From: | "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com> |
---|---|
To: | Rakesh Kumar <rakeshkumar464a3(at)gmail(dot)com>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Trying to understand page structures in PG |
Date: | 2016-04-06 14:00:20 |
Message-ID: | 57051674.4000002@commandprompt.com |
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Thread: | |
Lists: | pgsql-general |
On 04/06/2016 02:39 AM, Rakesh Kumar wrote:
> Hello
>
> I understand that when an update of say 100,000 rows are made, PG
> writes the updated rows as a new row. These new rows are not visible
> to any sessions except the one creating it. At commit time PG flips
> something internally to make these rows visible to all.
>
> My Q: what happens to those rows which use to contain the values
> before the update. Shouldn't something change in those rows to
> indicate that those rows are no longer valid. Who does it chain those
> rows to the new rows.
They are marked dead and at a threshold vacuum will come along
automatically and mark them reusable.
The vacuum and maintenance docs explain this pretty well.
Sincerely,
JD
>
> thanks.
>
>
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