南大通用GBase 8a 从集群层通过segment_id直接查询分片数据排查倾斜

发表于 作者 laozizhu

GBase 8a 数据库集群，将数据按照用户指定的分布格式切分，保存在多台数据计算节点上。本文介绍在新版9.5.3版本里，如果通过segment_id直接查询某些计算节点的数据。

参考

GBase 8a数据倾斜，不同节点磁盘占用差距的几个原因

GBase8a 集群查看某张表占用的磁盘空间大小

限制

从V9.5.3版本才支持，该版本也同时支持多分片部署，可以参考【多实例有关文章】

背景

GBase 8a数据库集群的每个数据计算节点，都是一台数据库单机，负责本地计算和存储。老版本集群，可以通过直接连接计算节点(比如通过命令行工具gncli， -h参数指定IP）访问每个节点的分片数据，在V953版本里，引入了segment_id功能，可以直接访问对应的节点分片，简化了操作。

当然，如果该功能只用于方位当前服务中的分片，如为了查询备份分片，还是需要用命令行工具指定IP进行。

参数

数据库参数

gcluster_segment_id_replace

0：不启用segment_id功能，默认值

1：开启segnemtn_id功能

SQL语法

where segment_id=分片号

其中分片号，从1开始，详细可以看gcadmin showdistribution的输出

[gbase@gbase_rh7_003 ~]$ gcadmin showdistribution

                                 Distribution ID: 2 | State: new | Total segment num: 4

             Primary Segment Node IP                   Segment ID                 Duplicate Segment node IP
========================================================================================================================
|                   10.0.2.103                   |         1          |                   10.0.2.104                   |
------------------------------------------------------------------------------------------------------------------------
|                   10.0.2.104                   |         2          |                   10.0.2.105                   |
------------------------------------------------------------------------------------------------------------------------
|                   10.0.2.105                   |         3          |                   10.0.2.106                   |
------------------------------------------------------------------------------------------------------------------------
|                   10.0.2.106                   |         4          |                   10.0.2.103                   |
========================================================================================================================

样例

环境

4分片集群。

[gbase@gbase_rh7_003 ~]$ gcadmin
CLUSTER STATE:         ACTIVE
VIRTUAL CLUSTER MODE:  NORMAL

====================================
| GBASE GCWARE CLUSTER INFORMATION |
====================================
| NodeName |  IpAddress   | gcware |
------------------------------------
| gcware1  |  10.0.2.103  |  OPEN  |
------------------------------------
====================================================
|      GBASE COORDINATOR CLUSTER INFORMATION       |
====================================================
|   NodeName   | IpAddress  | gcluster | DataState |
----------------------------------------------------
| coordinator1 | 10.0.2.103 |   OPEN   |     0     |
----------------------------------------------------
=========================================================================================================
|                                    GBASE DATA CLUSTER INFORMATION                                     |
=========================================================================================================
| NodeName |                IpAddress                 | DistributionId | gnode | syncserver | DataState |
---------------------------------------------------------------------------------------------------------
|  node1   |                10.0.2.103                |       2        | OPEN  |    OPEN    |     0     |
---------------------------------------------------------------------------------------------------------
|  node2   |                10.0.2.104                |       2        | OPEN  |    OPEN    |     0     |
---------------------------------------------------------------------------------------------------------
|  node3   |                10.0.2.105                |       2        | OPEN  |    OPEN    |     0     |
---------------------------------------------------------------------------------------------------------
|  node4   |                10.0.2.106                |       2        | OPEN  |    OPEN    |     0     |
---------------------------------------------------------------------------------------------------------

设置参数

本例设置session级别，只对当前连接有效。

gbase> show variables like '%seg%';
+-------------------------------------------------+------------+
| Variable_name                                   | Value      |
+-------------------------------------------------+------------+
| _gbase_segment_size                             | 2147483648 |
| _t_gcluster_single_segment_cluster_optimization | 0          |
| gcluster_segment_id_replace                     | 0          |
+-------------------------------------------------+------------+
3 rows in set (Elapsed: 00:00:00.00)

gbase> set gcluster_segment_id_replace=1;
Query OK, 0 rows affected (Elapsed: 00:00:00.00)

数据

16行数据的hash分布表

gbase> select count(*) from t1;
+----------+
| count(*) |
+----------+
|       16 |
+----------+
1 row in set (Elapsed: 00:00:00.08)

gbase> show create table t1;
+-------+----------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                           |
+-------+----------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE "t1" (
  "id" int(11) DEFAULT NULL
) ENGINE=EXPRESS DISTRIBUTED BY('id') DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' |
+-------+----------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)

查询分片

通过segment_id在4个分片里查询。注意分片号不进行校验，如不存在则返回空。

gbase> select * from t1 where segment_id=1;
+------+
| id   |
+------+
|    4 |
|    7 |
|    6 |
|   12 |
|   10 |
|   16 |
|   11 |
+------+
7 rows in set (Elapsed: 00:00:00.01)

gbase> select * from t1 where segment_id=2;
+------+
| id   |
+------+
|    8 |
+------+
1 row in set (Elapsed: 00:00:00.01)

gbase> select * from t1 where segment_id=3;
+------+
| id   |
+------+
|    1 |
|    9 |
|   13 |
+------+
3 rows in set (Elapsed: 00:00:00.01)

gbase> select * from t1 where segment_id=4;
+------+
| id   |
+------+
|    2 |
|   15 |
|    3 |
|    5 |
|   14 |
+------+
5 rows in set (Elapsed: 00:00:00.01)

gbase> select * from t1 where segment_id=5;
Empty set (Elapsed: 00:00:00.01)

gbase> select * from t1 where segment_id=6;
Empty set (Elapsed: 00:00:00.01)

在查询结果里包含分片

在select 里包含分片号，比如

gbase> select segment_id,t1.* from t1;
+------------+------+
| segment_id | id   |
+------------+------+
|          1 |    4 |
|          1 |    7 |
|          1 |    6 |
|          1 |   12 |
|          1 |   10 |
|          1 |   16 |
|          1 |   11 |
|          4 |    2 |
|          4 |   15 |
|          4 |    3 |
|          4 |    5 |
|          4 |   14 |
|          3 |    1 |
|          3 |    9 |
|          3 |   13 |
|          2 |    8 |
+------------+------+
16 rows in set (Elapsed: 00:00:00.02)

多表时，指定表名字

gbase> select segment_id(t1),segment_id(t2),t1.* from t1,t2 where t1.id=t2.id;
+------------+------------+------+
| segment_id | segment_id | id   |
+------------+------------+------+
|          1 |          1 |    4 |
|          4 |          1 |    2 |
|          4 |          1 |    3 |
|          3 |          1 |    1 |
+------------+------------+------+
4 rows in set (Elapsed: 00:00:00.06)

统计各个分片的数据量

注意，此处返回的是正常分片的数据，用于排查数据倾斜很有用处。

gbase> select segment_id,count(*) from t1 group by segment_id;
+------------+----------+
| segment_id | count(*) |
+------------+----------+
|          4 |        5 |
|          1 |        7 |
|          2 |        1 |
|          3 |        3 |
+------------+----------+
4 rows in set (Elapsed: 00:00:00.17)

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