Re: Inaccuracy in VACUUM's tuple count estimates

On Thu, Jun 12, 2014 at 01:40:59PM +0200, Andres Freund wrote:
> Hi Tom,
>
> On 2014-06-06 15:44:25 -0400, Tom Lane wrote:
> > I figured it'd be easy enough to get a better estimate by adding another
> > counter to count just LIVE and INSERT_IN_PROGRESS tuples (thus effectively
> > assuming that in-progress inserts and deletes will both commit).
>
> Did you plan to backpatch that? My inclination would be no...
>
> > I did
> > that, and found that it helped Tim's test case not at all :-(. A bit of
> > sleuthing revealed that HeapTupleSatisfiesVacuum actually returns
> > INSERT_IN_PROGRESS for any tuple whose xmin isn't committed, regardless of
> > whether the transaction has since marked it for deletion:
> >
> > /*
> > * It'd be possible to discern between INSERT/DELETE in progress
> > * here by looking at xmax - but that doesn't seem beneficial for
> > * the majority of callers and even detrimental for some. We'd
> > * rather have callers look at/wait for xmin than xmax. It's
> > * always correct to return INSERT_IN_PROGRESS because that's
> > * what's happening from the view of other backends.
> > */
> > return HEAPTUPLE_INSERT_IN_PROGRESS;
> >
> > It did not use to blow this question off: back around 8.3 you got
> > DELETE_IN_PROGRESS if the tuple had a delete pending. I think we need
> > less laziness + fuzzy thinking here. Maybe we should have a separate
> > HEAPTUPLE_INSERT_AND_DELETE_IN_PROGRESS result code? Is it *really*
> > the case that callers other than VACUUM itself are okay with failing
> > to make this distinction? I'm dubious: there are very few if any
> > callers that treat the INSERT and DELETE cases exactly alike.
>
> My current position on this is that we should leave the code as is <9.4
> and HEAPTUPLE_INSERT_IN_PROGRESS for the 9.4/master. Would you be ok
> with that? The second best thing imo would be to discern and return
> HEAPTUPLE_INSERT_IN_PROGRESS/HEAPTUPLE_DELETE_IN_PROGRESS for the
> respective cases.
> Which way would you like to go?

Did we ever come to a conclusion on this?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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