Re: EXTERNAL storage and substring on long strings

От: Scott Cain
Тема: Re: EXTERNAL storage and substring on long strings
Дата: ,
Msg-id: 1059682839.5415.53.camel@localhost.localdomain
(см: обсуждение, исходный текст)
Ответ на: EXTERNAL storage and substring on long strings  (Scott Cain)
Ответы: Re: EXTERNAL storage and substring on long strings  (Tom Lane)
Re: EXTERNAL storage and substring on long strings  (Joe Conway)
Список: pgsql-performance

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EXTERNAL storage and substring on long strings  (Scott Cain, )
 Re: EXTERNAL storage and substring on long strings  (Scott Cain, )
  Re: EXTERNAL storage and substring on long strings  (Tom Lane, )
  Re: EXTERNAL storage and substring on long strings  (Joe Conway, )
 Re: EXTERNAL storage and substring on long strings  (Scott Cain, )
  Re: EXTERNAL storage and substring on long strings  (Tom Lane, )
 Re: EXTERNAL storage and substring on long strings  (Joe Conway, )
  Re: EXTERNAL storage and substring on long strings  (Ron Johnson, )
   Re: EXTERNAL storage and substring on long strings  (Scott Cain, )
    Re: [SQL] EXTERNAL storage and substring on long strings  (Tom Lane, )
     Re: [SQL] EXTERNAL storage and substring on long strings  (Scott Cain, )
      Re: [SQL] EXTERNAL storage and substring on long strings  (Joe Conway, )
       Re: [SQL] EXTERNAL storage and substring on long strings  (Scott Cain, )
        Re: [SQL] EXTERNAL storage and substring on long strings  (Scott Cain, )
         Re: [SQL] EXTERNAL storage and substring on long strings  (Joe Conway, )
     Re: [SQL] EXTERNAL storage and substring on long strings  (Jan Wieck, )
      Re: [SQL] EXTERNAL storage and substring on long strings  (Tom Lane, )
       Re: [SQL] EXTERNAL storage and substring on long strings  (Scott Cain, )
       Re: [SQL] EXTERNAL storage and substring on long strings  (Jan Wieck, )
    Re: [SQL] EXTERNAL storage and substring on long strings  (Richard Huxton, )
     Re: [SQL] EXTERNAL storage and substring on long strings  (Scott Cain, )
      Re: [SQL] EXTERNAL storage and substring on long strings  ("Shridhar Daithankar", )
      Re: [SQL] EXTERNAL storage and substring on long strings  ("Matt Clark", )
  Re: EXTERNAL storage and substring on long strings  (Scott Cain, )
 Re: EXTERNAL storage and substring on long strings  (Tom Lane, )
 Re: EXTERNAL storage and substring on long strings  (Joe Conway, )
  Re: EXTERNAL storage and substring on long strings  (Scott Cain, )
 Re: EXTERNAL storage and substring on long strings  (Tom Lane, )
  Re: EXTERNAL storage and substring on long strings  (Joe Conway, )
   Re: EXTERNAL storage and substring on long strings  (Tom Lane, )
    Re: EXTERNAL storage and substring on long strings  (Scott Cain, )

On Thu, 2003-07-31 at 15:44, Tom Lane wrote:
> Scott Cain <> writes:
> > explain analyze select substring(residues from 1000000 for 20000)
> > from feature where feature_id=1;
>
> > where feature is a table with ~3 million rows, and residues is a text
> > column, where for the majority of the rows of feature, it is null, for a
> > large minority, it is shortish strings (a few thousand characters), and
> > for 6 rows, residues contains very long strings (~20 million characters
> > (it's chromosome DNA sequence from fruit flies)).
>
> I think the reason uncompressed storage loses here is that the runtime
> is dominated by the shortish strings, and you have to do more I/O to get
> at those if they're uncompressed, negating any advantage from not having
> to fetch all of the longish strings.

I'm not sure I understand what that paragraph means, but it sounds like,
if PG is working the way it is supposed to, tough for me, right?
>
> Or it could be that there's a bug preventing John Gray's substring-slice
> optimization from getting used.  The only good way to tell that I can
> think of is to rebuild PG with profiling enabled and try to profile the
> execution both ways.  Are you up for that?

I am not against recompiling.  I am currently using an RPM version, but
I could probably recompile; the compilation is probably straight forward
(adding something like `--with_profiling` to ./configure), but how
straight forward is actually doing the profiling?  Is there a document
somewhere that lays it out?
>
> (BTW, if you are using a multibyte database encoding, then that's your
> problem right there --- the optimization is practically useless unless
> character and byte indexes are the same.)

I shouldn't be, but since it is an RPM, I can't be sure.  It sure would
be silly since the strings consist only of [ATGCN].

Thanks,
Scott

--
------------------------------------------------------------------------
Scott Cain, Ph. D.                                         
GMOD Coordinator (http://www.gmod.org/)                     216-392-3087
Cold Spring Harbor Laboratory



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