Re: BUG #11025: could not access status of transaction 7

Sorry, I didn't put the code clearly.
I modified the pgstat_heap function, the SnapshotNow is changed to
HeapTupleSatisfiesMVCCCMDChkDB which is wrote by me.
Stack information and code
#0  SimpleLruReadPage (ctl=0x8464c60, pageno=43890, write_ok=1 '\001',
xid=1438198033) at slru.c:374
#1  0x080e8e4c in SimpleLruReadPage_ReadOnly (ctl=0x8464c60, pageno=43890,
xid=1438198033) at slru.c:494
#2  0x080c7d69 in TransactionIdGetStatus (xid=1, lsn=0xbfb59c7c) at
clog.c:184
#3  0x080c7f5f in TransactionLogFetch (transactionId=1438198033) at
transam.c:82
#4  0x080c8101 in TransactionIdDidCommit (transactionId=1438198033) at
transam.c:172
#5  0x0835997d in HeapTupleSatisfiesMVCCCMDChkDB (tuple=0xb10d6688,
snapshot=0x8461740, buffer=128176) at tqual.c:1481
#6  0x0831d689 in pgstat_heap (rel=0x6dbbffe0, fcinfo=0xbfb59f80) at
pgstattuple.c:671
#7  0x0831f257 in pgstattuplebyid (fcinfo=0xbfb59f80) at pgstattuple.c:583

pgstat_heap
        /* Disable syncscan because we assume we scan from block zero
upwards */
        scan = heap_beginscan_strat(rel, SnapshotAny, 0, NULL, true, false);

        *nblocks = scan->rs_nblocks; /* # blocks to be scanned */

        /* scan the relation */
        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
            /* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
            LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
            if (HeapTupleSatisfiesVisibility(tuple, SnapshotNow, scan->rs_cbuf))
            {
                stat.tuple_len += tuple->t_len;
                stat.tuple_count++;
            }

HeapTupleSatisfiesMVCCCMDChkDB
bool HeapTupleSatisfiesMVCCCMDChkDB(HeapTupleHeader tuple, Snapshot
snapshot,
                       Buffer buffer)
{
    TransactionId   xmin        = InvalidTransactionId;
    TransactionId   xmax        = InvalidTransactionId;
    bool            xminCommit  = false;
    bool            xmaxCommit  = false;
    bool            xminInvalid  = false;
    bool            xmaxInvalid  = false;

    volatile BufferDesc *bufHdr = NULL;

    xmin = HeapTupleHeaderGetXmin(tuple);
    xmax = HeapTupleHeaderGetXmax(tuple);

    if (InvalidTransactionId == xmin)
    {
         xminInvalid = true;
    }
    else
    {
         xminCommit = TransactionIdDidCommit(xmin);
    }

    if (InvalidTransactionId == xmax)
    {
         xmaxInvalid = true;
    }
    else
    {
         xmaxCommit = TransactionIdDidCommit(xmax);
    }



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