Hi, We're a team from Rome University (Italy) and we are working on an hacking of PostgreSQL MVCC. The basic idea is to have multiple instances of a same user transaction concurrently executing against the DB in order to achieve fault tolerance. We do not want to bother you with the details of our idea, but of course we'd be delighted to
provide you additional information wheter you were interested or even just curious! We're writing to you since we're studying the current MVCC implementation and we're stuck with a doubt that's driving us crazy since a couple of weeks so we'd be extremely grateful to anybody who could enlighten us. Our amlethic question is: tqual.c, HeapTupleSatisfies* (Self,Now etc). Let's focus on HeapTupleSatisfiesItself for simplicity. The piece of code that's unclear is the following: 144: else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(tuple)))145: {146: if (tuple->t_infomask & HEAP_XMAX_INVALID) /* xid invalid
*/147: return true;148: 149: if (tuple->t_infomask & HEAP_IS_LOCKED) /* not deleter */150: return true;151: 152: Assert(!(tuple->t_infomask & HEAP_XMAX_IS_MULTI));153: 154: /* deleting subtransaction aborted? */155:
if (TransactionIdDidAbort(HeapTupleHeaderGetXmax(tuple)))156: {157: tuple->t_infomask |= HEAP_XMAX_INVALID;158: SetBufferCommitInfoNeedsSave(buffer);159: return true; 160: }161: 162: Assert(TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmax(tuple)));163: 164: return false;165: } 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. This seems correct according to the comment in line 154, which refers to a "subtransaction". Are we understanding correctly what this code is doing and the related underlying MVCC mechanisms? |
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