Re: 9.2.1 & index-only scans : abnormal heap fetches after VACUUM FULL

On 2012-11-29 17:59:39 +0530, Pavan Deolasee wrote:
> On Thu, Nov 29, 2012 at 5:42 PM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:
>
> > On 2012-11-29 17:20:01 +0530, Pavan Deolasee wrote:
> >
> > > Now can CLUSTER or VACUUM FULL recreate the visibility map with all bits
> > > set to visible, thats an entirely different question. I don't think it
> > can,
> > > but then I haven't thought through this completely.
> >
> > It can't set everything to visible as it also copies RECENTLY_DEAD
> > tuples and tuples which are not yet visible to other transactions, but
> > it should be relatively easy to keep enough information about whether it
> > can set the current page to all visible.
>
>
> Yeah, that looks fairly easy to have. Thinking about it more, now that we
> have ability to skip WAL for the case when a table is created and populated
> in the same transaction, we could also set the visibility map bits for such
> a table (if we are not doing that already). That should be fairly safe too.

I don't think the latter would be safe at all. Every repeatable read
transaction that started before the table creation would see that tables
content based on the visibilitymap instead of seeing it as empty.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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