Postgres的外键深入使用
Postgres的外键深入使用 有开发同事问及postgresql外键的用法，这里普及一下。外键是一个很基础的概念，使用得当可以对事务的一致性有很好的保障，方法上和Oracle是很接近的，作用很简单地说就是保证子表的数据都能在主表中找到，可保证数据一致性。 建立主
Postgres的外键深入使用
有开发同事问及postgresql外键的用法，这里普及一下。外键是一个很基础的概念，使用得当可以对事务的一致性有很好的保障，方法上和Oracle是很接近的，作用很简单地说就是保证子表的数据都能在主表中找到，可保证数据一致性。
建立主表
postgres=# create table t_parent(
postgres(# id serial primary key,
postgres(# vname varchar(32),
postgres(# ctime timestamp without time zone);
NOTICE: CREATE TABLE will create implicit sequence "t_parent_id_seq" for serial column "t_parent.id"
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "t_parent_pkey" for table "t_parent"
CREATE TABLE
建立子表
postgres=# create table t_child(
postgres(# cid int4,
postgres(# vname varchar(32));
CREATE TABLE
查看表外键
postgres=# \d+ t_child
Table "public.t_child"
Column | Type | Modifiers | Storage | Stats target | Description
--------+-----------------------+-----------+----------+--------------+-------------
cid | integer | | plain | |
vname | character varying(32) | | extended | |
Foreign-key constraints:
"t_child_fk" FOREIGN KEY (cid) REFERENCES t_parent(id)
Has OIDs: no
在PGADMINIII中查看
CREATE TABLE t_child
(
cid integer,
vname character varying(32),
CONSTRAINT t_child_fk FOREIGN KEY (cid)
REFERENCES t_parent (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
OIDS=FALSE
);
ALTER TABLE t_child
OWNER TO postgres;
建立外键关联，如果子表有父表没有的数据，会报错
postgres=# alter table t_child add constraint t_child_fk foreign key(cid) references t_parent (id) ;
ALTER TABLE
--另一种情况，需要先清理数据
postgres=# alter table t_child add constraint t_child_fk foreign key(cid) references t_parent (id) ;
ERROR: insert or update on table "t_child" violates foreign key constraint "t_child_fk"
DETAIL: Key (cid)=(100001) is not present in table "t_parent".
查看外键的关联关系
postgres=# SELECT
postgres-# tc.constraint_name, tc.table_name, kcu.column_name,
postgres-# ccu.table_name AS foreign_table_name,
postgres-# ccu.column_name AS foreign_column_name,
postgres-# tc.is_deferrable,tc.initially_deferred
postgres-# FROM
postgres-# information_schema.table_constraints AS tc
postgres-# JOIN information_schema.key_column_usage AS kcu ON tc.constraint_name = kcu.constraint_name
postgres-# JOIN information_schema.constraint_column_usage AS ccu ON ccu.constraint_name = tc.constraint_name
postgres-# WHERE constraint_type = 'FOREIGN KEY' AND tc.table_name='t_child';
constraint_name | table_name | column_name | foreign_table_name | foreign_column_name | is_deferrable | initially_deferred
-----------------+------------+-------------+--------------------+---------------------+---------------+--------------------
t_child_fk | t_child | cid | t_parent | id | NO | NO
(1 row)
外键数据生成
postgres=# insert into t_parent select generate_series(1,100000),md5(random()::text),clock_timestamp();
INSERT 0 100000
postgres=# insert into t_child select id,md5(random()::text) from t_parent;
INSERT 0 100000
postgres=# select * from t_parent limit 10;
id | vname | ctime
----+----------------------------------+----------------------------
2 | f12c9b7d21f467a6c47b5adca5a5478e | 2013-05-20 20:51:08.678242
3 | ce758f15428d56be00ba5b0834daa5af | 2013-05-20 20:51:08.678284
4 | 55892bd9a81db1566c7fefb3e459dcd6 | 2013-05-20 20:51:08.678303
5 | 5c9dabb81782953fdfea3da0d7bafdbb | 2013-05-20 20:51:08.678322
6 | e5358f0c23d9042e599aa8d03b6b8944 | 2013-05-20 20:51:08.67834
7 | e51c3ab198d605699de5472dc7589712 | 2013-05-20 20:51:08.678357
8 | db8c0b2f7ad6579594f79abf2828f70e | 2013-05-20 20:51:08.678376
9 | 904630d3dcab4308edea4bed5f6b556d | 2013-05-20 20:51:08.678394
10 | 1c419398ac492b16be8a252a9c8e28ba | 2013-05-20 20:51:08.678411
11 | b774007d756a6c4b7c54d3854eb964b7 | 2013-05-20 20:51:08.678429
(10 rows)
外键对数据导入的影响测试
postgres=# \timing
Timing is on.
postgres=# copy t_child(cid,vname) to '/home/postgres/t_child.bak';
COPY 100000
Time: 207.030 ms
postgres=# truncate table t_child;
TRUNCATE TABLE
Time: 43.775 ms
postgres=# copy t_child(cid,vname) from '/home/postgres/t_child.bak';
COPY 100000
Time: 10325.357 ms
postgres=# truncate table t_child;
TRUNCATE TABLE
Time: 16.749 ms
postgres=# alter table t_child drop constraint t_child_fk;
ALTER TABLE
Time: 26.552 ms
postgres=# copy t_child(cid,vname) from '/home/postgres/t_child.bak';
COPY 100000
Time: 755.239 ms
postgres=#
可以看到加了外键后对数据的导入影响很大，这里只是测试了10W数据的COPY导入，数据量再大一点差别更明显，所以大数据的导入请先去掉各种约束，这对其他DB也适用。
UPDATE和DELETE的外键属性
上面建的外键默认是MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION，除了NO ACTION，还有cascade/restrict这两种常用的。
cascade则是级联的意思，如删除父表数据时子表也存在则会级联删除
cascade示例：
postgres=# alter table t_child add constraint t_child_fk foreign key(cid) references t_parent (id) match simple on update cascade on delete cascade;
ALTER TABLE
postgres=# select * from t_child where cid = 100003;
cid | vname
-----+-------
(0 rows)
postgres=# select * from t_parent where id = 100003;
id | vname | ctime
----+-------+-------
(0 rows)
postgres=# update t_parent set id = 100003 where id = 100002;
UPDATE 1
postgres=# select * from t_parent where id = 100003;
id | vname | ctime
--------+----------------------------------+----------------------------
100003 | 20e9c1b966bc9fc133339bad7d374dd8 | 2013-05-20 20:51:08.677156
(1 row)
postgres=# select * from t_child where cid = 100003;
cid | vname
--------+----------------------------------
100003 | 9fd9b9d977abcba5f8b38658b4116985
(1 row)
这对delete是一样的，主表数据被删，关联子表数据也被删
同样，匹配的方式也有三种match simple/match full/match partition，其实是两种
simple(默认)
full
partition(功能还未完成)
simple与full的区别是simple允许多字段外键的部分字段数据为Null,而full一般是不允许外键字段数据为Null,除非该外键的所有字段都为Null。示例：
postgres=# create table t_p(id1 int,id2 int);
CREATE TABLE
postgres=# create table t_c(id1 int,id2 int);
CREATE TABLE
postgres=# insert into t_p values(1,2),(1,3),(2,3);
INSERT 0 3
postgres=# alter table t_p add constraint dd unique(id1,id2);
NOTICE: ALTER TABLE / ADD UNIQUE will create implicit index "dd" for table "t_p"
ALTER TABLE
postgres=# alter table t_c add constraint fk_c foreign key(id1,id2) references t_p(id1,id2) match full;
ALTER TABLE
postgres=# insert into t_c values(1,2);
INSERT 0 1
postgres=# insert into t_c values(null,null);
INSERT 0 1
postgres=# insert into t_c values(1,null);
ERROR: insert or update on table "t_c" violates foreign key constraint "fk_c"
DETAIL: MATCH FULL does not allow mixing of null and nonnull key values.
--另外一种模式
postgres=# alter table t_c drop constraint fk_c;
ALTER TABLE
postgres=# alter table t_c add constraint fk_c foreign key(id1,id2) references t_p(id1,id2) match simple;
ALTER TABLE
postgres=# insert into t_c values(1,2);
INSERT 0 1
postgres=# insert into t_c values(1,null);
INSERT 0 1
postgres=# insert into t_c values(null,null);
INSERT 0 1 可以看到插空值入有明显的区别。