced
> in the visibility map patch, apparently as a way to know when to clear
> the VM bit when doing an update. It was then also used for scans, which
> showed a significant speedup. But I wonder: why not just use the
> visibilitymap directly from those places? It can be used for the scan
> because it is crash safe now (not possible before). And since it's only
> one lookup per scanned page, then I don't think it would be a measurable
> performance loss there. Inserts/updates/deletes also do a significant
> amount of work, so again, I doubt it's a big drop in performance there
> -- maybe under a lot of concurrency or something.
> The benefit of removing PD_ALL_VISIBLE would be significantly higher.
> It's quite common to load a lot of data, and then do some reads for a
> while (setting hint bits and flushing them to disk), and then do a
> VACUUM a while later, setting PD_ALL_VISIBLE and writing all of the
> pages again. Also, if I remember correctly, Robert went to significant
> effort when making the VM crash-safe to keep the PD_ALL_VISIBLE and VM
> bits consistent. Maybe this was all discussed before?

As far as I understand the code the crash-safety aspects of the
visibilitymap currently rely on on having the knowledge that ALL_VISIBLE
has been cleared during a heap_(insert|update|delete). That allows
management of the visibilitymap without it being xlogged itself which
seems pretty important to me.


Andres Freund

 Andres Freund	         
 PostgreSQL Development, 24x7 Support, Training & Services

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