Re: Scalability in postgres

От: Tom Lane
Тема: Re: Scalability in postgres
Дата: ,
Msg-id: 1261.1250292118@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Re: Scalability in postgres  (Jeff Janes)
Ответы: Re: Scalability in postgres  (Jeff Janes)
Список: pgsql-performance

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Scalability in postgres  (Fabrix, )
 Re: Scalability in postgres  (David Rees, )
  Re: Scalability in postgres  (Fabrix, )
   Re: Scalability in postgres  (Scott Mead, )
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   Re: Scalability in postgres  (Scott Marlowe, )
 Re: Scalability in postgres  (Scott Marlowe, )
  Re: Scalability in postgres  (Fabrix, )
  Re: Scalability in postgres  (Flavio Henrique Araque Gurgel, )
   Re: Scalability in postgres  (Fabrix, )
    Re: Scalability in postgres  (Scott Marlowe, )
    Re: Scalability in postgres  (Flavio Henrique Araque Gurgel, )
     Re: Scalability in postgres  (Greg Smith, )
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         Re: Scalability in postgres  (Scott Marlowe, )
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           Re: Scalability in postgres  (Scott Marlowe, )
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      Re: Scalability in postgres  (Fabrix, )
       Re: Scalability in postgres  (Greg Smith, )
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       Re: Scalability in postgres  (Greg Smith, )
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           Re: Scalability in postgres  (Robert Haas, )
            Re: Scalability in postgres  (Scott Carey, )
             Re: Scalability in postgres  (Robert Haas, )
              Re: Scalability in postgres  (Scott Carey, )
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              Re: Scalability in postgres  (, )
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                Re: Scalability in postgres  (Greg Smith, )
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                  Re: Scalability in postgres  (Robert Haas, )
                 Re: Scalability in postgres  (Mark Mielke, )
                  Re: Scalability in postgres  (Greg Smith, )
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           Re: Scalability in postgres  (Marc Cousin, )
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         Re: Scalability in postgres  ("Kevin Grittner", )
          Re: Scalability in postgres  (Mark Mielke, )
           Re: Scalability in postgres  (, )
            Re: Scalability in postgres  (Mark Mielke, )
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           Re: Scalability in postgres  ("Kevin Grittner", )
           Re: Scalability in postgres  (Greg Smith, )
          Re: Scalability in postgres  (Scott Carey, )
           Re: Scalability in postgres  ("Kevin Grittner", )
         Re: Scalability in postgres  ("Kevin Grittner", )
 Re: Scalability in postgres  (Jeff Janes, )
  Re: Scalability in postgres  (Tom Lane, )
   Re: Scalability in postgres  (Jeff Janes, )
  Re: Scalability in postgres  ("", )

Jeff Janes <> writes:
> I apologize if it is bad form to respond to a message that is two
> months old, but I did not see this question answered elsewhere and
> thought it would be helpful to have it answered.  This my rough
> understanding.  Oracle never "takes" a snapshot, it computes one the
> fly, if and when it is needed.  It maintains a structure of recently
> committed transactions, with the XID for when they committed.  If a
> process runs into a tuple that is neither from the future nor from the
> deep past, it consults this structure to see if that transaction has
> committed, and if so whether it did so before or after the current
> query was started.  The structure is partionable so it does not have
> one global lock to serialize on, and the lock is short as it only gets
> the info it needs, not the entire set of global open transactions.

Are you sure it's partitionable?  I've been told that Oracle's
transaction log is a serious scalability bottleneck.  (But I think
I first heard that in 2001, so maybe they've improved it in recent
releases.)  We know that Postgres' WAL log is a bottleneck --- check
for recent discussions involving XLogInsert.  But the WAL log is
only touched by read-write transactions, whereas in Oracle even
read-only transactions often have to go to the transaction log.

            regards, tom lane


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