Re: Rome university

letizia leo wrote:

> and the doubt is the following: how is it possible that -line 144- Xmin
> is the current transaction ( i.e. it has created this tuple, it is
> holding an exclusive lock on it since it has not committed yet) and
> that
> -line 149- there is a different (?) transaction that is also locking
> the
> tuple (HEAP_IS_LOCKED=(HEAP_XMAX_EXCL_LOCK||HEAP_XMAX_SHARED_LOCK) )?
> Unless we are missing something, this situation is possible exclusively
> in case the XMAX transaction is a subtransaction of XMIN, which can
> access the tuple despite the exclusive lock held by XMIN.

Exactly.

> This seems correct according to the comment in line 154, which refers
> to a "subtransaction".

It is correct.

> Are we understanding correctly what this code is doing and the related
> underlying MVCC mechanisms?

Can't say, but that part at least you got right :-)

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Alvaro Herrera http://www.CommandPrompt.com/
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