Greenplum创建表--分布键_PHP教程

Greenplum有两种分布策略：

1、hash分布。

Greenplum默认使用hash分布策略。该策略可选一个或者多个列作为分布键（distribution key，简称DK）。分布键做hash算法来确认数据存放到对应的segment上。相同分布键值会hash到相同的segment上。表上最好有唯一键或者主键，这样能保证数据均衡分不到各个segment上。语法，distributed by。

如果没有主键或者唯一键，默认选择第一列作为分布键。增加主键

2、随机（randomly）分布。

数据会被随机分不到segment上，相同记录可能会存放在不同的segment上。随机分布可以保证数据平均，但是Greenplum没有跨节点的唯一键约束数据，所以无法保证数据唯一。基于唯一性和性能考虑，推荐使用hash分布，性能部分会另开一篇文档详细介绍。语法，distributed randomly。

一、hash分布键

创建表，未指定分布列、分布类型，默认创建hash分布表，把第一列ID字段作为了分布键。

testDB=# create table t_hash(id int,name varchar(50)) distributed by (id);
CREATE TABLE
testDB=# 
 
testDB=# \d t_hash
           Table "public.t_hash"
 Column |         Type          | Modifiers 
--------+-----------------------+-----------
 id     | integer               | 
 name   | character varying(50) | 
Distributed by: (id)

添加主键后，主键升级为分布键替代了id列。

testDB=# alter table t_hash add primary key (name);
NOTICE:  updating distribution policy to match new primary key
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index "t_hash_pkey" for table "t_hash"
 
ALTER TABLE
testDB=# \d t_hash
           Table "public.t_hash"
 Column |         Type          | Modifiers 
--------+-----------------------+-----------
 id     | integer               | 
 name   | character varying(50) | not null
Indexes:
    "t_hash_pkey" PRIMARY KEY, btree (name)
Distributed by: (name)

验证hash分布表可实现主键或者唯一键值的唯一性

testDB=# insert into t_hash values(1,'szlsd1');
INSERT 0 1
testDB=#
testDB=# insert into t_hash values(2,'szlsd1');
ERROR:  duplicate key violates unique constraint "t_hash_pkey"(seg2 gp-s3:40000 pid=3855)

另外，主键列上依然能够创建唯一键

testDB=# create unique index u_id on t_hash(name);
CREATE INDEX
testDB=#
testDB=#
testDB=# \d t_hash
           Table "public.t_hash"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Indexes:
    "t_hash_pkey" PRIMARY KEY, btree (name)
    "u_id" UNIQUE, btree (name)
Distributed by: (name)

但是，非主键列无法单独创建唯一索引，想创建的话必须包含多有分布键列

testDB=#  create unique index uk_id on t_hash(id);
ERROR:  UNIQUE index must contain all columns in the distribution key of relation "t_hash"
testDB=#  create unique index uk_id on t_hash(id,name);
CREATE INDEX
testDB=# \d t_hash
           Table "public.t_hash"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Indexes:
    "t_hash_pkey" PRIMARY KEY, btree (name)
    "uk_id" UNIQUE, btree (id, name)
Distributed by: (name)

删除主键后，原hash分布键依然不变。

testDB=# alter table t_hash drop constraint t_hash_pkey;
ALTER TABLE
testDB=# \d t_hash
           Table "public.t_hash"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Distributed by: (name)

当分布键不是主键或者唯一键时，我们来验证分布键的相同值落在一个segment的结论。

下面的实验，name列是分布键，我们插入相同的name值，可以看到7条记录都落在了2号segment节点中。

testDB=#  insert into t_hash values(1,'szlsd');
INSERT 0 1
testDB=#  insert into t_hash values(2,'szlsd');
INSERT 0 1
testDB=#  insert into t_hash values(3,'szlsd');
INSERT 0 1
testDB=#  insert into t_hash values(4,'szlsd');
INSERT 0 1
testDB=#  insert into t_hash values(5,'szlsd');
INSERT 0 1
testDB=#  insert into t_hash values(6,'szlsd');
INSERT 0 1
testDB=#
testDB=#
testDB=# select gp_segment_id,count(*) from t_hash group by gp_segment_id; 
 gp_segment_id | count
---------------+-------
             2 |     7
(1 row)

二、随机分布键

创建随机分布表需加distributed randomly关键字，具体使用哪列作为分布键不得而知。

testDB=# create table t_random(id int ,name varchar(100)) distributed randomly;
CREATE TABLE
testDB=#
testDB=#
testDB=# \d t_random
           Table "public.t_random"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                |
 name   | character varying(100) |
Distributed randomly

验证主键/唯一键的唯一性，可以看到随机分布表不能创建主键和唯一键

testDB=# alter table t_random add primary key (id,name);
ERROR:  PRIMARY KEY and DISTRIBUTED RANDOMLY are incompatible
testDB=#
testDB=# create unique index uk_r_id on t_random(id);
ERROR:  UNIQUE and DISTRIBUTED RANDOMLY are incompatible
testDB=#

从实验中可以看出无法实现数据的唯一性。并且，数据插入随机分布表，并不是轮询插入，实验中共有3个segment，但是在1号插入3条记录，在2号segment节点插入2条记录后，才在0号segment中插入数据。随机分布表如何实现数据平均分配不得而知。这个实验也验证了随机分布表的相同值分布在不同segment的结论。

testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             1 |     1
(1 row)
 
testDB=#
testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             2 |     1
             1 |     1
(2 rows)
 
testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             2 |     1
             1 |     2
(2 rows)
 
testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             2 |     2
             1 |     2
(2 rows)
 
testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             2 |     2
             1 |     3
(2 rows)
 
testDB=# insert into t_random values(1,'szlsd3');
INSERT 0 1
testDB=# select gp_segment_id,count(*) from t_random group by gp_segment_id;
 gp_segment_id | count
---------------+-------
             2 |     2
             1 |     3
             0 |     1
(3 rows)

三、CTAS继承原表分布键

Greenplum中有两种CTAS语法，无论哪种语法，都默认继承原表的分布键。但是，不会继承表的一些特殊属性，如主键、唯一键、APPENDONLY、COMPRESSTYPE（压缩）等。

testDB=# \d t_hash;
           Table "public.t_hash"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Indexes:
    "t_hash_pkey" PRIMARY KEY, btree (name)
    "uk_id" UNIQUE, btree (id, name)
Distributed by: (name)
 
testDB=#
testDB=#
testDB=# create table t_hash_1 as select * from t_hash;
NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column(s) named 'name' as the Greenplum 
Database data distribution key for this table.
HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the 
optimal data distribution key to minimize skew.
SELECT 0
testDB=# \d t_hash_1
          Table "public.t_hash_1"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) |
Distributed by: (name)
 
testDB=#
testDB=# create table t_hash_2 (like t_hash);
NOTICE:  Table doesn't have 'distributed by' clause, defaulting to distribution columns from LIKE table
CREATE TABLE
testDB=# \d t_hash_2
          Table "public.t_hash_2"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Distributed by: (name)

如果CTAS创建表改变分布键，加上distributed by即可。

testDB=# create table t_hash_3 as select * from t_hash distributed by (id);
SELECT 0
testDB=#
testDB=# \d t_hash_3
          Table "public.t_hash_3"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) |
Distributed by: (id)
 
testDB=#
testDB=#
testDB=# create table t_hash_4 (like t_hash) distributed by (id);
CREATE TABLE
testDB=#
testDB=# \d t_hash4
Did not find any relation named "t_hash4".
testDB=# \d t_hash_4
          Table "public.t_hash_4"
 Column |         Type          | Modifiers
--------+-----------------------+-----------
 id     | integer               |
 name   | character varying(50) | not null
Distributed by: (id)

CTAS时，randomly随机分布键要特别注意，一定要加上distributed randomly，不然原表是hash分布键，CTAS新表则是随机分布键。

testDB=# \d t_random
           Table "public.t_random"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                |
 name   | character varying(100) |
Distributed randomly
 
testDB=#
testDB=# \d t_random_1
          Table "public.t_random_1"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                |
 name   | character varying(100) |
Distributed by: (id)

testDB=# create table t_random_2 as select * from t_random distributed randomly;
SELECT 7
testDB=#
testDB=# \d t_random_2
          Table "public.t_random_2"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                |
 name   | character varying(100) |
Distributed randomly

参考：

《Greenplum企业应用实战》

《Greenplum4.2.2管理员指南》

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