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Re: More tablescanning fun

From: "Jim C(dot) Nasby" <jim(at)nasby(dot)net>
To: Hannu Krosing <hannu(at)tm(dot)ee>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-performance(at)postgresql(dot)org
Subject: Re: More tablescanning fun
Date: 2003-04-25 16:56:13
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Lists: pgsql-performance
On Fri, Apr 25, 2003 at 07:28:06PM +0300, Hannu Krosing wrote:
> I have been pondering if keeping pages half-empty (or even 70% empty)
> could solve both clustering problems and longish updates for much data.
> If we could place the copy in the same page than original, most big
> updates would be possible by one sweep of disk heads and also clustering
> order would be easier to keep if pages were kept intentionally half
> empty.
> So "VACUUM FULL 65% EMPTY;" could make sense ?
That's actually a recommended practice, at least for sybase when you're
using a clustered index, depending on what you're using it for. If you
cluster a table in such a way that inserts will happen across the entire
table, you'll actually end up with a fillratio (amount of data v. empty
space on a page) of 75% over time, because of page splits. When sybase
goes to insert, if it can't find room on the page it needs to insert
into (keep in mind this is a clustered table, so a given row *must* go
into a given position), it will split that single page into two pages,
each of which will then have a fillratio of 50%. Of course they'll
eventually approach 100%, so the average fill ratio across all pages for
the table would be 75%.

I'm not familiar enough with pgsql's guts to know how big an impact
updates across pages are, or if they even happen often at all. If you're
not maintaining a clustered table, couldn't all updates just occur
in-place? Or are you thinking of the case where you have a lot of
variable-length stuff?
Jim C. Nasby (aka Decibel!)                    jim(at)nasby(dot)net
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