Have we made a decision on this issue?  Should I apply my patch that
still forces a split unless 10% of the page has been freed?


Tom Lane wrote:
> ITAGAKI Takahiro <[EMAIL PROTECTED]> writes:
> > This is a revised patch originated by Junji TERAMOTO for HEAD.
> >   [BTree vacuum before page splitting]
> >   http://archives.postgresql.org/pgsql-patches/2006-01/msg00301.php
> > I think we can resurrect his idea because we will scan btree pages
> > at-atime now; the missing-restarting-point problem went away.
> I've applied this but I'm now having some second thoughts about it,
> because I'm seeing an actual *decrease* in pgbench numbers from the
> immediately prior CVS HEAD code.  Using
>       pgbench -i -s 10 bench
>       pgbench -c 10 -t 1000 bench     (repeat this half a dozen times)
> with fsync off but all other settings factory-stock, what I'm seeing
> is that the first run looks really good but subsequent runs tail off in
> spectacular fashion :-(  Pre-patch there was only minor degradation in
> successive runs.
> What I think is happening is that because pgbench depends so heavily on
> updating existing records, we get into a state where an index page is
> about full and there's one dead tuple on it, and then for each insertion
> we have
>       * check for uniqueness marks one more tuple dead (the
>         next-to-last version of the tuple)
>       * newly added code removes one tuple and does a write
>       * now there's enough room to insert one tuple
>       * lather, rinse, repeat, never splitting the page.
> The problem is that we've traded splitting a page every few hundred
> inserts for doing a PageIndexMultiDelete, and emitting an extra WAL
> record, on *every* insert.  This is not good.
> Had you done any performance testing on this patch, and if so what
> tests did you use?  I'm a bit hesitant to try to fix it on the basis
> of pgbench results alone.
> One possible fix that comes to mind is to only perform the cleanup
> if we are able to remove more than one dead tuple (perhaps about 10
> would be good).  Or do the deletion anyway, but then go ahead and
> split the page unless X amount of space has been freed (where X is
> more than just barely enough for the incoming tuple).
> After all the thought we've put into this, it seems a shame to
> just abandon it :-(.  But it definitely needs more tweaking.
>                       regards, tom lane
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  Bruce Momjian   [EMAIL PROTECTED]
  EnterpriseDB    http://www.enterprisedb.com

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