Re: Plans for solving the VACUUM problem - Mailing list pgsql-hackers

On Fri, 18 May 2001, Tom Lane wrote:

> Oleg Bartunov <oleg@sai.msu.su> writes:
> >> The comment says
> >>
> >> ** Take a compressed entry, and install it on a page.  Since we now know
> >> ** where the entry will live, we decompress it and recompress it using
> >> ** that knowledge (some compression routines may want to fish around
> >> ** on the page, for example, or do something special for leaf nodes.)
> >>
> >> Are you prepared to say that you will no longer support the ability for
> >> GIST compression routines to do those things?  That seems shortsighted.
>
> > No-no !!! we don't intend to lose that (compression) functionality.
>
> > there are several reason we want to eliminate gistPageAddItem:
> > 1. It seems there are no examples where compress uses information about
> >    the page.
>
> We have none now, perhaps, but the original GIST authors seemed to think
> it would be a useful capability.  I'm hesitant to rip out functionality
> that they put in --- I don't think we understand GIST better than they
> did ;-)

ok. we save the code for future. Probably we could ask original author
btw, who is the orig, author (Hellerstein, Aoki) ?

>
> > 2. There is some discrepancy between calculation of free space on page and
> >    the size of tuple saved on page - calculation of free space on page
> >    by gistNoSpace uses compressed tuple but tuple itself saved after
> >    recompression. It's possible that size of tupple could changed
> >    after recompression.
>
> Yes, that's a serious problem with the idea.  We'd have to suppose that
> recompression could not increase the size of the tuple --- or else be
> prepared to back up and find another page and do it again (ugh).
>

We have to keep in mind this when return to gistPageAddItem.

> > 3. decompress/compress could slowdown insert  because it happens
> >    for every tuple.
>
> Seems like there should be a flag somewhere that tells whether the
> compression function actually cares about the page context or not.
> If not, you could skip the useless processing.
>

ok. see above.

> > 4. Currently gistPageAddItem is broken because it's not toast safe
> >    (see call gist_tuple_replacekey in gistPageAddItem)
>
> Not sure I see the problem.  gist_tuple_replacekey is kinda ugly, but
> what's it got to do with TOAST?
>

tuple could be formed not by index_formtuple which is a correct way.


>             regards, tom lane
>
Regards,    Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83