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Re: Need to update all my 60 million rows at oncewithout transactional integrity

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: christian_behrens(at)gmx(dot)net
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Need to update all my 60 million rows at oncewithout transactional integrity
Date: 2008-04-23 18:15:52
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On Mon, 2008-04-21 at 00:19 +0200, christian_behrens(at)gmx(dot)net wrote:

> How can I make a Update of a column in a very large table for all rows
> without using the double amount of disc space and without any need for
> atomic operation?
> I have a very large table with about 60 million rows. I sometimes need
> to do a simple update to ALL rows that resets a status-flag to zero.
> I don't need to have transactional integrity (but of course if the
> system crashes, there should be no data corruption. 

No such thing. Without transactions you have no sensible definition of
what constitutes data corruption.

> A separate flag in the file system can well save the fact that that
> bulk update was in progress) for this, I don't care or want a abort or
> "all or nothing" like SQL mandates. The idea is basically that either
> this update succeeds or it succeeds or - there is no "not". It must
> succeed. It must be tried until it works. If the update is halfway
> finished, that's okay.

Don't reset them to zero, just redefine the meaning of the counter. Take
the max value in the table and then have the app understand that
anything <= the previous max value means the same thing as whatever
"status = 0" means now. The app would need to read the new baseline
value before performing any work.

This is roughly the technique used by Slony to avoid needing to update
every row in the log table to show that it has successfully replicated
it. It's also the technique MVCC relies upon internally.

It's less work and crash safe in all cases.

  Simon Riggs

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