Hi,
I have a couple questions about how update, truncate and vacuum would
work together.

1) If I update a table foo (id int, value numeric (20, 6))
with
update foo set value = 100 where id = 1

Would a vacuum be necessary after this type of operation since the
updated value is a numeric? (as opposed to a sql type where its size
could potentially change i.e varchar)

2) After several updates/deletes to a table, if I truncate it, would
it be necessary to run vacuum in order to reclaim the space?

thanks,
Scott

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> From: Scott Feldstein
> Subject: [PERFORM] update, truncate and vacuum
>
> Hi,
> I have a couple questions about how update, truncate and
> vacuum would work together.
>
> 1) If I update a table foo (id int, value numeric (20, 6))
> with update foo set value = 100 where id = 1
>
> Would a vacuum be necessary after this type of operation
> since the updated value is a numeric? (as opposed to a sql
> type where its size could potentially change i.e varchar)

Yes a vacuum is still necessary. The type doesn't really matter. Postgres
effectively does a delete and insert on all updates.


> 2) After several updates/deletes to a table, if I truncate
> it, would it be necessary to run vacuum in order to reclaim the space?

No a vacuum is not necessary after a truncate because the whole data file is
deleted once a truncate commits. There aren't any dead rows because there
aren't any rows.

Dave


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