为什么80%的码农都做不了架构师？>>> 
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。
no action和restrict对于操作都会检查，如果不符合约束则会报ERROR并退出，数据还是不变，唯一的区别是no action可以设置约束延迟生效，而restrict不允许，见http://my.oschina.net/Kenyon/blog/126360
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_parent where id = 100003; postgres=# update t_parent set id = 100003 where id = 100002; postgres=# select * from t_child where cid = 100003; |
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 |