From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: "Wong, Yi Wen" <yiwong@amazon.com>,
        Michael Paquier <michael.paquier@gmail.com>,
        Peter Geoghegan <pg@bowt.ie>, "Wood, Dan" <hexpert@amazon.com>,
        "pgsql-bugs@postgresql.org" <pgsql-bugs@postgresql.org>
Subject: Re: Old row version in hot chain become visible after a freeze
In-reply-to: <20170908164244.5oi6rcvy5ut7dpkb@alvherre.pgsql>
References: <20170908164244.5oi6rcvy5ut7dpkb@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Fri, 08 Sep 2017 18:42:44 +0200"
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Date: Fri, 08 Sep 2017 13:04:10 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Wong, Yi Wen wrote:
>> lazy_record_dead_tuple may fail to record the heap for later pruning
>> for lazy_vacuum_heap if there is already a sufficiently large number of =
dead tuples
>> in the array:

> Hmm, ouch, good catch.

> AFAICS this is a shouldn't-happen condition, since we bail out of the
> loop pessimistically as soon as we would be over the array limit if the
> next page were to be full of dead tuples (i.e., we never give the chance
> for overflow to actually happen).  So unless I misunderstand, this could
> only fail if you give maint_work_mem smaller than necessary for one
> pageful of dead tuples, which should be about 1800 bytes ...

> If we wanted to be very sure about this we could add a test and perhaps
> abort the vacuum, but I'm not sure it's worth the trouble.

I think if we're going to depend on that, we should change the logic from
"don't record tuple if no space" to "throw error on no space".

			regards, tom lane


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