scott.marlowe wrote:
>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....
>>
>>
So I think you have shown that 7.2 achieves this by skiping current
transactions....
Thank you
>>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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>
>
>
