对PostgreSQL xmin的深入学习

当PostgreSQL需要insert 一条记录的时候，它会把记录头放入xmin,xmax等字段。

xmin的值，就是当前的Transaction的TransactionId。这是为了满足MVCC的需要。

跟踪程序进行了解：

/*
 * Allocate the next XID for a new transaction or subtransaction.
 *
 * The new XID is also stored into MyProc before returning.
 *
 * Note: when this is called, we are actually already inside a valid
 * transaction, since XIDs are now not allocated until the transaction
 * does something.    So it is safe to do a database lookup if we want to
 * issue a warning about XID wrap.
 */
TransactionId
GetNewTransactionId(bool isSubXact)
{
    TransactionId xid;

    /*
     * During bootstrap initialization, we return the special bootstrap
     * transaction id.
     */
    if (IsBootstrapProcessingMode())
    {
        Assert(!isSubXact);
        MyProc->xid = BootstrapTransactionId;
        return BootstrapTransactionId;
    }

    /* safety check, we should never get this far in a HS slave */
    if (RecoveryInProgress())
        elog(ERROR, "cannot assign TransactionIds during recovery");

    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

    xid = ShmemVariableCache->nextXid;

    //fprintf(stderr,"In GetNewTransactionId--------1, xid is :%d\n",xid);

    /*----------
     * Check to see if it's safe to assign another XID.  This protects against
     * catastrophic data loss due to XID wraparound.  The basic rules are:
     *
     * If we're past xidVacLimit, start trying to force autovacuum cycles.
     * If we're past xidWarnLimit, start issuing warnings.
     * If we're past xidStopLimit, refuse to execute transactions, unless
     * we are running in a standalone backend (which gives an escape hatch
     * to the DBA who somehow got past the earlier defenses).
     *----------
     */
    if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
    {
        /*
         * For safety's sake, we release XidGenLock while sending signals,
         * warnings, etc.  This is not so much because we care about
         * preserving concurrency in this situation, as to avoid any
         * possibility of deadlock while doing get_database_name(). First,
         * copy all the shared values we'll need in this path.
         */
        TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
        TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
        TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
        Oid            oldest_datoid = ShmemVariableCache->oldestXidDB;

        LWLockRelease(XidGenLock);

        /*
         * To avoid swamping the postmaster with signals, we issue the autovac
         * request only once per 64K transaction starts.  This still gives
         * plenty of chances before we get into real trouble.
         */
        if (IsUnderPostmaster && (xid % ) == )
            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

        if (IsUnderPostmaster &&
            TransactionIdFollowsOrEquals(xid, xidStopLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
                                oldest_datname),
                         errhint("Stop the postmaster and use a standalone backend to vacuum that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
            else
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
                                oldest_datoid),
                         errhint("Stop the postmaster and use a standalone backend to vacuum that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
        }
        else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(WARNING,
                        (errmsg("database \"%s\" must be vacuumed within %u transactions",
                                oldest_datname,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
            else
                ereport(WARNING,
                        (errmsg("database with OID %u must be vacuumed within %u transactions",
                                oldest_datoid,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
        }

        /* Re-acquire lock and start over */
        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
        xid = ShmemVariableCache->nextXid;
    }

    /*
     * If we are allocating the first XID of a new page of the commit log,
     * zero out that commit-log page before returning. We must do this while
     * holding XidGenLock, else another xact could acquire and commit a later
     * XID before we zero the page.  Fortunately, a page of the commit log
     * holds 32K or more transactions, so we don't have to do this very often.
     *
     * Extend pg_subtrans too.
     */
    ExtendCLOG(xid);
    ExtendSUBTRANS(xid);

    /*
     * Now advance the nextXid counter.  This must not happen until after we
     * have successfully completed ExtendCLOG() --- if that routine fails, we
     * want the next incoming transaction to try it again.    We cannot assign
     * more XIDs until there is CLOG space for them.
     */
    TransactionIdAdvance(ShmemVariableCache->nextXid);

    /*
     * We must store the new XID into the shared ProcArray before releasing
     * XidGenLock.    This ensures that every active XID older than
     * latestCompletedXid is present in the ProcArray, which is essential for
     * correct OldestXmin tracking; see src/backend/access/transam/README.
     *
     * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are
     * relying on fetch/store of an xid to be atomic, else other backends
     * might see a partially-set xid here.    But holding both locks at once
     * would be a nasty concurrency hit.  So for now, assume atomicity.
     *
     * Note that readers of PGPROC xid fields should be careful to fetch the
     * value only once, rather than assume they can read a value multiple
     * times and get the same answer each time.
     *
     * The same comments apply to the subxact xid count and overflow fields.
     *
     * A solution to the atomic-store problem would be to give each PGPROC its
     * own spinlock used only for fetching/storing that PGPROC's xid and
     * related fields.
     *
     * If there's no room to fit a subtransaction XID into PGPROC, set the
     * cache-overflowed flag instead.  This forces readers to look in
     * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
     * race-condition window, in that the new XID will not appear as running
     * until its parent link has been placed into pg_subtrans. However, that
     * will happen before anyone could possibly have a reason to inquire about
     * the status of the XID, so it seems OK.  (Snapshots taken during this
     * window *will* include the parent XID, so they will deliver the correct
     * answer later on when someone does have a reason to inquire.)
     */
    {
        /*
         * Use volatile pointer to prevent code rearrangement; other backends
         * could be examining my subxids info concurrently, and we don't want
         * them to see an invalid intermediate state, such as incrementing
         * nxids before filling the array entry.  Note we are assuming that
         * TransactionId and int fetch/store are atomic.
         */
        volatile PGPROC *myproc = MyProc;

        if (!isSubXact)
            myproc->xid = xid;
        else
        {
            int            nxids = myproc->subxids.nxids;

            if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
            {
                myproc->subxids.xids[nxids] = xid;
                myproc->subxids.nxids = nxids + ;
            }
            else
                myproc->subxids.overflowed = true;
        }
    }

    LWLockRelease(XidGenLock);

    //fprintf(stderr,"In GetNewTransactionId--------2, xid is :%d\n",xid);

    return xid;
}

函数 GetNewTransactionId 为 AssignTransactionId 调用：

/*
 * AssignTransactionId
 *
 * Assigns a new permanent XID to the given TransactionState.
 * We do not assign XIDs to transactions until/unless this is called.
 * Also, any parent TransactionStates that don't yet have XIDs are assigned
 * one; this maintains the invariant that a child transaction has an XID
 * following its parent's.
 */
static void
AssignTransactionId(TransactionState s)
{

    fprintf(stderr,"---------------------In AssignTransactionId\n");
    bool        isSubXact = (s->parent != NULL);
    ResourceOwner currentOwner;

    /* Assert that caller didn't screw up */
    Assert(!TransactionIdIsValid(s->transactionId));
    Assert(s->state == TRANS_INPROGRESS);

    /*
     * Ensure parent(s) have XIDs, so that a child always has an XID later
     * than its parent.  Musn't recurse here, or we might get a stack overflow
     * if we're at the bottom of a huge stack of subtransactions none of which
     * have XIDs yet.
     */
    if (isSubXact && !TransactionIdIsValid(s->parent->transactionId))
    {
        TransactionState p = s->parent;
        TransactionState *parents;
        size_t        parentOffset = ;

        parents = palloc(sizeof(TransactionState) * s->nestingLevel);
        while (p != NULL && !TransactionIdIsValid(p->transactionId))
        {
            parents[parentOffset++] = p;
            p = p->parent;
        }

        /*
         * This is technically a recursive call, but the recursion will never
         * be more than one layer deep.
         */
        while (parentOffset != )
            AssignTransactionId(parents[--parentOffset]);

        pfree(parents);
    }

    /*
     * Generate a new Xid and record it in PG_PROC and pg_subtrans.
     *
     * NB: we must make the subtrans entry BEFORE the Xid appears anywhere in
     * shared storage other than PG_PROC; because if there's no room for it in
     * PG_PROC, the subtrans entry is needed to ensure that other backends see
     * the Xid as "running".  See GetNewTransactionId.
     */
    s->transactionId = GetNewTransactionId(isSubXact);

    fprintf(stderr,"In AssignTransactionId transaction is: %d \n",s->transactionId);

    if (isSubXact)
        SubTransSetParent(s->transactionId, s->parent->transactionId, false);

    /*
     * If it's a top-level transaction, the predicate locking system needs to
     * be told about it too.
     */
    if (!isSubXact)
        RegisterPredicateLockingXid(s->transactionId);

    /*
     * Acquire lock on the transaction XID.  (We assume this cannot block.) We
     * have to ensure that the lock is assigned to the transaction's own
     * ResourceOwner.
     */
    currentOwner = CurrentResourceOwner;
    PG_TRY();
    {
        CurrentResourceOwner = s->curTransactionOwner;
        XactLockTableInsert(s->transactionId);
    }
    PG_CATCH();
    {
        /* Ensure CurrentResourceOwner is restored on error */
        CurrentResourceOwner = currentOwner;
        PG_RE_THROW();
    }
    PG_END_TRY();
    CurrentResourceOwner = currentOwner;

    /*
     * Every PGPROC_MAX_CACHED_SUBXIDS assigned transaction ids within each
     * top-level transaction we issue a WAL record for the assignment. We
     * include the top-level xid and all the subxids that have not yet been
     * reported using XLOG_XACT_ASSIGNMENT records.
     *
     * This is required to limit the amount of shared memory required in a hot
     * standby server to keep track of in-progress XIDs. See notes for
     * RecordKnownAssignedTransactionIds().
     *
     * We don't keep track of the immediate parent of each subxid, only the
     * top-level transaction that each subxact belongs to. This is correct in
     * recovery only because aborted subtransactions are separately WAL
     * logged.
     */
    if (isSubXact && XLogStandbyInfoActive())
    {
        unreportedXids[nUnreportedXids] = s->transactionId;
        nUnreportedXids++;

        /*
         * ensure this test matches similar one in
         * RecoverPreparedTransactions()
         */
        if (nUnreportedXids >= PGPROC_MAX_CACHED_SUBXIDS)
        {
            XLogRecData rdata[];
            xl_xact_assignment xlrec;

            /*
             * xtop is always set by now because we recurse up transaction
             * stack to the highest unassigned xid and then come back down
             */
            xlrec.xtop = GetTopTransactionId();
            Assert(TransactionIdIsValid(xlrec.xtop));
            xlrec.nsubxacts = nUnreportedXids;

            rdata[].data = (char *) &xlrec;
            rdata[].len = MinSizeOfXactAssignment;
            rdata[].buffer = InvalidBuffer;
            rdata[].next = &rdata[];

            rdata[].data = (char *) unreportedXids;
            rdata[].len = PGPROC_MAX_CACHED_SUBXIDS * sizeof(TransactionId);
            rdata[].buffer = InvalidBuffer;
            rdata[].next = NULL;

            (void) XLogInsert(RM_XACT_ID, XLOG_XACT_ASSIGNMENT, rdata);

            nUnreportedXids = ;
        }
    }
}

而  AssignTransactionId 函数，为  所调用

/*
 *    GetCurrentTransactionId
 *
 * This will return the XID of the current transaction (main or sub
 * transaction), assigning one if it's not yet set.  Be careful to call this
 * only inside a valid xact.
 */
TransactionId
GetCurrentTransactionId(void)
{
    TransactionState s = CurrentTransactionState;
    if (!TransactionIdIsValid(s->transactionId))
        fprintf(stderr,"transaction id invalid.......\n");
    else
        fprintf(stderr,"transaction id OK!.......\n");
    if (!TransactionIdIsValid(s->transactionId))
        AssignTransactionId(s);
    return s->transactionId;
}

而 GetCurrentTransactionId 函数为 heap_insert  函数所调用：

/*
 *    heap_insert        - insert tuple into a heap
 *
 * The new tuple is stamped with current transaction ID and the specified
 * command ID.
 *
 * If the HEAP_INSERT_SKIP_WAL option is specified, the new tuple is not
 * logged in WAL, even for a non-temp relation.  Safe usage of this behavior
 * requires that we arrange that all new tuples go into new pages not
 * containing any tuples from other transactions, and that the relation gets
 * fsync'd before commit.  (See also heap_sync() comments)
 *
 * The HEAP_INSERT_SKIP_FSM option is passed directly to
 * RelationGetBufferForTuple, which see for more info.
 *
 * Note that these options will be applied when inserting into the heap's
 * TOAST table, too, if the tuple requires any out-of-line data.
 *
 * The BulkInsertState object (if any; bistate can be NULL for default
 * behavior) is also just passed through to RelationGetBufferForTuple.
 *
 * The return value is the OID assigned to the tuple (either here or by the
 * caller), or InvalidOid if no OID.  The header fields of *tup are updated
 * to match the stored tuple; in particular tup->t_self receives the actual
 * TID where the tuple was stored.    But note that any toasting of fields
 * within the tuple data is NOT reflected into *tup.
 */
Oid
heap_insert(Relation relation, HeapTuple tup, CommandId cid,
            int options, BulkInsertState bistate)
{

    fprintf(stderr,"In heap_insert------------------------------1\n");

    TransactionId xid = GetCurrentTransactionId();

    fprintf(stderr,"xid is :%d.......\n",(int)xid);

    HeapTuple    heaptup;
    Buffer        buffer;
    bool        all_visible_cleared = false;

    if (relation->rd_rel->relhasoids)
    {
#ifdef NOT_USED
        /* this is redundant with an Assert in HeapTupleSetOid */
        Assert(tup->t_data->t_infomask & HEAP_HASOID);
#endif

        /*
         * If the object id of this tuple has already been assigned, trust the
         * caller.    There are a couple of ways this can happen.  At initial db
         * creation, the backend program sets oids for tuples. When we define
         * an index, we set the oid.  Finally, in the future, we may allow
         * users to set their own object ids in order to support a persistent
         * object store (objects need to contain pointers to one another).
         */
        if (!OidIsValid(HeapTupleGetOid(tup)))
            HeapTupleSetOid(tup, GetNewOid(relation));
    }
    else
    {
        /* check there is not space for an OID */
        Assert(!(tup->t_data->t_infomask & HEAP_HASOID));
    }

    tup->t_data->t_infomask &= ~(HEAP_XACT_MASK);
    tup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);
    tup->t_data->t_infomask |= HEAP_XMAX_INVALID;
    HeapTupleHeaderSetXmin(tup->t_data, xid);
    HeapTupleHeaderSetCmin(tup->t_data, cid);
    HeapTupleHeaderSetXmax(tup->t_data, );        /* for cleanliness */
    tup->t_tableOid = RelationGetRelid(relation);

    /*
     * If the new tuple is too big for storage or contains already toasted
     * out-of-line attributes from some other relation, invoke the toaster.
     *
     * Note: below this point, heaptup is the data we actually intend to store
     * into the relation; tup is the caller's original untoasted data.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(tup));
        heaptup = tup;
    }
    else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
        heaptup = toast_insert_or_update(relation, tup, NULL, options);
    else
        heaptup = tup;

    /*
     * We're about to do the actual insert -- but check for conflict first,
     * to avoid possibly having to roll back work we've just done.
     *
     * For a heap insert, we only need to check for table-level SSI locks.
     * Our new tuple can't possibly conflict with existing tuple locks, and
     * heap page locks are only consolidated versions of tuple locks; they do
     * not lock "gaps" as index page locks do.  So we don't need to identify
     * a buffer before making the call.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    /* Find buffer to insert this tuple into */
    buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
                                       InvalidBuffer, options, bistate);

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    RelationPutHeapTuple(relation, buffer, heaptup);

    if (PageIsAllVisible(BufferGetPage(buffer)))
    {
        all_visible_cleared = true;
        PageClearAllVisible(BufferGetPage(buffer));
    }

    /*
     * XXX Should we set PageSetPrunable on this page ?
     *
     * The inserting transaction may eventually abort thus making this tuple
     * DEAD and hence available for pruning. Though we don't want to optimize
     * for aborts, if no other tuple in this page is UPDATEd/DELETEd, the
     * aborted tuple will never be pruned until next vacuum is triggered.
     *
     * If you do add PageSetPrunable here, add it in heap_xlog_insert too.
     */

    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
    {
        xl_heap_insert xlrec;
        xl_heap_header xlhdr;
        XLogRecPtr    recptr;
        XLogRecData rdata[];
        Page        page = BufferGetPage(buffer);
        uint8        info = XLOG_HEAP_INSERT;

        xlrec.all_visible_cleared = all_visible_cleared;
        xlrec.target.node = relation->rd_node;
        xlrec.target.tid = heaptup->t_self;
        rdata[].data = (char *) &xlrec;
        rdata[].len = SizeOfHeapInsert;
        rdata[].buffer = InvalidBuffer;
        rdata[].next = &(rdata[]);

        xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
        xlhdr.t_infomask = heaptup->t_data->t_infomask;
        xlhdr.t_hoff = heaptup->t_data->t_hoff;

        /*
         * note we mark rdata[1] as belonging to buffer; if XLogInsert decides
         * to write the whole page to the xlog, we don't need to store
         * xl_heap_header in the xlog.
         */
        rdata[].data = (char *) &xlhdr;
        rdata[].len = SizeOfHeapHeader;
        rdata[].buffer = buffer;
        rdata[].buffer_std = true;
        rdata[].next = &(rdata[]);

        /* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
        rdata[].data = (char *) heaptup->t_data + offsetof(HeapTupleHeaderData, t_bits);
        rdata[].len = heaptup->t_len - offsetof(HeapTupleHeaderData, t_bits);
        rdata[].buffer = buffer;
        rdata[].buffer_std = true;
        rdata[].next = NULL;

        /*
         * If this is the single and first tuple on page, we can reinit the
         * page instead of restoring the whole thing.  Set flag, and hide
         * buffer references from XLogInsert.
         */
        if (ItemPointerGetOffsetNumber(&(heaptup->t_self)) == FirstOffsetNumber &&
            PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
        {
            info |= XLOG_HEAP_INIT_PAGE;
            rdata[].buffer = rdata[].buffer = InvalidBuffer;
        }

        recptr = XLogInsert(RM_HEAP_ID, info, rdata);

        PageSetLSN(page, recptr);
        PageSetTLI(page, ThisTimeLineID);
    }

    END_CRIT_SECTION();

    UnlockReleaseBuffer(buffer);

    /* Clear the bit in the visibility map if necessary */
    if (all_visible_cleared)
        visibilitymap_clear(relation,
                            ItemPointerGetBlockNumber(&(heaptup->t_self)));

    /*
     * If tuple is cachable, mark it for invalidation from the caches in case
     * we abort.  Note it is OK to do this after releasing the buffer, because
     * the heaptup data structure is all in local memory, not in the shared
     * buffer.
     */
    CacheInvalidateHeapTuple(relation, heaptup);

    pgstat_count_heap_insert(relation);

    /*
     * If heaptup is a private copy, release it.  Don't forget to copy t_self
     * back to the caller's image, too.
     */
    if (heaptup != tup)
    {
        tup->t_self = heaptup->t_self;
        heap_freetuple(heaptup);
    }

    fprintf(stderr,"In heap_insert------------------------------2\n");
    return HeapTupleGetOid(tup);
}

而heap_insert 函数为   所调用：

/* ----------------------------------------------------------------
 *        ExecInsert
 *
 *        For INSERT, we have to insert the tuple into the target relation
 *        and insert appropriate tuples into the index relations.
 *
 *        Returns RETURNING result if any, otherwise NULL.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecInsert(TupleTableSlot *slot,
           TupleTableSlot *planSlot,
           EState *estate,
           bool canSetTag)
{
    HeapTuple    tuple;
    ResultRelInfo *resultRelInfo;
    Relation    resultRelationDesc;
    Oid            newId;
    List       *recheckIndexes = NIL;

    /**
    if (slot == NULL)
        fprintf(stderr,"---In ExecInsert...slot is null\n");
    else
        fprintf(stderr,"---In ExecInsert...slot is not null\n");

    fprintf(stderr,"IN ExecInsert-----------------------------100\n");

    if (slot->tts_isempty)
        fprintf(stderr,"slot. tts_isempty!\n");
    else
    {
        fprintf(stderr,"slot, tts not empty!\n");

        HeapTuple htp = slot->tts_tuple;

        if (htp == NULL)
            fprintf(stderr,"htp is NULL\n");
        else
            fprintf(stderr,"htp is NOT NULL\n");
    }

    fprintf(stderr,"--------------------------------------------\n");
    */
    //////////////////////////

    /*
     * get the heap tuple out of the tuple table slot, making sure we have a
     * writable copy
     */
    tuple = ExecMaterializeSlot(slot);

    fprintf(stderr,"--------------------------------------------150\n");

    if (slot->tts_isempty)
        fprintf(stderr,"slot. tts_isempty!\n");
    else
    {
        ///fprintf(stderr,"slot, tts not empty!\n");

        HeapTuple htp = slot->tts_tuple;

        HeapTupleHeader theader = htp->t_data;

        if (theader == NULL)
            fprintf(stderr,"heap tuple header is NULL\n");
        else{

            ////fprintf(stderr,"heap tuple header is NOT NULL\n");

            HeapTupleFields htfds = theader->t_choice.t_heap;

            TransactionId txmin = htfds.t_xmin;
            TransactionId txmax = htfds.t_xmax;

            fprintf(stderr,"t_xmin is :%d ", (int)txmin );
            fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

        }
    }

    /*
     * get information on the (current) result relation
     */
    resultRelInfo = estate->es_result_relation_info;
    resultRelationDesc = resultRelInfo->ri_RelationDesc;

    /*
     * If the result relation has OIDs, force the tuple's OID to zero so that
     * heap_insert will assign a fresh OID.  Usually the OID already will be
     * zero at this point, but there are corner cases where the plan tree can
     * return a tuple extracted literally from some table with the same
     * rowtype.
     *
     * XXX if we ever wanted to allow users to assign their own OIDs to new
     * rows, this'd be the place to do it.  For the moment, we make a point of
     * doing this before calling triggers, so that a user-supplied trigger
     * could hack the OID if desired.
     */
    if (resultRelationDesc->rd_rel->relhasoids)
        HeapTupleSetOid(tuple, InvalidOid);

    /* BEFORE ROW INSERT Triggers */
    if (resultRelInfo->ri_TrigDesc &&
        resultRelInfo->ri_TrigDesc->trig_insert_before_row)
    {
        slot = ExecBRInsertTriggers(estate, resultRelInfo, slot);

        if (slot == NULL)        /* "do nothing" */
            return NULL;

        /* trigger might have changed tuple */
        tuple = ExecMaterializeSlot(slot);
    }

    fprintf(stderr,"--------------------------------------------200\n");

    if (slot->tts_isempty)
        fprintf(stderr,"slot. tts_isempty!\n");
    else
    {
        ///fprintf(stderr,"slot, tts not empty!\n");

        HeapTuple htp = slot->tts_tuple;

        HeapTupleHeader theader = htp->t_data;

        if (theader == NULL)
            fprintf(stderr,"heap tuple header is NULL\n");
        else{

            ////fprintf(stderr,"heap tuple header is NOT NULL\n");

            HeapTupleFields htfds = theader->t_choice.t_heap;

            TransactionId txmin = htfds.t_xmin;
            TransactionId txmax = htfds.t_xmax;

            fprintf(stderr,"t_xmin is :%d ", (int)txmin );
            fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

        }
    }

    /* INSTEAD OF ROW INSERT Triggers */
    if (resultRelInfo->ri_TrigDesc &&
        resultRelInfo->ri_TrigDesc->trig_insert_instead_row)
    {
        ////fprintf(stderr,"x--------------------------1\n");
        slot = ExecIRInsertTriggers(estate, resultRelInfo, slot);

        if (slot == NULL)        /* "do nothing" */
            return NULL;

        /* trigger might have changed tuple */
        tuple = ExecMaterializeSlot(slot);
        newId = InvalidOid;
    }
    else
    {
        fprintf(stderr,"x--------------------------2\n");
        /*
         * Check the constraints of the tuple
         */
        if (resultRelationDesc->rd_att->constr)
            ExecConstraints(resultRelInfo, slot, estate);

        fprintf(stderr,"IN ExecInsert-----------------------------210\n");

        if (slot->tts_isempty)
            fprintf(stderr,"slot. tts_isempty!\n");
        else
        {
            ///fprintf(stderr,"slot, tts not empty!\n");
            HeapTuple htp = slot->tts_tuple;

            HeapTupleHeader theader = htp->t_data;

            if (theader == NULL)
                fprintf(stderr,"heap tuple header is NULL\n");
            else{
               ///fprintf(stderr,"heap tuple header is NOT NULL\n");
                HeapTupleFields htfds = theader->t_choice.t_heap;

                TransactionId txmin = htfds.t_xmin;
                TransactionId txmax = htfds.t_xmax;

                fprintf(stderr,"t_xmin is :%d ", (int)txmin );
                fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

                /**
                if ( htfds == NULL )
                    fprintf(stderr,"t_heap is NULL\n");
                else
                    fprintf(stderr,"t_heap is not NULL\n");
                */

            }
        }

        /*
         * insert the tuple
         *
         * Note: heap_insert returns the tid (location) of the new tuple in
         * the t_self field.
         */
        newId = heap_insert(resultRelationDesc, tuple,
                            estate->es_output_cid, , NULL);

        fprintf(stderr,"IN ExecInsert-----------------------------230\n");

        if (slot->tts_isempty)
            fprintf(stderr,"slot. tts_isempty!\n");
        else
        {
            ///fprintf(stderr,"slot, tts not empty!\n");

            HeapTuple htp = slot->tts_tuple;

            HeapTupleHeader theader = htp->t_data;

            if (theader == NULL)
                fprintf(stderr,"heap tuple header is NULL\n");
            else{
                ///fprintf(stderr,"heap tuple header is NOT NULL\n");
                HeapTupleFields htfds = theader->t_choice.t_heap;
                TransactionId txmin = htfds.t_xmin;
                TransactionId txmax = htfds.t_xmax;

                fprintf(stderr,"t_xmin is :%d ", (int)txmin );
                fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

                /**
                if ( htfds == NULL )
                    fprintf(stderr,"t_heap is NULL\n");
                else
                    fprintf(stderr,"t_heap is not NULL\n");
                */
            }
        }

        /*
         * insert index entries for tuple
         */
        if (resultRelInfo->ri_NumIndices > )
            recheckIndexes = ExecInsertIndexTuples(slot, &(tuple->t_self),
                                                   estate);
    }

    fprintf(stderr,"IN ExecInsert-----------------------------250\n");

    if (slot->tts_isempty)
        fprintf(stderr,"slot. tts_isempty!\n");
    else
    {
        ///fprintf(stderr,"slot, tts not empty!\n");
        HeapTuple htp = slot->tts_tuple;
        HeapTupleHeader theader = htp->t_data;

        if (theader == NULL)
            fprintf(stderr,"heap tuple header is NULL\n");
        else{
            ///fprintf(stderr,"heap tuple header is NOT NULL\n");
            HeapTupleFields htfds = theader->t_choice.t_heap;
            TransactionId txmin = htfds.t_xmin;
            TransactionId txmax = htfds.t_xmax;
            fprintf(stderr,"t_xmin is :%d ", (int)txmin );
            fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

            /**
            if ( htfds == NULL )
                fprintf(stderr,"t_heap is NULL\n");
            else
                fprintf(stderr,"t_heap is not NULL\n");
            */

        }
    }

    if (canSetTag)
    {
        (estate->es_processed)++;
        estate->es_lastoid = newId;
        setLastTid(&(tuple->t_self));
    }

    /* AFTER ROW INSERT Triggers */
    ExecARInsertTriggers(estate, resultRelInfo, tuple, recheckIndexes);

    fprintf(stderr,"IN ExecInsert-----------------------------300\n");

    if (slot->tts_isempty)
        fprintf(stderr,"slot. tts_isempty!\n");
    else
    {
        ///fprintf(stderr,"slot, tts not empty!\n");
        HeapTuple htp = slot->tts_tuple;
        HeapTupleHeader theader = htp->t_data;
        if (theader == NULL)
            fprintf(stderr,"heap tuple header is NULL\n");
        else{
            ///fprintf(stderr,"heap tuple header is NOT NULL\n");
            HeapTupleFields htfds = theader->t_choice.t_heap;

            TransactionId txmin = htfds.t_xmin;
            TransactionId txmax = htfds.t_xmax;

            fprintf(stderr,"t_xmin is :%d ", (int)txmin );
            fprintf(stderr,"t_xmax is :%d \n", (int)txmax );

            /**
            if ( htfds == NULL )
                fprintf(stderr,"t_heap is NULL\n");
            else
                fprintf(stderr,"t_heap is not NULL\n");
            */
        }
    }

    list_free(recheckIndexes);

    /* Process RETURNING if present */
    if (resultRelInfo->ri_projectReturning)
        return ExecProcessReturning(resultRelInfo->ri_projectReturning,
                                    slot, planSlot);

    return NULL;
}

接着要看这个：

typedef struct VariableCacheData
{
    /*
     * These fields are protected by OidGenLock.
     */
    Oid            nextOid;        /* next OID to assign */
    uint32        oidCount;        /* OIDs available before must do XLOG work */

    /*
     * These fields are protected by XidGenLock.
     */
    TransactionId nextXid;        /* next XID to assign */

    TransactionId oldestXid;    /* cluster-wide minimum datfrozenxid */
    TransactionId xidVacLimit;    /* start forcing autovacuums here */
    TransactionId xidWarnLimit; /* start complaining here */
    TransactionId xidStopLimit; /* refuse to advance nextXid beyond here */
    TransactionId xidWrapLimit; /* where the world ends */
    Oid            oldestXidDB;    /* database with minimum datfrozenxid */

    /*
     * These fields are protected by ProcArrayLock.
     */
    TransactionId latestCompletedXid;    /* newest XID that has committed or
                                         * aborted */
} VariableCacheData;

typedef VariableCacheData *VariableCache;

现在已经知道，xmin是从 ShmemVairableCache->nextXid得来。

而且，即使数据库重新启动后，xmin也能在上一次的基础上继续增加。

可以知道，对其应当是进行了初始化。

而xmin的来源是 checkPoint数据，它是从数据库文件中来。

目前所知：调用关系

PostmasterMain-->StartupDataBase

StartupDataBase是一个宏，展开后是 ： StartChildProcess(StartupProcess)

StartupProcess-->StartupXLOG

在StartupXLOG中，有如下的代码：

        /*
         * Get the last valid checkpoint record.  If the latest one according
         * to pg_control is broken, try the next-to-last one.
         */
        checkPointLoc = ControlFile->checkPoint;
        RedoStartLSN = ControlFile->checkPointCopy.redo;
        record = ReadCheckpointRecord(checkPointLoc, );

    ShmemVariableCache->nextXid = checkPoint.nextXid;
    ShmemVariableCache->nextOid = checkPoint.nextOid;

为何说 ShmemVariableCache是在共享内存中呢，下面代码会有所启示：

/*
 *    InitShmemAllocation() --- set up shared-memory space allocation.
 *
 * This should be called only in the postmaster or a standalone backend.
 */
void
InitShmemAllocation(void)
{

    fprintf(stderr,"In InitShmemAllocation.....start by process %d\n",getpid());

    PGShmemHeader *shmhdr = ShmemSegHdr;

    Assert(shmhdr != NULL);

    /*
     * Initialize the spinlock used by ShmemAlloc.    We have to do the space
     * allocation the hard way, since obviously ShmemAlloc can't be called
     * yet.
     */
    ShmemLock = (slock_t *) (((char *) shmhdr) + shmhdr->freeoffset);
    shmhdr->freeoffset += MAXALIGN(sizeof(slock_t));
    Assert(shmhdr->freeoffset <= shmhdr->totalsize);

    SpinLockInit(ShmemLock);

    /* ShmemIndex can't be set up yet (need LWLocks first) */
    shmhdr->index = NULL;
    ShmemIndex = (HTAB *) NULL;

    /*
     * Initialize ShmemVariableCache for transaction manager. (This doesn't
     * really belong here, but not worth moving.)
     */
    ShmemVariableCache = (VariableCache)
        ShmemAlloc(sizeof(*ShmemVariableCache));
    memset(ShmemVariableCache, , sizeof(*ShmemVariableCache));

    fprintf(stderr,"When Initiating, nextXid is: %d \n", (int)ShmemVariableCache->nextXid);

    fprintf(stderr,"In InitShmemAllocation.....end by process %d\n\n",getpid());
}

而ShmemAlloc是关键：

/*
 * ShmemAlloc -- allocate max-aligned chunk from shared memory
 *
 * Assumes ShmemLock and ShmemSegHdr are initialized.
 *
 * Returns: real pointer to memory or NULL if we are out
 *        of space.  Has to return a real pointer in order
 *        to be compatible with malloc().
 */
void *
ShmemAlloc(Size size)
{
    Size        newStart;
    Size        newFree;
    void       *newSpace;

    /* use volatile pointer to prevent code rearrangement */
    volatile PGShmemHeader *shmemseghdr = ShmemSegHdr;

    /*
     * ensure all space is adequately aligned.
     */
    size = MAXALIGN(size);

    Assert(shmemseghdr != NULL);

    SpinLockAcquire(ShmemLock);

    newStart = shmemseghdr->freeoffset;

    /* extra alignment for large requests, since they are probably buffers */
    if (size >= BLCKSZ)
        newStart = BUFFERALIGN(newStart);

    newFree = newStart + size;
    if (newFree <= shmemseghdr->totalsize)
    {
        newSpace = (void *) ((char *) ShmemBase + newStart);
        shmemseghdr->freeoffset = newFree;
    }
    else
        newSpace = NULL;

    SpinLockRelease(ShmemLock);

    if (!newSpace)
        ereport(WARNING,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory")));

    return newSpace;
}

/* shared memory global variables */

static PGShmemHeader *ShmemSegHdr;        /* shared mem segment header */

static void *ShmemBase;            /* start address of shared memory */

static void *ShmemEnd;            /* end+1 address of shared memory */

slock_t    *ShmemLock;            /* spinlock for shared memory and LWLock
                                 * allocation */

static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */

/*
 *    InitShmemAccess() --- set up basic pointers to shared memory.
 *
 * Note: the argument should be declared "PGShmemHeader *seghdr",
 * but we use void to avoid having to include ipc.h in shmem.h.
 */
void
InitShmemAccess(void *seghdr)
{
    PGShmemHeader *shmhdr = (PGShmemHeader *) seghdr;

    ShmemSegHdr = shmhdr;
    ShmemBase = (void *) shmhdr;
    ShmemEnd = (char *) ShmemBase + shmhdr->totalsize;
}

数据库系统启动的时候的情形已经有所了解了。那么运行中，transaction id 是如何递增的呢。如果我运行两次 GetNewTransactionId，就可以发现 transactionid 每次加2了。

/*
 * AssignTransactionId
 *
 * Assigns a new permanent XID to the given TransactionState.
 * We do not assign XIDs to transactions until/unless this is called.
 * Also, any parent TransactionStates that don't yet have XIDs are assigned
 * one; this maintains the invariant that a child transaction has an XID
 * following its parent's.
 */
static void
AssignTransactionId(TransactionState s)
{

    fprintf(stderr,"************---------------------In AssignTransactionId..start by process %d\n",getpid());
    bool        isSubXact = (s->parent != NULL);
    ResourceOwner currentOwner;

    /* Assert that caller didn't screw up */
    Assert(!TransactionIdIsValid(s->transactionId));
    Assert(s->state == TRANS_INPROGRESS);

    /*
     * Ensure parent(s) have XIDs, so that a child always has an XID later
     * than its parent.  Musn't recurse here, or we might get a stack overflow
     * if we're at the bottom of a huge stack of subtransactions none of which
     * have XIDs yet.
     */
    if (isSubXact && !TransactionIdIsValid(s->parent->transactionId))
    {
        TransactionState p = s->parent;
        TransactionState *parents;
        size_t        parentOffset = ;

        parents = palloc(sizeof(TransactionState) * s->nestingLevel);
        while (p != NULL && !TransactionIdIsValid(p->transactionId))
        {
            parents[parentOffset++] = p;
            p = p->parent;
        }

        /*
         * This is technically a recursive call, but the recursion will never
         * be more than one layer deep.
         */
        while (parentOffset != )
            AssignTransactionId(parents[--parentOffset]);

        pfree(parents);
    }

    /*
     * Generate a new Xid and record it in PG_PROC and pg_subtrans.
     *
     * NB: we must make the subtrans entry BEFORE the Xid appears anywhere in
     * shared storage other than PG_PROC; because if there's no room for it in
     * PG_PROC, the subtrans entry is needed to ensure that other backends see
     * the Xid as "running".  See GetNewTransactionId.
     */
    s->transactionId = GetNewTransactionId(isSubXact);

　　 ////#####added by gaojian
    fprintf(stderr,"In AssignTransactionId ....1.... transaction is: %d \n",s->transactionId);

    s->transactionId = GetNewTransactionId(isSubXact);
    fprintf(stderr,"In AssignTransactionId ....2.... transaction is: %d \n",s->transactionId);

    /////#####added by gaojian

    if (isSubXact)
        SubTransSetParent(s->transactionId, s->parent->transactionId, false);

    /*
     * If it's a top-level transaction, the predicate locking system needs to
     * be told about it too.
     */
    if (!isSubXact)
        RegisterPredicateLockingXid(s->transactionId);

    /*
     * Acquire lock on the transaction XID.  (We assume this cannot block.) We
     * have to ensure that the lock is assigned to the transaction's own
     * ResourceOwner.
     */
    currentOwner = CurrentResourceOwner;
    PG_TRY();
    {
        CurrentResourceOwner = s->curTransactionOwner;
        XactLockTableInsert(s->transactionId);
    }
    PG_CATCH();
    {
        /* Ensure CurrentResourceOwner is restored on error */
        CurrentResourceOwner = currentOwner;
        PG_RE_THROW();
    }
    PG_END_TRY();
    CurrentResourceOwner = currentOwner;

    /*
     * Every PGPROC_MAX_CACHED_SUBXIDS assigned transaction ids within each
     * top-level transaction we issue a WAL record for the assignment. We
     * include the top-level xid and all the subxids that have not yet been
     * reported using XLOG_XACT_ASSIGNMENT records.
     *
     * This is required to limit the amount of shared memory required in a hot
     * standby server to keep track of in-progress XIDs. See notes for
     * RecordKnownAssignedTransactionIds().
     *
     * We don't keep track of the immediate parent of each subxid, only the
     * top-level transaction that each subxact belongs to. This is correct in
     * recovery only because aborted subtransactions are separately WAL
     * logged.
     */
    if (isSubXact && XLogStandbyInfoActive())
    {
        unreportedXids[nUnreportedXids] = s->transactionId;
        nUnreportedXids++;

        /*
         * ensure this test matches similar one in
         * RecoverPreparedTransactions()
         */
        if (nUnreportedXids >= PGPROC_MAX_CACHED_SUBXIDS)
        {
            XLogRecData rdata[];
            xl_xact_assignment xlrec;

            /*
             * xtop is always set by now because we recurse up transaction
             * stack to the highest unassigned xid and then come back down
             */
            xlrec.xtop = GetTopTransactionId();
            Assert(TransactionIdIsValid(xlrec.xtop));
            xlrec.nsubxacts = nUnreportedXids;

            rdata[].data = (char *) &xlrec;
            rdata[].len = MinSizeOfXactAssignment;
            rdata[].buffer = InvalidBuffer;
            rdata[].next = &rdata[];

            rdata[].data = (char *) unreportedXids;
            rdata[].len = PGPROC_MAX_CACHED_SUBXIDS * sizeof(TransactionId);
            rdata[].buffer = InvalidBuffer;
            rdata[].next = NULL;

            (void) XLogInsert(RM_XACT_ID, XLOG_XACT_ASSIGNMENT, rdata);

            nUnreportedXids = ;
        }
    }

    fprintf(stderr,"---------------------In AssignTransactionId end..by process %d\n\n",getpid());
}

关键在这个：GetNewTransactionId函数中，调用了 TransactionIdAdvance(ShmemVariableCache->nextXid)

/*
 * Allocate the next XID for a new transaction or subtransaction.
 *
 * The new XID is also stored into MyProc before returning.
 *
 * Note: when this is called, we are actually already inside a valid
 * transaction, since XIDs are now not allocated until the transaction
 * does something.    So it is safe to do a database lookup if we want to
 * issue a warning about XID wrap.
 */
TransactionId
GetNewTransactionId(bool isSubXact)
{
    fprintf(stderr,"*********In GetNewTransactionId.....start by process %d\n",getpid());

    TransactionId xid;

    /*
     * During bootstrap initialization, we return the special bootstrap
     * transaction id.
     */
    if (IsBootstrapProcessingMode())
    {
        Assert(!isSubXact);
        MyProc->xid = BootstrapTransactionId;
        return BootstrapTransactionId;
    }

    /* safety check, we should never get this far in a HS slave */
    if (RecoveryInProgress())
        elog(ERROR, "cannot assign TransactionIds during recovery");

    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

    xid = ShmemVariableCache->nextXid;

    fprintf(stderr,"In GetNewTransactionId--------1, xid is :%d\n",xid);

    /*----------
     * Check to see if it's safe to assign another XID.  This protects against
     * catastrophic data loss due to XID wraparound.  The basic rules are:
     *
     * If we're past xidVacLimit, start trying to force autovacuum cycles.
     * If we're past xidWarnLimit, start issuing warnings.
     * If we're past xidStopLimit, refuse to execute transactions, unless
     * we are running in a standalone backend (which gives an escape hatch
     * to the DBA who somehow got past the earlier defenses).
     *----------
     */
    if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
    {
        /*
         * For safety's sake, we release XidGenLock while sending signals,
         * warnings, etc.  This is not so much because we care about
         * preserving concurrency in this situation, as to avoid any
         * possibility of deadlock while doing get_database_name(). First,
         * copy all the shared values we'll need in this path.
         */
        TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
        TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
        TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
        Oid            oldest_datoid = ShmemVariableCache->oldestXidDB;

        LWLockRelease(XidGenLock);

        /*
         * To avoid swamping the postmaster with signals, we issue the autovac
         * request only once per 64K transaction starts.  This still gives
         * plenty of chances before we get into real trouble.
         */
        if (IsUnderPostmaster && (xid % ) == )
            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

        if (IsUnderPostmaster &&
            TransactionIdFollowsOrEquals(xid, xidStopLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
                                oldest_datname),
                         errhint("Stop the postmaster and use a standalone backend to vacuum that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
            else
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
                                oldest_datoid),
                         errhint("Stop the postmaster and use a standalone backend to vacuum that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
        }
        else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);

            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(WARNING,
                        (errmsg("database \"%s\" must be vacuumed within %u transactions",
                                oldest_datname,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
            else
                ereport(WARNING,
                        (errmsg("database with OID %u must be vacuumed within %u transactions",
                                oldest_datoid,
                                xidWrapLimit - xid),
                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions.")));
        }

        /* Re-acquire lock and start over */
        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
        xid = ShmemVariableCache->nextXid;
    }

    /*
     * If we are allocating the first XID of a new page of the commit log,
     * zero out that commit-log page before returning. We must do this while
     * holding XidGenLock, else another xact could acquire and commit a later
     * XID before we zero the page.  Fortunately, a page of the commit log
     * holds 32K or more transactions, so we don't have to do this very often.
     *
     * Extend pg_subtrans too.
     */
    ExtendCLOG(xid);
    ExtendSUBTRANS(xid);

    /*
     * Now advance the nextXid counter.  This must not happen until after we
     * have successfully completed ExtendCLOG() --- if that routine fails, we
     * want the next incoming transaction to try it again.    We cannot assign
     * more XIDs until there is CLOG space for them.
     */
    TransactionIdAdvance(ShmemVariableCache->nextXid);

   /*
     * We must store the new XID into the shared ProcArray before releasing
     * XidGenLock.    This ensures that every active XID older than
     * latestCompletedXid is present in the ProcArray, which is essential for
     * correct OldestXmin tracking; see src/backend/access/transam/README.
     *
     * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are
     * relying on fetch/store of an xid to be atomic, else other backends
     * might see a partially-set xid here.    But holding both locks at once
     * would be a nasty concurrency hit.  So for now, assume atomicity.
     *
     * Note that readers of PGPROC xid fields should be careful to fetch the
     * value only once, rather than assume they can read a value multiple
     * times and get the same answer each time.
     *
     * The same comments apply to the subxact xid count and overflow fields.
     *
     * A solution to the atomic-store problem would be to give each PGPROC its
     * own spinlock used only for fetching/storing that PGPROC's xid and
     * related fields.
     *
     * If there's no room to fit a subtransaction XID into PGPROC, set the
     * cache-overflowed flag instead.  This forces readers to look in
     * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
     * race-condition window, in that the new XID will not appear as running
     * until its parent link has been placed into pg_subtrans. However, that
     * will happen before anyone could possibly have a reason to inquire about
     * the status of the XID, so it seems OK.  (Snapshots taken during this
     * window *will* include the parent XID, so they will deliver the correct
     * answer later on when someone does have a reason to inquire.)
     */
    {
        /*
         * Use volatile pointer to prevent code rearrangement; other backends
         * could be examining my subxids info concurrently, and we don't want
         * them to see an invalid intermediate state, such as incrementing
         * nxids before filling the array entry.  Note we are assuming that
         * TransactionId and int fetch/store are atomic.
         */
        volatile PGPROC *myproc = MyProc;

        if (!isSubXact)
            myproc->xid = xid;
        else
        {
            int            nxids = myproc->subxids.nxids;

            if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
            {
                myproc->subxids.xids[nxids] = xid;
                myproc->subxids.nxids = nxids + ;
            }
            else
                myproc->subxids.overflowed = true;
        }
    }

    LWLockRelease(XidGenLock);

    fprintf(stderr,"****************In GetNewTransactionId...xid is:%d..end by process %d\n\n",xid,getpid());

    return xid;
}