On Thu, Jul 20, 2017 at 12:08 AM, Peter Geoghegan <p...@bowt.ie> wrote:
>> The traditional
>> wisdom about btrees, for instance, is that no matter how full you pack
>> them to start with, the steady state is going to involve something like
>> 1/3rd free space.  You can call that bloat if you want, but it's not
>> likely that you'll be able to reduce the number significantly without
>> paying exorbitant costs.
> For the purposes of this discussion, I'm mostly talking about
> duplicates within a page on a unique index. If the keyspace owned by
> an int4 unique index page only covers 20 distinct values, it will only
> ever cover 20 distinct values, now and forever, despite the fact that
> there is room for about 400 (a 90/10 split leaves you with 366 items +
> 1 high key).

Microvacuum could also help.

If during a scan you find pointers that point to dead (in vacuum terms)
tuples, the pointers in the index could be deleted. That could be done
during insert into unique indexes before a split, to avoid the split.

Chances are, if there are duplicates, at least a few of them will be dead.

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