作者
digoal
日期
2019-06-21
标签
PostgreSQL , 分区表 , PostgreSQL , attach , detach , 多级分区 , 非平衡多级分区
背景
已有分区表，修改分区的范围。
例如拆分分区，合并分区。
语法如下，PG支持非常灵活的分区布局，看本文提到的HASH分区拆分，支持任意层级的分区，支持每个分区的层级深度不一样。特别适合某些数据分布不均匀的情况。例如id=1落在同一个分区但是数据量非常庞大，可以对这个分区再进行二级分区（使用其他分区方法，其他字段都可以，非常灵活）。
```
ALTER TABLE [ IF EXISTS ] name
ATTACH PARTITION partition_name { FOR VALUES partition_bound_spec | DEFAULT }
ALTER TABLE [ IF EXISTS ] name
DETACH PARTITION partition_name
and partition_bound_spec is:
IN ( partition_bound_expr [, ...] ) |
FROM ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] )
TO ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS numeric_literal, REMAINDER numeric_literal )
```
拆分已有分区例子(hash)
1、创建一个哈希分区表，4个分区
postgres=# create table t_hash (id int , info text) PARTITION BY hash (id);
CREATE TABLE
postgres=# create table t_hash0 partition of t_hash FOR VALUES with (MODULUS 4, REMAINDER 0);
CREATE TABLE
postgres=# create table t_hash1 partition of t_hash FOR VALUES with (MODULUS 4, REMAINDER 1);
CREATE TABLE
postgres=# create table t_hash2 partition of t_hash FOR VALUES with (MODULUS 4, REMAINDER 2);
CREATE TABLE
postgres=# create table t_hash3 partition of t_hash FOR VALUES with (MODULUS 4, REMAINDER 3);
CREATE TABLE
2、查看分区表
postgres=# \d+ t_hash
Partitioned table "public.t_hash"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+----------+--------------+-------------
id | integer | | | | plain | |
info | text | | | | extended | |
Partition key: HASH (id)
Partitions: t_hash0 FOR VALUES WITH (modulus 4, remainder 0),
t_hash1 FOR VALUES WITH (modulus 4, remainder 1),
t_hash2 FOR VALUES WITH (modulus 4, remainder 2),
t_hash3 FOR VALUES WITH (modulus 4, remainder 3)
3、插入一些记录
postgres=# insert into t_hash select generate_series(1,10);
INSERT 0 10
4、查看每一条记录属于哪个分区
postgres=# select tableoid::regclass,* from t_hash;
tableoid | id | info
----------+----+------
t_hash0 | 1 |
t_hash1 | 3 |
t_hash1 | 5 |
t_hash1 | 8 |
t_hash1 | 9 |
t_hash2 | 2 |
t_hash3 | 4 |
t_hash3 | 6 |
t_hash3 | 7 |
t_hash3 | 10 |
(10 rows)
5、将1号分区拆分为2个分区。按8取模，把1号分区拆分成一个分区表（即1号分区被一个耳机分区表代替。而其他分区是直接的分区表，所以看起来就像一颗非平衡树）
```
4(0) , 4(1) , 4(2) , 4(3)
拆分为
4(0) , 4(1){8(1) , 8(5)} , 4(2) , 4(3)
```
解绑分区
postgres=# alter table t_hash DETACH PARTITION t_hash1;
ALTER TABLE
创建二级分区
postgres=# create table t_hash1_subp (id int, info text) PARTITION BY hash (id);;
CREATE TABLE
postgres=# create table t_hash1_subp1 partition of t_hash1_subp FOR VALUES with (MODULUS 8, REMAINDER 1);
CREATE TABLE
postgres=# create table t_hash1_subp5 partition of t_hash1_subp FOR VALUES with (MODULUS 8, REMAINDER 5);
CREATE TABLE
绑定二级分区到一级分区。
postgres=# alter table t_hash attach partition t_hash1_subp FOR VALUES WITH ( MODULUS 4, REMAINDER 1 );
ALTER TABLE
将原来子分区的数据写入新的二级分区表(数据迁移)
postgres=# insert into t_hash1_subp select * from t_hash1;
INSERT 0 4
查看记录，并列出记录所在分区名
postgres=# select tableoid::regclass,* from t_hash;
tableoid | id | info
---------------+----+------
t_hash0 | 1 |
t_hash1_subp1 | 3 |
t_hash1_subp5 | 5 |
t_hash1_subp5 | 8 |
t_hash1_subp5 | 9 |
t_hash2 | 2 |
t_hash3 | 4 |
t_hash3 | 6 |
t_hash3 | 7 |
t_hash3 | 10 |
(10 rows)
列出非平衡的分区表
postgres=# \d+ t_hash
Partitioned table "public.t_hash"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+----------+--------------+-------------
id | integer | | | | plain | |
info | text | | | | extended | |
Partition key: HASH (id)
Partitions: t_hash0 FOR VALUES WITH (modulus 4, remainder 0),
t_hash1_subp FOR VALUES WITH (modulus 4, remainder 1), PARTITIONED,
t_hash2 FOR VALUES WITH (modulus 4, remainder 2),
t_hash3 FOR VALUES WITH (modulus 4, remainder 3)
扩展阅读，甚至我们可以把其他分区改成别的分区方法，例如将t_hash2改成list分区
``` postgres=# alter table t_hash detach partition t_hash2; ALTER TABLE
postgres=# create table t_hash2_subp (id int, info text) partition by list (info); CREATE TABLE postgres=# create table t_hash2_supb1 partition of t_hash2_subp FOR VALUES in ('hello'); CREATE TABLE postgres=# create table t_hash2_supb2 partition of t_hash2_subp FOR VALUES in ('abc','cde'); CREATE TABLE postgres=# create table t_hash2_supb3 partition of t_hash2_subp DEFAULT; CREATE TABLE
postgres=# alter table t_hash attach partition t_hash2_subp for values with (modulus 4, remainder 2); ALTER TABLE
postgres=# \d+ t_hash Partitioned table "public.t_hash" Column | Type | Collation | Nullable | Default | Storage | Stats target | Description --------+---------+-----------+----------+---------+----------+--------------+------------- id | integer | | | | plain | | info | text | | | | extended | | Partition key: HASH (id) Partitions: t_hash0 FOR VALUES WITH (modulus 4, remainder 0), t_hash1_subp FOR VALUES WITH (modulus 4, remainder 1), PARTITIONED, t_hash2_subp FOR VALUES WITH (modulus 4, remainder 2), PARTITIONED, t_hash3 FOR VALUES WITH (modulus 4, remainder 3)
postgres=# insert into t_hash select id,'abc' from t_hash2; INSERT 0 1 postgres=# insert into t_hash select id,'def' from t_hash2; INSERT 0 1 postgres=# insert into t_hash select id,'hello' from t_hash2; INSERT 0 1
postgres=# select tableoid::regclass,* from t_hash;
tableoid | id | info
---------------+----+-------
t_hash0 | 1 |
t_hash1_subp1 | 3 |
t_hash1_subp5 | 5 |
t_hash1_subp5 | 8 |
t_hash1_subp5 | 9 |
t_hash2_supb2 | 2 | abc
t_hash2_supb1 | 2 | hello
t_hash2_supb3 | 2 | def
t_hash3 | 4 |
t_hash3 | 6 |
t_hash3 | 7 |
t_hash3 | 10 |
(12 rows)
```
拆分已有分区例子(range)
与之类似，无非就是partition_bound_spec的用法不一样，略。
```
and partition_bound_spec is:
IN ( partition_bound_expr [, ...] ) |
FROM ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] )
TO ( { partition_bound_expr | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS numeric_literal, REMAINDER numeric_literal )
```
合并已有分区例子(hash)
与之类似，略
合并已有分区例子(range)
与之类似，略
小结
通过attach, detach支持分区的拆分，合并。PG支持非平衡分区表，深度不一定一致。例如本文，
```
4(0) , 4(1) , 4(2) , 4(3)
拆分为
4(0) , 4(1){8(1) , 8(5)} , 4(2) , 4(3)
```
参考
https://www.postgresql.org/docs/12/sql-altertable.html
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