From: | Peter Geoghegan <pg(at)heroku(dot)com> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Marti Raudsepp <marti(at)juffo(dot)org>, Stephen Frost <sfrost(at)snowman(dot)net>, Greg Stark <stark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: B-Tree support function number 3 (strxfrm() optimization) |
Date: | 2015-01-21 01:46:37 |
Message-ID: | CAM3SWZTvVbiwQmWJ2yJ--Ei+x0fxfU-jGZ-xv3S_5As2UaYhGQ@mail.gmail.com |
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On Tue, Jan 20, 2015 at 5:42 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Jan 20, 2015 at 8:39 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
>> On Tue, Jan 20, 2015 at 5:32 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> I was assuming we were going to fix this by undoing the abbreviation
>>> (as in the abort case) when we spill to disk, and not bothering with
>>> it thereafter.
>>
>> The spill-to-disk case is at least as compelling at the internal sort
>> case. The overhead of comparisons is much higher for tapesort.
>
> First, we need to unbreak this. Then, we can look at optimizing it.
> The latter task will require performance testing.
I don't see that any alternative isn't a performance trade-off. My
patch accomplishes unbreaking this. I agree that it needs still needs
review from that perspective, but it doesn't seem any worse than any
other alternative. Would you prefer it if the spill-to-disk case
aborted in the style of low entropy keys? That doesn't seem
significantly safer than this, and it certainly not acceptable from a
performance perspective.
--
Peter Geoghegan
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