Re: Skip all-visible pages during second HeapScan of CIC

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Stephen Frost <sfrost(at)snowman(dot)net>, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Skip all-visible pages during second HeapScan of CIC
Date: 2017-03-08 03:03:40
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On 2017-03-07 21:38:40 -0500, Robert Haas wrote:
> > I wonder however, if careful snapshot managment couldn't solve this as
> > well. I have *not* thought a lot about this, but afaics we can easily
> > prevent all-visible from being set in cases it'd bother us by having an
> > "appropriate" xmin / registered snapshot.
> Yeah, but that's a tax on the whole system.

I'm not sure I can buy that argument. CIC *already* holds a snapshot
during each of the two scans. By reducing the amount of time that's held
in the second scan, the systemwide impact is reduced, because it's held
for a shorter amount of time. We need to hold a slightly "more
aggressive" snapshot, that's true, but if you have high xid throughput
those effects should roughly balance each other.


Andres Freund

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