PG vacuum可见性

Posted 2021-03-03

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

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;
}