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Re: update performance 8.1.4

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <pgsql-admin(at)postgresql(dot)org>,"Thomas Markus" <t(dot)markus(at)proventis(dot)net>
Subject: Re: update performance 8.1.4
Date: 2008-01-18 23:04:54
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Lists: pgsql-admin
>>> On Tue, Jan 15, 2008 at  3:33 AM, in message <478C7DEC(dot)1020806(at)proventis(dot)net>,
Thomas Markus <t(dot)markus(at)proventis(dot)net> wrote: 
>   update tablename set datecol=null
> ~ 2.3 mio rows
> After 6 hours, this was still not finished. selects and inserts are ok.
> System is a dual xeon, 8gb ram, debian 64bit, pg 8.1.4
You do realize that every row updated is a delete and an insert,
right?  So this should take as long as a single database transaction
which deleted all the rows in the table and inserted them again.
If datecol is null in any rows, you should try:
update tablename set datecol=null where datecol is not null;
If you have canceled the update because of the long run time, you
will have left a lot of dead rows in the table, which will make the
next attempt run even longer.  Be sure you are doing vacuums.

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