Re: Pre-allocated free space for row updating (like PCTFREE)

Satoshi Nagayasu <nagayasus@nttdata.co.jp> writes:
> I've done a quick hack to implement PCTFREE on PostgreSQL.
> ...
> According to my experiments, pgbench score was improved 10% or more
> with 1024 bytes free space.

I'm not very enthused about this.  Enforcing 12.5% PCTFREE means that
you pay 12.5% extra I/O costs across the board for INSERT and SELECT
and then hope you can make it back (plus some more) on UPDATEs.
pgbench is a completely UPDATE-dominated benchmark and thus it makes
such a patch look much better than it would on other workloads.

I think the reason Oracle offers this has to do with their
overwrite-based storage management; it's not obvious that the tradeoff
is as useful for us.  There are some relevant threads in our archives
here, here, and here:
http://archives.postgresql.org/pgsql-patches/2005-04/msg00078.php
http://archives.postgresql.org/pgsql-performance/2004-08/msg00402.php
http://archives.postgresql.org/pgsql-performance/2003-10/msg00618.php
        regards, tom lane