Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM)

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От Pavan Deolasee
Тема Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM)
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Msg-id CABOikdPqPvoVUYmZ2TsMgTHQ76+9jrwogXBGJJP9fyF+UAS1CQ@mail.gmail.com
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Ответ на Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM)  (Robert Haas <robertmhaas@gmail.com>)
Ответы Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM)  (Robert Haas <robertmhaas@gmail.com>)
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On Fri, Feb 24, 2017 at 3:23 PM, Robert Haas <robertmhaas@gmail.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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