GBase 8s savepoint

        GBase 8s 在事务中支持（savepoint）保存点，可以在事务中实现类似于快照的功能。

可以使用rollback语句，将事务返回至该保存点的状态。

保存点的使用方法以及效果如下：

> create database t1_savepoint with log;

Database closed.

Database created.

> database t1_savepoint;

Database closed.

Database selected.

> begin;

Started transaction.

> create table t1(id int,name varchar(20));

Table created.

> insert into t1 values(1,'liming');

1 row(s) inserted.

> savepoint p1;

Savepoint set.

> select * from t1;

         id name                 

          1 liming              

1 row(s) retrieved.

> insert into t1 values(2,'lily');

1 row(s) inserted.

> select * from t1;

         id name                 

          1 liming              
          2 lily                

2 row(s) retrieved.

> rollback to savepoint p1;

Transaction rolled back to savepoint.

> select * from t1;

         id name                 

          1 liming              

1 row(s) retrieved.