On Wed, 20 Nov 2002, Medi Montaseri wrote:
> Its my understanding that vacuum actually removes tuples that have been
> updated or deleted.
> Sort of like emptying your trash .... whence a tuple has been removed,
> no rollback can set the
> state back. If you have logically removed a tuple (not vacuumed yet),
> then one can rollback,
> but if you vacuum then you can not rollback.
>
> Now suppose transaction A decides to delete some tuples, a vacuum job
> comes along and
> deletes things (in parallel), trans A decides to rollback....engines who
> support parallel
> vacuum-ing and transactions such as PG 7.2 better have a way of
> protecting themselves
> against this....
>
> Correct me if ...
Yes, you are wrong. Postgresql's vacuuming does NOT free tuples that are
still in a transaction, hence a full vacuum will hand waiting for the
transaction to complete or roll back. A normal 7.2 vacuum will simply
skip the in transaction tuples.
For proof, try this: (Note A> and B> are used to represent two different
sessions)
A> create table test (a text, id int);
A> insert into test (a,id) values ('abc',123);
A> begin;
A> delete from test where id=123;
B> vacuum;
A> rollback;
A> select * from test;
a | id
--------
abc| 123
Still there.
>
> Neil Conway wrote:
>
> >Medi Montaseri <medi.montaseri@intransa.com> writes:
> >
> >
> >>I think from the data integrity point of view, vacuum is more
> >>important than vacuum full.
> >>
> >>
> >
> >Why would VACUUM have any effect on data integrity, either positive or
> >negative?
> >
> >Cheers,
> >
> >Neil
> >
> >
> >
>
>
>
>
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