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Re: MVCC performance issue

From: Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>
To: Marti Raudsepp <marti(at)juffo(dot)org>
Cc: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, Kyriacos Kyriacou <kyriacosk(at)prime-tel(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: MVCC performance issue
Date: 2010-11-15 00:32:54
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Marti Raudsepp wrote:
> Another advantage of Oracle's approach seems that they need much less
> tuple-level overhead. IMO the 23-byte tuple overhead is a much bigger
> drawback in Postgres than table fragmentation.
> Regards,
> Marti
Oracle, however, does have a problem with "ORA-1555 Snapshot too old", 
precisely because of their implementation of MVCC. In other words, if 
your query is running long and Oracle is not able to reconstruct the old 
rows from the UNDO segments, you're out of luck and your query will die. 
The greatest burden of the Postgres implementation is the fact that 
there is no row id, so that the table header and the indexes need to be 
updated much more frequently than is the case with Oracle.

Mladen Gogala 
Sr. Oracle DBA
1500 Broadway
New York, NY 10036
(212) 329-5251 

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