| From: | Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Jaime Casanova <jaime(dot)casanova(at)2ndquadrant(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Patch: Write Amplification Reduction Method (WARM) |
| Date: | 2017-02-24 10:01:08 |
| Message-ID: | CABOikdPqPvoVUYmZ2TsMgTHQ76+9jrwogXBGJJP9fyF+UAS1CQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, Feb 24, 2017 at 3:23 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> I don't immediately see how this will work with index-only scans. If
> the tuple is HOT updated several times, HOT-pruned back to a single
> version, and then the page is all-visible, the index entries are
> guaranteed to agree with the remaining tuple, so it's fine to believe
> the data in the index tuple. But with WARM, that would no longer be
> true, unless you have some trick for that...
>
>
Well the trick is to not allow index-only scans on such pages by not
marking them all-visible. That's why when a tuple is WARM updated, we carry
that information in the subsequent versions even when later updates are HOT
updates. The chain conversion algorithm will handle this by clearing those
bits and thus allowing index-only scans again.
Thanks,
Pavan
--
Pavan Deolasee http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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