PostgreSQL分区表（Table Partitioning）应用

一、简介

　　在数据库日渐庞大的今天，为了方便对数据库数据的管理，比如按时间，按地区去统计一些数据时，基数过于庞大，多有不便。很多商业数据库都提供分区的概念，按不同的维度去存放数据，便于后期的管理，PostgreSQL也不例外。

　　PostgresSQL分区的意思是把逻辑上的一个大表分割成物理上的几块儿。分区不仅能带来访问速度的提升，关键的是，它能带来管理和维护上的方便。

　　分区的具体好处是：

• 某些类型的查询性能可以得到极大提升。
• 更新的性能也可以得到提升，因为表的每块的索引要比在整个数据集上的索引要小。如果索引不能全部放在内存里，那么在索引上的读和写都会产生更多的磁盘访问。
• 批量删除可以用简单的删除某个分区来实现。
• 可以将很少用的数据移动到便宜的、转速慢的存储介质上。

　　在PG里表分区是通过表继承来实现的，一般都是建立一个主表，里面是空，然后每个分区都去继承它。无论何时，都应保证主表里面是空的。

　　小表分区不实际，表在多大情况下才考虑分区呢？PostgresSQL官方给出的建议是：当表本身大小超过了机器物理内存的实际大小时（the size of the table should exceed the physical memory of the database server），可以考虑分区。

　　PG目前（9.2.2）仅支持范围分区和列表分区，尚未支持散列分区。

二、环境

系统环境：CentOS release 6.3 (Final)

PostgreSQL版本：PostgreSQL 9.2.2 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4), 64-bit

三、实现分区

3.1 创建主表

david=# create table tbl_partition (
david(# id integer,
david(# name varchar(20),
david(# gender boolean,
david(# join_date date,
david(# dept char(4));
CREATE TABLE
david=#

3.2 创建分区表

david=# create table tbl_partition_201211 (
check ( join_date >= DATE '2012-11-01' AND join_date < DATE '2012-12-01' )       
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201212 (
check ( join_date >= DATE '2012-12-01' AND join_date < DATE '2013-01-01' )      
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201301 (
check ( join_date >= DATE '2013-01-01' AND join_date < DATE '2013-02-01' )      
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201302 (
check ( join_date >= DATE '2013-02-01' AND join_date < DATE '2013-03-01' )  
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201303 (
check ( join_date >= DATE '2013-03-01' AND join_date < DATE '2013-04-01' ) 
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201304 (
check ( join_date >= DATE '2013-04-01' AND join_date < DATE '2013-05-01' )  
) INHERITS (tbl_partition);
CREATE TABLE
david=# create table tbl_partition_201305 (
check ( join_date >= DATE '2013-05-01' AND join_date < DATE '2013-06-01' )  
) INHERITS (tbl_partition);
CREATE TABLE
david=# 

3.3 分区键上建索引

david=# create index tbl_partition_201211_joindate on tbl_partition_201211 (join_date);
CREATE INDEX
david=# create index tbl_partition_201212_joindate on tbl_partition_201212 (join_date); 
CREATE INDEX
david=# create index tbl_partition_201301_joindate on tbl_partition_201301 (join_date);    
CREATE INDEX
david=# create index tbl_partition_201302_joindate on tbl_partition_201302 (join_date); 
CREATE INDEX
david=# create index tbl_partition_201303_joindate on tbl_partition_201303 (join_date); 
CREATE INDEX
david=# create index tbl_partition_201304_joindate on tbl_partition_201304 (join_date); 
CREATE INDEX
david=# create index tbl_partition_201305_joindate on tbl_partition_201305 (join_date); 
CREATE INDEX
david=# 

对于开发人员来说，希望数据库是透明的，只管 insert into tbl_partition。对于数据插向哪个分区，则希望由DB决定。这点，ORACLE实现了，但是PG不行，需要前期人工处理下。

3.4 创建触发器函数

david=# CREATE OR REPLACE FUNCTION tbl_partition_insert_trigger()                      
RETURNS TRIGGER AS $$  
BEGIN  
    IF ( NEW.join_date >= DATE '2012-11-01' AND    
         NEW.join_date < DATE '2012-12-01' ) THEN  
        INSERT INTO tbl_partition_201211 VALUES (NEW.*);  
    ELSIF ( NEW.join_date >= DATE '2012-12-01' AND  
            NEW.join_date < DATE '2013-01-01' ) THEN  
        INSERT INTO tbl_partition_201212 VALUES (NEW.*);  
    ELSIF ( NEW.join_date >= DATE '2013-01-01' AND  
            NEW.join_date < DATE '2013-02-01' ) THEN  
        INSERT INTO tbl_partition_201301 VALUES (NEW.*);  
    ELSIF ( NEW.join_date >= DATE '2013-02-01' AND  
            NEW.join_date < DATE '2013-03-01' ) THEN  
        INSERT INTO tbl_partition_201302 VALUES (NEW.*);  
    ELSIF ( NEW.join_date >= DATE '2013-03-01' AND  
            NEW.join_date < DATE '2013-04-01' ) THEN  
        INSERT INTO tbl_partition_201303 VALUES (NEW.*);  
    ELSIF ( NEW.join_date >= DATE '2013-04-01' AND  
            NEW.join_date < DATE '2013-05-01' ) THEN  
        INSERT INTO tbl_partition_201304 VALUES (NEW.*);
    ELSIF ( NEW.join_date >= DATE '2013-05-01' AND
            NEW.join_date < DATE '2013-06-01' ) THEN
        INSERT INTO tbl_partition_201305 VALUES (NEW.*); 
    ELSE  
        RAISE EXCEPTION 'Date out of range. Fix the tbl_partition_insert_trigger() function!';  
    END IF;  
    RETURN NULL;  
END;  
$$  
LANGUAGE plpgsql;
CREATE FUNCTION
david=# 

说明：如果不想丢失数据，上面的ELSE 条件可以改成 INSERT INTO tbl_partition_error_join_date VALUES (NEW.*); 同时需要创建一张结构和tbl_partition 一样的表tbl_partition_error_join_date，这样，错误的join_date 数据就可以插入到这张表中而不是报错了。

3.5 创建触发器

david=# CREATE TRIGGER insert_tbl_partition_trigger
david-#     BEFORE INSERT ON tbl_partition
david-#     FOR EACH ROW EXECUTE PROCEDURE tbl_partition_insert_trigger();
CREATE TRIGGER
david=# 

四、查看表

4.1 查看所有表

david=# \dt
                List of relations
 Schema |         Name         | Type  |  Owner   
--------+----------------------+-------+----------
 public | tbl_partition        | table | postgres
 public | tbl_partition_201211 | table | postgres
 public | tbl_partition_201212 | table | postgres
 public | tbl_partition_201301 | table | postgres
 public | tbl_partition_201302 | table | postgres
 public | tbl_partition_201303 | table | postgres
 public | tbl_partition_201304 | table | postgres
 public | tbl_partition_201305 | table | postgres
(8 rows)

david=# 

4.2 查看主表

david=# \d tbl_partition
         Table "public.tbl_partition"
  Column   |         Type          | Modifiers 
-----------+-----------------------+-----------
 id        | integer               | 
 name      | character varying(20) | 
 gender    | boolean               | 
 join_date | date                  | 
 dept      | character(4)          | 
Triggers:
    insert_tbl_partition_trigger BEFORE INSERT ON tbl_partition FOR EACH ROW EXECUTE PROCEDURE tbl_partition_insert_trigger()
Number of child tables: 7 (Use \d+ to list them.)

david=# 

4.3 查看分区表

david=# \d tbl_partition_201304
      Table "public.tbl_partition_201304"
  Column   |         Type          | Modifiers 
-----------+-----------------------+-----------
 id        | integer               | 
 name      | character varying(20) | 
 gender    | boolean               | 
 join_date | date                  | 
 dept      | character(4)          | 
Indexes:
    "tbl_partition_201304_joindate" btree (join_date)
Check constraints:
    "tbl_partition_201304_join_date_check" CHECK (join_date >= '2013-04-01'::date AND join_date < '2013-05-01'::date)
Inherits: tbl_partition

david=#

五、测试

5.1 插入数据

david=# insert into tbl_partition values (1, 'David', '1', '2013-01-10', 'TS');
INSERT 0 0
david=# insert into tbl_partition values (2, 'Sandy', '0', '2013-02-10', 'TS');
INSERT 0 0
david=# insert into tbl_partition values (3, 'Eagle', '1', '2012-11-01', 'TS');
INSERT 0 0
david=# insert into tbl_partition values (4, 'Miles', '1', '2012-12-15', 'SD');
INSERT 0 0
david=# insert into tbl_partition values (5, 'Simon', '1', '2012-12-10', 'SD');
INSERT 0 0
david=# insert into tbl_partition values (6, 'Rock', '1', '2012-11-10', 'SD');
INSERT 0 0
david=# insert into tbl_partition values (7, 'Peter', '1', '2013-01-11', 'SD');
INSERT 0 0
david=# insert into tbl_partition values (8, 'Sally', '0', '2013-03-10', 'BCSC');
INSERT 0 0
david=# insert into tbl_partition values (9, 'Carrie', '0', '2013-04-02', 'BCSC');
INSERT 0 0
david=# insert into tbl_partition values (10, 'Lee', '1', '2013-01-05', 'BMC');
INSERT 0 0
david=# insert into tbl_partition values (11, 'Nicole', '0', '2012-11-10', 'PROJ');
INSERT 0 0
david=# insert into tbl_partition values (12, 'Renee', '0', '2013-01-10', 'TS');
INSERT 0 0
david=# 

5.2 查看主表数据

david=# select * from tbl_partition;
 id |  name  | gender | join_date  | dept 
----+--------+--------+------------+------
  3 | Eagle  | t      | 2012-11-01 | TS  
  6 | Rock   | t      | 2012-11-10 | SD  
 11 | Nicole | f      | 2012-11-10 | PROJ
  4 | Miles  | t      | 2012-12-15 | SD  
  5 | Simon  | t      | 2012-12-10 | SD  
  1 | David  | t      | 2013-01-10 | TS  
  7 | Peter  | t      | 2013-01-11 | SD  
 10 | Lee    | t      | 2013-01-05 | BMC 
 12 | Renee  | f      | 2013-01-10 | TS  
  2 | Sandy  | f      | 2013-02-10 | TS  
  8 | Sally  | f      | 2013-03-10 | BCSC
  9 | Carrie | f      | 2013-04-02 | BCSC
(12 rows)

david=# 

5.3 查看分区表数据

david=# select * from tbl_partition_201301 ;
 id | name  | gender | join_date  | dept 
----+-------+--------+------------+------
  1 | David | t      | 2013-01-10 | TS  
  7 | Peter | t      | 2013-01-11 | SD  
 10 | Lee   | t      | 2013-01-05 | BMC 
 12 | Renee | f      | 2013-01-10 | TS  
(4 rows)

david=# 

六、管理分区

6.1 移除数据/分区

实现分区表之后，我们就可以很容易地移除不再使用的旧数据了，最简单的方法就是：

david=# drop table tbl_partition_201304;

这样可以快速移除大量数据，而不是逐条删除数据。

另一个推荐做法是将分区从分区表中移除，但是保留访问权限。

david=# alter table tbl_partition_201304 no inherit tbl_partition;
ALTER TABLE
david=#

和直接DROP 相比，该方式仅仅是使子表脱离了原有的主表，而存储在子表中的数据仍然可以得到访问，因为此时该表已经被还原成一个普通的数据表了。这样对于数据库的DBA来说，就可以在此时对该表进行必要的维护操作，如数据清理、归档等，在完成诸多例行性的操作之后，就可以考虑是直接删除该表(DROP TABLE)，还是先清空该表的数据(TRUNCATE TABLE)，之后再让该表重新继承主表。

david=# alter table tbl_partition_201304 inherit tbl_partition;                                                            
ALTER TABLE
david=# 

6.2 增加分区

我们可以像之前那样增加一个分区

david=# create table tbl_partition_201306 (
check ( join_date >= DATE '2013-06-01' AND join_date < DATE '2013-07-01' )  
) INHERITS (tbl_partition);

david=# create index tbl_partition_201306_joindate on tbl_partition_201306 (join_date);

同时，需要修改触发器函数，将插入条件改成相应的值。

说明：创建触发器函数时，最好把插入条件写更未来一点，比如多写十年，这样以后增加新分区时就不需要重新创建触发器函数了，也可以避免一些不必要的错误。

另外，还可以如下增加新的分区：

david=# create table tbl_partition_201307
david-# (LIKE tbl_partition INCLUDING DEFAULTS INCLUDING CONSTRAINTS);
CREATE TABLE
david=#

david=# alter table tbl_partition_201307 add constraint tbl_partition_201307_join_date_check 
david-# check ( join_date >= DATE '2013-07-01' AND join_date < DATE '2013-08-01' );
ALTER TABLE
david=#

david=# create index tbl_partition_201307_joindate on tbl_partition_201307 (join_date);

david=# copy tbl_partition_201307 from '/tmp/tbl_partition_201307.sql';      //从文件中拷贝数据，这些数据可以是事前准备的

david=# alter table tbl_partition_201307 inherit tbl_partition;

七、约束排除

约束排除（Constraint exclusion）是一种查询优化技巧，它改进了用上面方法定义的表分区的性能。

确保postgresql.conf 里的配置参数constraint_exclusion 是打开的。没有这个参数，查询不会按照需要进行优化。这里我们需要做的是确保该选项在配置文件中没有被注释掉。

如果没有约束排除，查询会扫描tbl_partition 表中的每一个分区。打开了约束排除之后，规划器将检查每个分区的约束然后再试图证明该分区不需要被扫描，因为它不能包含任何符合WHERE子句条件的数据行。如果规划器可以证明这个，它就把该分区从查询规划里排除出去。

可以使用EXPLAIN 命令显示一个规划在constraint_exclusion 关闭和打开情况下的不同：

7.1 约束排除关闭

david=# set constraint_exclusion = off;                                                 
SET
david=# explain select count(*) from tbl_partition where join_date >= DATE '2013-04-01';
                                           QUERY PLAN                                            
-------------------------------------------------------------------------------------------------
 Aggregate  (cost=172.80..172.81 rows=1 width=0)
   ->  Append  (cost=0.00..167.62 rows=2071 width=0)
         ->  Seq Scan on tbl_partition  (cost=0.00..0.00 rows=1 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201211 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201212 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201301 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201302 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201303 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201305 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201304 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201306 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201307 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
(22 rows)

david=# 

从上面的查询计划中可以看出，PostgreSQL 扫描了所有分区。下面我们再看一下打开约束排除之后的查询计划： 

7.2 约束排除开启

david=# set constraint_exclusion = on;                                                  
SET
david=# explain select count(*) from tbl_partition where join_date >= DATE '2013-04-01';
                                           QUERY PLAN                                            
-------------------------------------------------------------------------------------------------
 Aggregate  (cost=76.80..76.81 rows=1 width=0)
   ->  Append  (cost=0.00..74.50 rows=921 width=0)
         ->  Seq Scan on tbl_partition  (cost=0.00..0.00 rows=1 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201305 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201304 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201306 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
         ->  Seq Scan on tbl_partition_201307 tbl_partition  (cost=0.00..18.62 rows=230 width=0)
               Filter: (join_date >= '2013-04-01'::date)
(12 rows)

david=#

可以看到，PostgreSQL 只扫描四月份以后的分区表。

八、可选的分区方式

还可以通过建立规则的方式进行分区。

CREATE RULE insert_tbl_partition_201211 AS
ON INSERT TO tbl_partition WHERE
    ( join_date >= DATE '2012-11-01' AND join_date < DATE '2012-12-01' )
DO INSTEAD
    INSERT INTO tbl_partition_201211 VALUES (NEW.*);

CREATE RULE insert_tbl_partition_201212 AS
ON INSERT TO tbl_partition WHERE
    ( join_date >= DATE '2012-12-01' AND join_date < DATE '2013-01-01' )
DO INSTEAD
    INSERT INTO tbl_partition_201212 VALUES (NEW.*);

...
    
CREATE RULE insert_tbl_partition_201306 AS
ON INSERT TO tbl_partition WHERE
    ( join_date >= DATE '2013-06-01' AND join_date < DATE '2013-07-01' )
DO INSTEAD
    INSERT INTO tbl_partition_201306 VALUES (NEW.*);
    
CREATE RULE insert_tbl_partition_201307 AS
ON INSERT TO tbl_partition WHERE
    ( join_date >= DATE '2013-07-01' AND join_date < DATE '2013-08-01' )
DO INSTEAD
    INSERT INTO tbl_partition_201307 VALUES (NEW.*);
    
CREATE RULE insert_tbl_partition_error_join_date AS
ON INSERT TO tbl_partition WHERE
    ( join_date >= DATE '2013-08-01' OR join_date < DATE '2012-11-01' )
DO INSTEAD
    INSERT INTO tbl_partition_error_join_date VALUES (NEW.*);

九、注意事项

VACUUM 或 ANALYZE tbl_partition 只会对主表起作用，要想分析表，需要分别分析每个分区表。

十、参考资料

• PostgreSQL官方说明：http://www.postgresql.org/docs/9.2/static/ddl-partitioning.html
• ITEYE：http://diegoball.iteye.com/blog/713826
• kenyon(君羊)：http://my.oschina.net/Kenyon/blog/59455