| From: | "Simon Riggs" <simon(at)2ndquadrant(dot)com> |
|---|---|
| To: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Gavin Sherry" <swm(at)linuxworld(dot)com(dot)au>, "ITAGAKI Takahiro" <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Dead Space Map for vacuum |
| Date: | 2006-12-29 22:18:14 |
| Message-ID: | 1167430694.3903.275.camel@silverbirch.site |
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| Lists: | pgsql-hackers |
On Fri, 2006-12-29 at 16:41 -0500, Tom Lane wrote:
> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> > On Fri, 2006-12-29 at 10:49 -0500, Tom Lane wrote:
> >> Counterexample: table in which all tuples exceed half a page.
>
> > Current FSM code will ignore those too, if they are less than the
> > average size of the tuple so far requested. Thats a pretty wierd
> > counterexample, even if it is a case that needs handling.
>
> Better read it again. The number that's passed to the FSM is the
> free space *after* vacuuming, which in this scenario will be
> BLCKSZ-less-page-header. This case is not broken now, but it will
> be if we adopt your proposal.
The case doesn't is extremely rare, since
#define TOAST_TUPLE_THRESHOLD (MaxTupleSize / 4)
Even so, I'm fairly certain that an if () statement is OK to handle
that case. So I don't really understand that as a limit to the proposal,
which is a small change in the scheme of things.
DSM has my support; I would like it to be as efficient as possible.
--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com
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