# 一、数据分布

## 查看表的分布键方式

```sql
tpch1000s=# select c.relname, policytype, numsegments, distkey 
from pg_class c, pg_catalog.gp_distribution_policy gpp 
where c.oid = gpp.localoid;
 relname  | policytype | numsegments | distkey 
----------+------------+-------------+---------
 nation   | p          |         192 | 1
 region   | p          |         192 | 1
 part     | p          |         192 | 1
```

注：如果为 RANDOMLY分布，则 distkey为空。

## 查看数据分布

```sql
tpch1000s=# select gp_segment_id, count(*) from supplier group by gp_segment_id order by 1;
 gp_segment_id | count 
---------------+-------
             0 | 51716
             1 | 52062
             2 | 51880
```

## 数据分布倾斜程度分析

```sql
tpch1s=# select * from gp_toolkit.gp_skew_coefficients;
 skcoid | skcnamespace | skcrelname |         skccoeff         
--------+--------------+------------+--------------------------
  16386 | public       | nation     | 28.284271247461900800000
  16393 | public       | region     | 28.284271247461900976000
  16400 | public       | part       |  0.118793939239339984000
  16407 | public       | supplier   |  0.367695526217004712000
```

skccoeff 表示变异系数（coefficient of variation, CV）值，是标准差除以均值。值越小越好（分布越均匀），值越大表示分布越倾斜。
$$
{标准差\ } \sigma = \sum_{i=1}^N\sqrt{\frac{(x_i-\overline{x})^2}{n}}
$$

$$
{变异系数\ } C_V = \frac{\sigma}{\overline{x}}
$$

# 二、集群状态

## segment状态

```sql
tpch1000s=# SELECT * FROM gp_segment_configuration order by dbid;
 dbid | content | role | preferred_role | mode | status | port | hostname | address |         datadir          
------+---------+------+----------------+------+--------+------+----------+---------+--------------------------
    1 |      -1 | p    | p              | n    | u      | 7456 | test-4   | gp10g_7 | /opt/gp/gpmaster/gpseg-1
    2 |       0 | p    | p              | n    | u      | 6345 | test-1   | gp10g_4 | /opt/gp/gpdata/gpseg0
    3 |       1 | p    | p              | n    | u      | 6346 | test-1   | gp10g_4 | /opt/gp/gpdata/gpseg1
    4 |       2 | p    | p              | n    | u      | 6347 | test-1   | gp10g_4 | /opt/gp/gpdata/gpseg2
```

参考：[PostgreSQL、Greenplum 日常监控 和 维护任务](https://developer.aliyun.com/article/210407)

## 查询当前session的backend进程id

```sql
tpch1s=> select pg_backend_pid();
 pg_backend_pid 
----------------
           2543
```

可以用ps命令验证。

# 三、配置参数

## reload config

有些配置参数修改后，需要刷新参数值，即重新加载postgres.conf配置文件，但无需重启集群。则使用“gpstop -u”命令，例如：

```bash
[gpadmin@bogon ~]$ gpconfig -s gp_resqueue_memory_policy
Values on all segments are consistent
GUC          : gp_resqueue_memory_policy
Master  value: none
Segment value: none
[gpadmin@bogon ~]$ gpconfig -c gp_resqueue_memory_policy -v eager_free
……
[gpadmin@bogon ~]$ gpconfig -s gp_resqueue_memory_policy
Values on all segments are consistent
GUC          : gp_resqueue_memory_policy
Master  value: none
Segment value: none
[gpadmin@bogon ~]$ gpstop -u
……
[gpadmin@bogon ~]$ gpconfig -s gp_resqueue_memory_policy
Values on all segments are consistent
GUC          : gp_resqueue_memory_policy
Master  value: eager_free
Segment value: eager_free
```