对PostgreSQL xmax的理解

柔情只为你懂 2022-03-27 01:28 98阅读 0赞

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][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 (9.1.2)
    Type "help" for help.
    
    pgsql=# begin;
    BEGIN
    
    pgsql=# select xmin,xmax,* from tab01;
     xmin | xmax | id | cd 
    ------+------+----+----
     1866 |    0 |  3 | 3
     1867 |    0 |  4 | 4
     1868 |    0 |  5 | 5
     1870 |    0 |  6 | 6
     1872 |    0 |  7 | 7
     1873 |    0 |  8 | 8
     1874 |    0 |  9 | 9
    (7 rows)
    
    pgsql=#

我再开一个终端B，此时，终端B的transactionId为：1876。

[pgsql@localhost bin]$ ./psql
    psql (9.1.2)
    Type "help" for help.
    
    pgsql=# begin;
    BEGIN
    pgsql=# select xmin,xmax,* from tab01;
     xmin | xmax | id | cd 
    ------+------+----+----
     1866 |    0 |  3 | 3
     1867 |    0 |  4 | 4
     1868 |    0 |  5 | 5
     1870 |    0 |  6 | 6
     1872 |    0 |  7 | 7
     1873 |    0 |  8 | 8
     1874 |    0 |  9 | 9
    (7 rows)
    
    pgsql=#

回到终端A，执行 delete 操作：

pgsql=# delete from tab01 where id=9;
    DELETE 1
    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 
    ------+------+----+----
     1866 |    0 |  3 | 3
     1867 |    0 |  4 | 4
     1868 |    0 |  5 | 5
     1870 |    0 |  6 | 6
     1872 |    0 |  7 | 7
     1873 |    0 |  8 | 8
    (6 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 
    ------+------+----+----
     1866 |    0 |  3 | 3
     1867 |    0 |  4 | 4
     1868 |    0 |  5 | 5
     1870 |    0 |  6 | 6
     1872 |    0 |  7 | 7
     1873 |    0 |  8 | 8
     1874 | 1875 |  9 | 9
    (7 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 
    ------+------+----+----
     1866 |    0 |  3 | 3
     1867 |    0 |  4 | 4
     1868 |    0 |  5 | 5
     1870 |    0 |  6 | 6
     1872 |    0 |  7 | 7
     1873 |    0 |  8 | 8
    (6 rows)
    
    pgsql=#

再回到终端B，查看：

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 (9.1.2)
    Type "help" for help.
    
    pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
     xmin | xmax | cmin | cmax | id | cd 
    ------+------+------+------+----+----
     1878 |    0 |    0 |    0 |  1 | 1
     1879 |    0 |    0 |    0 |  2 | 2
    (2 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 (9.1.2)
    Type "help" for help.
    
    pgsql=# begin;
    BEGIN
    pgsql=# update tab01 set id=3 where cd='2';
    UPDATE 1
    pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
     xmin | xmax | cmin | cmax | id | cd 
    ------+------+------+------+----+----
     1878 |    0 |    0 |    0 |  1 | 1
     1880 |    0 |    0 |    0 |  3 | 2
    (2 rows)
    
    pgsql=#

此时，进入终端B:

[pgsql@localhost bin]$ ./psql
    psql (9.1.2)
    Type "help" for help.
    
    pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
     xmin | xmax | cmin | cmax | id | cd 
    ------+------+------+------+----+----
     1878 |    0 |    0 |    0 |  1 | 1
     1879 | 1880 |    0 |    0 |  2 | 2
    (2 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 
    ------+------+------+------+----+----
     1878 |    0 |    0 |    0 |  1 | 1
     1880 |    0 |    0 |    0 |  3 | 2
    (2 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, 0);    /* 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(60);
        }
    
        ...
    
        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, 0);    /* 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=2 where cd = '2';
    ERROR:  could not access status of transaction 8888
    DETAIL:  Could not read from file "pg_subtrans/0000" at offset 32768: 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 (9.1.2)
    Type "help" for help.
    
    pgsql=# select xmin,xmax,cmin,cmax, * from tab01;
     xmin | xmax | cmin | cmax |    id     | cd 
    ------+------+------+------+-----------+----
     1878 |    0 |    0 |    0 |         1 | 1
     1884 | 8888 |    0 |    0 | 999888777 | 2
     1885 |    0 |    0 |    0 |         2 | 2
    (3 rows)
    
    pgsql=#

[http_www.postgresql.org_docs_9.1_static_ddl-system-columns.html]: http://www.postgresql.org/docs/9.1/static/ddl-system-columns.html