对PostgreSQL xmax的理解

xmax

The identity (transaction ID) of the deleting transaction, or zero for an undeleted row version. It is possible for this column to be nonzero in a visible row version. That usually indicates that the deleting transaction hasn't committed yet, or that an attempted deletion was rolled back.

http://www.postgresql.org/docs/9.1/static/ddl-system-columns.html

作一个实验：

我数据库关闭前，最后一条transaction的id是 1874。

我开一个终端A，此时终端A的当前transactionId为 1875。

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# begin;
BEGIN

pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
( rows)

pgsql=# 

我再开一个终端B，此时，终端B的transactionId为：1876。

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# begin;
BEGIN
pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
( rows)

pgsql=# 

回到终端A，执行 delete 操作：

pgsql=# delete from tab01 where id=;
DELETE
pgsql=# 

此时，在终端A中，已经看不到删除后的数据：

pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
( rows)

pgsql=# 

此时，由于终端A尚未提交，所以，可以在终端B中看到如下的情形：

pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |  |   |
( rows)

pgsql=# 

也就是说，id为9的那条记录，其xmax为1875，表明其为 transactionid为 1875的事务所删除。

回到终端A，进行提交：

pgsql=# commit;
COMMIT
pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
( rows)

pgsql=# 

再回到终端B，查看：

pgsql=# select xmin,xmax,* from tab01;
 xmin | xmax | id | cd
------+------+----+----
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
  |     |   |
( rows)

pgsql=# 

让我来再进一步，看一看：

重新开两个终端：

终端A和终端B。

在终端A中：

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
 xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
  |     |     |     |   |
  |     |     |     |   |
( rows)

pgsql=# \q

可以看到两条由不同的事务提交所形成的记录。

然后再次使用psql: 经过update 后，自己所看到的是 xmin的变化，这时尚未提交，别的终端看到就不一样了。

终端A:

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# begin;
BEGIN
pgsql=# update tab01 set id= where cd='';
UPDATE
pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
 xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
  |     |     |     |   |
  |     |     |     |   |
( rows)

pgsql=# 

此时，进入终端B:

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
 xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
  |     |     |     |   |
  |  |     |     |   |
( rows)

pgsql=# 

我推测，update的时候，自身事务所看到的是内存中的影像。同时它也已经提交到了物理文件上。而别的事务是从物理文件来读取的。

在A终端提交以后：

pgsql=# commit;
COMMIT
pgsql=# 

B终端所看到的：

pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
 xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
  |     |     |     |   |
  |     |     |     |   |
( rows)

pgsql=# 

继续从代码上进行分析：

/*
 *    heap_update - replace a tuple
 *
 * NB: do not call this directly unless you are prepared to deal with
 * concurrent-update conditions.  Use simple_heap_update instead.
 *
 *    relation - table to be modified (caller must hold suitable lock)
 *    otid - TID of old tuple to be replaced
 *    newtup - newly constructed tuple data to store
 *    ctid - output parameter, used only for failure case (see below)
 *    update_xmax - output parameter, used only for failure case (see below)
 *    cid - update command ID (used for visibility test, and stored into
 *        cmax/cmin if successful)
 *    crosscheck - if not InvalidSnapshot, also check old tuple against this
 *    wait - true if should wait for any conflicting update to commit/abort
 *
 * Normal, successful return value is HeapTupleMayBeUpdated, which
 * actually means we *did* update it.  Failure return codes are
 * HeapTupleSelfUpdated, HeapTupleUpdated, or HeapTupleBeingUpdated
 * (the last only possible if wait == false).
 *
 * On success, the header fields of *newtup are updated to match the new
 * stored tuple; in particular, newtup->t_self is set to the TID where the
 * new tuple was inserted, and its HEAP_ONLY_TUPLE flag is set iff a HOT
 * update was done.  However, any TOAST changes in the new tuple's
 * data are not reflected into *newtup.
 *
 * In the failure cases, the routine returns the tuple's t_ctid and t_xmax.
 * If t_ctid is the same as otid, the tuple was deleted; if different, the
 * tuple was updated, and t_ctid is the location of the replacement tuple.
 * (t_xmax is needed to verify that the replacement tuple matches.)
 */
HTSU_Result
heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
            ItemPointer ctid, TransactionId *update_xmax,
            CommandId cid, Snapshot crosscheck, bool wait)
{
    HTSU_Result result;
    TransactionId xid = GetCurrentTransactionId();
    Bitmapset  *hot_attrs;
    ItemId        lp;
    HeapTupleData oldtup;
    HeapTuple    heaptup;
    Page        page;
    Buffer        buffer,
                newbuf;
    bool        need_toast,
                already_marked;
    Size        newtupsize,
                pagefree;
    bool        have_tuple_lock = false;
    bool        iscombo;
    bool        use_hot_update = false;
    bool        all_visible_cleared = false;
    bool        all_visible_cleared_new = false;

    ...

    ////////////////First Phase marked by gaojian

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

    ...

    if (!already_marked)
    {
/* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_COMMITTED |
                                       HEAP_XMAX_INVALID |
                                       HEAP_XMAX_IS_MULTI |
                                       HEAP_IS_LOCKED |
                                       HEAP_MOVED);
        /* ... and store info about transaction updating this tuple */

        ///HeapTupleHeaderSetXmax(oldtup.t_data, xid);
        /////>>>>>added by gaojian for testing.
        ////xid = (TransactionId)8888;

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

        HeapTupleHeaderSetXmax(oldtup.t_data, xid);
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);

        sleep();
    }

    ...

    return HeapTupleMayBeUpdated;
}

可以看到，第一段的 ：

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

写的很明白， HeapTupleHeaderSetXmin(newtup->t_data, xid);

而第二段的 ：

    if (!already_marked)
    {
        /* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_COMMITTED |
                                       HEAP_XMAX_INVALID |
                                       HEAP_XMAX_IS_MULTI |
                                       HEAP_IS_LOCKED |
                                       HEAP_MOVED);
        /* ... and store info about transaction updating this tuple */

        HeapTupleHeaderSetXmax(oldtup.t_data, xid);
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
    }

然后，我再来验证一下，加点调试代码，这样，我执行sql时会出错：

    if (!already_marked)
    {
        /* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_COMMITTED |
                                       HEAP_XMAX_INVALID |
                                       HEAP_XMAX_IS_MULTI |
                                       HEAP_IS_LOCKED |
                                       HEAP_MOVED);
        /* ... and store info about transaction updating this tuple */

        ///HeapTupleHeaderSetXmax(oldtup.t_data, xid);
        /////>>>>>added by gaojian for testing.
        ////xid = (TransactionId)8888;

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

        HeapTupleHeaderSetXmax(oldtup.t_data, xid);
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);

    }

执行结果：

pgsql=# begin;
BEGIN
pgsql=# update tab01 set id= where cd = '';
ERROR:  could not access status of transaction
DETAIL:  Could not read from file "pg_subtrans/0000" at offset : Success.
pgsql=# \q
[pgsql@loca

再次登陆进来看看，哈，这证明我所看到的代码部分，就是实现写入xmax的部分。

总结来说，就是，在update（可能delete也是），用向oldtup写入transaction id的方式，来记录哪个transaction改动了记录。

[pgsql@localhost bin]$ ./psql
psql (9.1.)
Type "help" for help.

pgsql=# select xmin,xmax,cmin,cmax, * from tab01;
 xmin | xmax | cmin | cmax |    id     | cd
------+------+------+------+-----------+----
  |     |     |     |          |
  |  |     |     |  |
  |     |     |     |          |
( rows)

pgsql=#