PG vacuum可見性

分兩種狀況，一是XMIN事務未提交，一個是xmin事務已提交。
針對xmin未提交的事務，即當前的tuple剛insert還未提交：
1）元組不合法，即壞元組時，返回HEAPTUPLE_DEAD
2）該tuple是當前事務產生的：此時這個記錄在這個事務未刪除或只是被鎖住或進行了delete可是delete abort了，那返回HAPTUPLE_INSERT_IN_PROGRESS；若則記錄又被刪除了，那返回HEAPTUPLE_DELETE_IN_PROGRESS
3）該tuple是其餘事務產生的，tuple頭中標記未提交：HEAP_INSERT_IN_POGRESS
4）該tuple是其餘事務產生，clog中顯示xmin已提交：則標記tuple頭爲HEAP_XMIN_COMMITTED。
5）其餘狀況下，這個xmin事務確實未提交，abort或損壞了：返回HEAPTUPLE_DEAD
針對xmin已提交的事務，即當前的tuple insert已提交了：
1）xmax未提交，返回HEAPTUPLE_LIVE
2）tuple只是被鎖着：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：標記tuple頭HEAP_XMAX_COMMITTED
5）其餘狀況delete確實未提交：HEAPTUPLE_LIVE
6）剩下的狀況，tuple.t_xmax >= OldestXmin表示有事務還能看到insert的值：HEAPTUPLE_RECENTLY_DEAD
7）其餘狀況XMAX已提交了：返回HEAPTUPLE_DEAD

HeapTupleSatisfiesVacuum
{
    HeapTupleHeader tuple = htup->t_data;

    if (!HeapTupleHeaderXminCommitted(tuple))//未提交
    {
        if (HeapTupleHeaderXminInvalid(tuple))//元組不合法，壞的元組
            return HEAPTUPLE_DEAD;
        else if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmin(tuple)))
        {//當前事務產生的元組
            if (tuple->t_infomask & HEAP_XMAX_INVALID)//未被刪除，當前事務正在insert
                return HEAPTUPLE_INSERT_IN_PROGRESS;
            //元組被鎖住，未被刪除 或者 
            if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) || HeapTupleHeaderIsOnlyLocked(tuple))
                return HEAPTUPLE_INSERT_IN_PROGRESS;
            //insert後當前事務又刪除
            if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(tuple)))
                return HEAPTUPLE_DELETE_IN_PROGRESS;
            //delete的子事務被abort，即insert正在進行
            return HEAPTUPLE_INSERT_IN_PROGRESS;
        }
        else if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmin(tuple)))
        {//其餘事務正在insert
            return HEAPTUPLE_INSERT_IN_PROGRESS;
        }//從clog中讀取tuple的事務狀態，爲提交則標記提交
        else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))
            SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,HeapTupleHeaderGetRawXmin(tuple));
        else
        {
            //真的tuple未提交、abort或者損壞
            SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,InvalidTransactionId);
            return HEAPTUPLE_DEAD;
        }
        //此時，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;//未被刪除則該記錄是活的

    if (HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask))
    {//只是鎖住，未進行delete
        if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED))
        {//delete事務未提交
            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正在進行
        else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmax(tuple)))//從clog中檢查delete提交了
            SetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,HeapTupleHeaderGetRawXmax(tuple));
        else
        {
            //真的未提交、abort或者損壞
            SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,InvalidTransactionId);
            return HEAPTUPLE_LIVE;
        }
        //此時delete已提交
    }
    //有事務還可能看到他
    //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;
}