Tom Lane wrote:
> Alvaro Herrera <alvherre@dcc.uchile.cl> writes:
> > I think what Tom is concerned about is that this hasn't been tested
> > enough with big datasets. Also there a little loss of index pages but
> > it's much less (orders of magnitude, I think) than what was before.
> > This is because the index won't shrink "vertically".
>
> The fact that we won't remove levels shouldn't be meaningful at all ---
> I mean, if the index was once big enough to require a dozen btree
> levels, and you delete everything, are you going to be upset that it
> drops to 13 pages rather than 2? I doubt it.
>
> The reason I'm waffling about whether the problem is completely fixed or
> not is that the existing code will only remove-and-recycle completely
> empty btree pages. As long as you have one key left on a page it will
> stay there. So you could end up with ridiculously low percentage-filled
> situations. This could be fixed by collapsing together adjacent
> more-than-half-empty pages, but we ran into a lot of problems trying to
> do that in a concurrent fashion. So I'm waiting to find out if real
> usage patterns have a significant issue with this or not.
Though the new code will put empty index pages into the free-space map,
will it also shrink the index file to remove those pages? For example,
if I have 200M rows in a table, and I delete all of them except 100,
does the index shrink, or the pages just become available for reuse.
With VACUUM FULL, we have a way to shrink the heap. Do we shrink the
index?
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