Re: reloption to prevent VACUUM from truncating empty pages at theend of relation

On 17 April 2018 at 20:09, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> Andres was working on a radix tree structure to fix this problem, but
>> that seems to be abandoned now, and it seems a major undertaking.  While
>> I agree that the proposed solution is a wart, it seems much better than
>> no solution at all.  Can we consider Fujii's proposal as a temporary
>> measure until we fix shared buffers?  I'm +1 on it myself.
>
> Once we've introduced a user-visible reloption it's going to be
> practically impossible to get rid of it, so I'm -1.  I'd much rather
> see somebody put some effort into the radix-tree idea than introduce
> a kluge that we'll be stuck with, and that doesn't even provide a
> good user experience.  Disabling vacuum truncation is *not* something
> that I think we should recommend.

The truncation at the end of VACUUM takes an AccessExclusiveLock,
which is already user visible. Using a radix tree won't alter that.

ISTM the user might be interested in having the *lock* NOT happen, so
I am +1 for the suggestion regardless of whether radix tree ever
happens.

The lock itself can be cancelled, so the user would also be interested
in explicitly requesting a retry with a separate command/function.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
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