数据库内核月报 - 2019 / 06

PgSQL · 应用案例 · 如何修改PostgreSQL分区表分区范围

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