| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)heroku(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 01:32:08 |
| Message-ID: | CA+TgmoaqO8=opewOjH_wVBDspsVvMNaiZxuVv9S1qWigkouOqA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Jan 20, 2015 at 7:07 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> On Tue, Jan 20, 2015 at 3:57 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
>> It's certainly possible to fix Andrew's test case with the attached.
>> I'm not sure that that's the appropriate fix, though: there is
>> probably a case to be made for not bothering with abbreviation once
>> we've read tuples in for the final merge run. More likely, the
>> strongest case is for storing the abbreviated keys on disk too, and
>> reading those back.
>
> Maybe not, though: An extra 8 bytes per tuple on disk is not free.
> OTOH, if we're I/O bound on the final merge, as we ought to be, then
> recomputing the abbreviated keys could make sense, since there may
> well be an idle CPU core anyway.
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.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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