Re: Synthetic keys and index fillfactor

On Thu, Jan 12, 2023 at 4:33 PM Gavan Schneider <list(dot)pg(dot)gavan(at)pendari(dot)org>
wrote:

> On 13 Jan 2023, at 9:54, David G. Johnston wrote:
>
> On Thu, Jan 12, 2023 at 3:45 PM Ron <ronljohnsonjr(at)gmail(dot)com> wrote:
>
> (This mostly pertains to recreating a PK on an existing table.)
>
> Is there any reason to have the PK index on an ever-increasing field (for
> example SERIAL, sequence or timestamp fed by clock_timestamp() at time
> zone
> 'UTC') be anything but fillfactor=100?
>
> New records will always be added to the "lower right hand corner" of the
> tree, so having 20% empty space in the rest of the tree would just waste
> space (mainly buffers, but disk space could even start to add up on Very
> Big Tables).
>
> Yes, at least that is what I gather from the advice on the CREATE INDEX
> page.
>
> https://www.postgresql.org/docs/15/sql-createindex.html
>
> My reading would say otherwise even in the context of monotonic serial
> additions
>
> Quote —
> You should only consider [fillfactor 100] when you are *completely* sure
> that the table is static
> (i.e. that it will *never* be affected by either inserts or updates). A
> fillfactor setting of 100 otherwise
> risks harming performance: even a few updates or inserts will cause a
> sudden flood of page splits.
> (My emphasis added)
>
>
> Yep, and as the question was: is there any reason to use a fillfactor
other than 100, the quoted material provides said reasons and thus "Yes" is
the correct answer. We interpreted the same material in the same way, I
just let the documentation speak for itself.

I'll admit it seems a bit counter-intuitive, and there really isn't a "why"
offered in the user-facing documentation, but for 10% I'm not too worried
about it.

David J.