PostgreSQL的外鍵深刻使用

有開發同事問及postgresql外鍵的用法，這裏普及一下。外鍵是一個很基礎的概念，使用得當能夠對事務的一致性有很好的保障，方法上和Oracle是很接近的，做用很簡單地說就是保證子表的數據都能在主表中找到，可保證數據一致性。

創建主表

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

創建子表

postgres=# create table t_child( postgres(# cid int4, postgres(# vname varchar(32)); CREATE TABLE

查看錶外鍵

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;

創建外鍵關聯，若是子表有父表沒有的數據，會報錯

postgres=# alter table t_child add constraint t_child_fk foreign key(cid) references t_parent (id) ; ALTER TABLE --另外一種狀況，須要先清理數據 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".

查看外鍵的關聯關係

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)

外鍵數據生成

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)

外鍵對數據導入的影響測試

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=#

能夠看到加了外鍵後對數據的導入影響很大，這裏只是測試了10W數據的COPY導入，數據量再大一點差異更明顯，因此大數據的導入請先去掉各類約束，這對其餘DB也適用。


UPDATE和DELETE的外鍵屬性
上面建的外鍵默認是MATCH SIMPLE ON UPDATE NO ACTION ON DELETE NO ACTION，除了NO ACTION，還有cascade/restrict這兩種經常使用的。
no action和restrict對於操做都會檢查，若是不符合約束則會報ERROR並退出，數據仍是不變，惟一的區別是no action能夠設置約束延遲生效，而restrict不容許，見http://my.oschina.net/Kenyon/blog/126360
cascade則是級聯的意思，如刪除父表數據時子表也存在則會級聯刪除
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) 這對delete是同樣的，主表數據被刪，關聯子表數據也被刪

一樣，匹配的方式也有三種match simple/match full/match partition，實際上是兩種
simple(默認)
full
partition(功能還未完成)
simple與full的區別是simple容許多字段外鍵的部分字段數據爲Null,而full通常是不容許外鍵字段數據爲Null,除非該外鍵的全部字段都爲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. --另一種模式 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

能夠看到插空值入有明顯的區別。