对PostgreSQL cmin和cmax的理解

看例子：

开两个终端来对比：

在终端A:

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

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

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

pgsql=# 

此时的终端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=# 

然后再在终端A进行提交：

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

pgsql=# 

此时，再在终端B进行观察：

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

pgsql=# 

继续研究cmin是咋回事：

pgsql=# begin;
BEGIN
pgsql=# insert into tab01(id,cd) values(generate_series(,),'xx');
INSERT
pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
 xmin | xmax | cmin | cmax |    id     | cd
------+------+------+------+-----------+----
  |     |     |     |          |
  |     |     |     |  |
  |     |     |     |          |
  |     |     |     |          |
  |     |     |     |          | xx
  |     |     |     |          | xx
( rows)

pgsql=# 

可以说cmin可理解为一个事务里，执行了几次sql命令的顺序。

那么cmax呢？在前面的基础上继续执行，居然没有看到区别：

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

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

pgsql=# 

经过反复折腾，终于发现，其实 cmin和 cmax就是一个东西：

看源代码：

/* ----------------
 *        heap_getsysattr
 *
 *        Fetch the value of a system attribute for a tuple.
 *
 * This is a support routine for the heap_getattr macro.  The macro
 * has already determined that the attnum refers to a system attribute.
 * ----------------
 */
Datum
heap_getsysattr(HeapTuple tup, int attnum, TupleDesc tupleDesc, bool *isnull)
{
    Datum        result;

    Assert(tup);

    /* Currently, no sys attribute ever reads as NULL. */
    *isnull = false;

    switch (attnum)
    {
        case SelfItemPointerAttributeNumber:
            /* pass-by-reference datatype */
            result = PointerGetDatum(&(tup->t_self));
            break;
        case ObjectIdAttributeNumber:
            result = ObjectIdGetDatum(HeapTupleGetOid(tup));
            break;
        case MinTransactionIdAttributeNumber:
            result = TransactionIdGetDatum(HeapTupleHeaderGetXmin(tup->t_data));
            break;
        case MaxTransactionIdAttributeNumber:
            result = TransactionIdGetDatum(HeapTupleHeaderGetXmax(tup->t_data));
            break;
        case MinCommandIdAttributeNumber:
        case MaxCommandIdAttributeNumber:

            /*
             * cmin and cmax are now both aliases for the same field, which
             * can in fact also be a combo command id.    XXX perhaps we should
             * return the "real" cmin or cmax if possible, that is if we are
             * inside the originating transaction?
             */
            result = CommandIdGetDatum(HeapTupleHeaderGetRawCommandId(tup->t_data));
            break;
        case TableOidAttributeNumber:
            result = ObjectIdGetDatum(tup->t_tableOid);
            break;
        default:
            elog(ERROR, "invalid attnum: %d", attnum);
            result = ;            /* keep compiler quiet */
            break;
    }
    return result;
}