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 Shouldn't something change in those rows to indicate that those rows are no longer valid.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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