PostgreSQL vacuum可见性

分两种情况，一是XMIN事务未提交，一个是xmin事务已提交。

针对xmin未提交的事务，即当前的tuple刚insert还未提交：

1）元组不合法，即坏元组时，返回HEAPTUPLE_DEAD

2）该tuple是当前事务产生的：此时这个记录在这个事务未删除或只是被锁住或进行了delete但是delete abort了，那返回HAPTUPLE_INSERT_IN_PROGRESS；若则记录又被删除了，那返回HEAPTUPLE_DELETE_IN_PROGRESS

3）该tuple是其他事务产生的，tuple头中标记未提交：HEAP_INSERT_IN_POGRESS

4）该tuple是其他事务产生，clog中显示xmin已提交：则标记tuple头为HEAP_XMIN_COMMITTED。

5）其他情况下，这个xmin事务确实未提交，abort或损坏了：返回HEAPTUPLE_DEAD

针对xmin已提交的事务，即当前的tuple insert已提交了：

1）xmax未提交，返回HEAPTUPLE_LIVE

2）tuple只是被锁着：xmax未提交：返回HEAPTUPLE_LIVE

3）tuple正在delete：返回HEAPTUPLE_DELETE_IN_PROGRESS

4）clog中查到xmax已提交：标记tuple头HEAP_XMAX_COMMITTED

5）其他情况delete确实未提交：HEAPTUPLE_LIVE

6）剩下的情况，tuple.t_xmax >= OldestXmin表示有事务还能看到insert的值：HEAPTUPLE_RECENTLY_DEAD

7）其他情况XMAX已提交了：返回HEAPTUPLE_DEAD

函数为：HeapTupleSatisfiesVacuum

HeapTupleSatisfiesVacuum
{
  HeapTupleHeader tuple = htup->t_data;

  if (!HeapTupleHeaderXminCommitted(tuple))//未提交
  {
    if (HeapTupleHeaderXminInvalid(tuple))//元组不合法，坏的元组
      return HEAPTUPLE_DEAD;
    else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))
    {//当前事务产生的元组
      if (tuple->t_infomask & HEAP_XMAX_INVALID)//未被删除，当前事务正在insert
        return HEAPTUPLE_INSERT_IN_PROGRESS;
      //元组被锁住，未被删除 或者 
      if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) || HeapTupleHeaderIsOnlyLocked(tuple))
        return HEAPTUPLE_INSERT_IN_PROGRESS;
      //insert后当前事务又删除
      if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tuple)))
        return HEAPTUPLE_DELETE_IN_PROGRESS;
      //delete的子事务被abort，即insert正在进行
      return HEAPTUPLE_INSERT_IN_PROGRESS;
    }
    else if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmin(tuple)))
    {//其他事务正在insert
      return HEAPTUPLE_INSERT_IN_PROGRESS;
    }//从clog中读取tuple的事务状态，为提交则标记提交
    else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))
      SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,HeapTupleHeaderGetRawXmin(tuple));
    else
    {
      //真的tuple未提交、abort或者损坏
      SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,InvalidTransactionId);
      return HEAPTUPLE_DEAD;
    }
    //此时，xmin已提交了。
  }

  /*
   * Okay, the inserter committed, so it was good at some point.  Now what
   * about the deleting transaction?
   */
  if (tuple->t_infomask & HEAP_XMAX_INVALID)
    return HEAPTUPLE_LIVE;//未被删除则该记录是活的

  if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask))
  {//只是锁住，未进行delete
    if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
    {//delete事务未提交
      if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmax(tuple)))
        return HEAPTUPLE_LIVE;
      SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
      
    }
    return HEAPTUPLE_LIVE;
  }
  //delete未提交
  if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
  {
    if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmax(tuple)))
      return HEAPTUPLE_DELETE_IN_PROGRESS;//delete正在进行
    else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple)))//从clog中检查delete提交了
      SetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,HeapTupleHeaderGetRawXmax(tuple));
    else
    {
      //真的未提交、abort或者损坏
      SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
      return HEAPTUPLE_LIVE;
    }
    //此时delete已提交
  }
  //有事务还可能看到他
  //tuple->t_choice.t_heap.t_xmax >= OldestXmin
  if (!TransactionIdPrecedes(HeapTupleHeaderGetRawXmax(tuple), OldestXmin))
    return HEAPTUPLE_RECENTLY_DEAD;
  /* Otherwise, it's dead and removable */
  return HEAPTUPLE_DEAD;
}