On Fri, Feb 24, 2017 at 3:42 PM, Robert Haas <robertmh...@gmail.com> wrote:

> Wow, OK.  In my view, that makes the chain conversion code pretty much
> essential, because if you had WARM without chain conversion then the
> visibility map gets more or less irrevocably less effective over time,
> which sounds terrible.

Yes. I decided to complete chain conversion patch when I realised that IOS
will otherwise become completely useful if large percentage of rows are
updated just once. So I agree. It's not an optional patch and should get in
with the main WARM patch.

> But it sounds to me like even with the chain
> conversion, it might take multiple vacuum passes before all visibility
> map bits are set, which isn't such a great property (thus e.g.
> fdf9e21196a6f58c6021c967dc5776a16190f295).
The chain conversion algorithm first converts the chains during vacuum and
then checks if the page can be set all-visible. So I'm not sure why it
would take multiple vacuums before a page is set all-visible. The commit
you quote was written to ensure that we make another attempt to set the
page all-visible after al dead tuples are removed from the page. Similarly,
we will convert all WARM chains to HOT chains and then check for
all-visibility of the page.


 Pavan Deolasee                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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