GBase 8a数据库支持在线扩容，其中重分布步骤rebalance，将老数据重分布到新的分布策略（distirbution id），在某些情况下，也可以通过rebalance to 将新策略数据，回退到老的策略上。
目录导航
参考
命令
rebalance {instance|database dbname|table tbname} [to distributionID]说明
该重分布命令，默认是将老的分布策略(distributionid 较小的)，重分布到新的(较大的)，在某些情况下，也存在要回退的可能性，比如随机分布表默认是快速重分布，但现场因为磁盘空间问题做的扩容，参数忘记设置了，而此时只有部分表做了重分布操作。后续有2个解决方案：
1、继续重分布，将所有操作做完，然后再新建一个distribution, 设置好参数，重新做一次重分布。
2、清掉现有任务，将已经重分布完成的表(data_distirbution_id已经变成新的),回退到老的版本。然后设置参数，重新开始重分布。
如果已经完成的任务比例很小，而整个重分布过程要耗时很久，比如几天，几星期，那么方案1将需要双倍的时间，此时方案2就成了更好的选择。
样例
清理现有任务，避免后续的重分布。
gbase> delete from rebalancing_status;
Query OK, 70 rows affected (Elapsed: 00:00:00.26)
gbase> select * from rebalancing_status;
Empty set (Elapsed: 00:00:00.01)
查看现有已经完成的表
如下看到有3个表的data_distribution_id=3。
gbase> select * from gbase.table_distribution order by data_distribution_id;;
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
| index_name | dbName | tbName | isReplicate | hash_column | lmt_storage_size | table_storage_size | is_nocopies | data_distribution_id | vc_id | mirror_vc_id |
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
| testdb.t1 | testdb | t1 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testmirror.t1 | testmirror | t1 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t2 | testdb | t2 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t3 | testdb | t3 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t_encrypt | testdb | t_encrypt | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
....
| gclusterdb.rebalancing_status | gclusterdb | rebalancing_status | NO | index_name | NULL | NULL | NO | 3 | vc00001 | NULL |
| gclusterdb.dual | gclusterdb | dual | YES | NULL | NULL | NULL | NO | 3 | vc00001 | NULL |
| testdb.t4 | testdb | t4 | NO | NULL | NULL | NULL | NO | 3 | vc00001 | |
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
82 rows in set (Elapsed: 00:00:00.00)回退到老的分布策略
通过rebalance table to 方法，将表回退到1，注意其中的gclusterdb.rebalancing_status不能手工回退。
gbase> rebalance table testdb.t4 to 1;
Query OK, 0 rows affected (Elapsed: 00:00:00.05)
gbase> rebalance table gclusterdb.dual to 1;
Query OK, 0 rows affected (Elapsed: 00:00:00.06)
gbase> rebalance table gclusterdb.rebalancing_status to 1;
ERROR 1707 (HY000): gcluster command error: system table can't be rebalanced.
gbase> select * from gbase.table_distribution order by data_distribution_id;;
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
| index_name | dbName | tbName | isReplicate | hash_column | lmt_storage_size | table_storage_size | is_nocopies | data_distribution_id | vc_id | mirror_vc_id |
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
| gclusterdb.dual | gclusterdb | dual | YES | NULL | NULL | NULL | NO | 1 | vc00001 | NULL |
| gclusterdb.rebalancing_status | gclusterdb | rebalancing_status | NO | index_name | NULL | NULL | NO | 1 | vc00001 | NULL |
| testdb.t1 | testdb | t1 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testmirror.t1 | testmirror | t1 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t2 | testdb | t2 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t3 | testdb | t3 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
| testdb.t4 | testdb | t4 | NO | NULL | NULL | NULL | NO | 1 | vc00001 | |
| testdb.t_encrypt | testdb | t_encrypt | NO | NULL | NULL | NULL | NO | 1 | vc00001 | vc00002 |
。。。。。
+------------------------------------+------------+-------------------------+-------------+-------------+------------------+--------------------+-------------+----------------------+---------+--------------+
82 rows in set (Elapsed: 00:00:00.00)再次清理重分布任务表
gbase> select * from gclusterdb.rebalancing_status;
+-----------------+------------+------------+----------+----------------------------+----------------------------+-----------+------------+----------+------------+-----------------+
| index_name | db_name | table_name | tmptable | start_time | end_time | status | percentage | priority | host | distribution_id |
+-----------------+------------+------------+----------+----------------------------+----------------------------+-----------+------------+----------+------------+-----------------+
| testdb.t4 | testdb | t4 | | 2021-03-22 15:52:12.401000 | 2021-03-22 15:52:12.809000 | COMPLETED | 100 | 5 | 10.0.2.101 | 1 |
| gclusterdb.dual | gclusterdb | dual | | 2021-03-22 15:52:42.051000 | 2021-03-22 15:52:42.515000 | COMPLETED | 100 | 5 | 10.0.2.101 | 1 |
+-----------------+------------+------------+----------+----------------------------+----------------------------+-----------+------------+----------+------------+-----------------+
2 rows in set (Elapsed: 00:00:00.02)
gbase> delete from rebalancing_status;
Query OK, 2 rows affected (Elapsed: 00:00:00.26)
gbase> select * from rebalancing_status;
Empty set (Elapsed: 00:00:00.01)重新开始重分布
参考重分布方案，不再重复。