Postgresql 非主键上的POSTGRES序列触发器

我有一个PostgreSQL数据库版本9.5.0

创建的表

UserComments

如下所示：

create table UserComments (  
  userId integer, 
  commentNr integer, 
  comment text,  
  UNIQUE (userId, commentNr)
);

该表不能更改，我也不能添加额外的表

我想根据

userId

条目的数量在

commentNr

上创建一个序列（换句话说：一个自动递增的值）。 对于具有相同用户ID的每个新条目，其值应递增。
例如：

userId | commentNr | comment -------|-----------|--------- 1 | 1 | blabla 1 | 2 | more bla 2 | 1 | myownbla 1 | 3 | evenmorebla

---

任何帮助都将不胜感激

对于这样的简单情况，不需要创建触发器。可以在CTE或子查询中计算并插入正确的序列

注意：如果使用下面的sql，则必须指定两次插入的

userId

INSERT INTO "user" ("userId", "commentNr", "comment") (
WITH 
  nextval AS (
  SELECT COALESCE(MAX("commentNr"),0)+1 commentNr
  FROM "user"
  WHERE "userId" = '1'
)
SELECT '1', nextval.commentNr, 'bla'
FROM nextval
);

---

要自动计算commentNr字段，可以使用触发器：

CREATE INDEX i1 ON UserComments (userId, commentNr DESC);

CREATE OR REPLACE FUNCTION set_commentNr() RETURNS TRIGGER AS $sql$
BEGIN
    NEW.commentNr = coalesce(max(commentNr) + 1, 1) FROM UserComments WHERE UserId = NEW.userID;
    RETURN NEW;
END;
$sql$ LANGUAGE plpgsql STABLE;


CREATE TRIGGER ins_comment BEFORE INSERT ON UserComments FOR EACH ROW EXECUTE PROCEDURE set_commentNr();

小型测试：

insert into userComments (userId, comment) values ('1', 'bla');
insert into userComments (userId, comment) values ('1', 'bla');
insert into userComments (userId, comment) values ('2', 'bla');
insert into userComments (userId, comment) values ('1', 'bla');
insert into userComments (userId, comment) values ('2', 'bla');

SELECT * FROM userComments;

 userid | commentnr | comment 
--------+-----------+---------
      1 |         1 | bla
      1 |         2 | bla
      2 |         1 | bla
      1 |         3 | bla
      2 |         2 | bla
(5 rows)

为提高计算下一个值的性能而创建的索引i1：

                                                             QUERY PLAN                                                              
-------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=4.17..4.18 rows=1 width=4) (actual time=0.032..0.032 rows=1 loops=1)
   InitPlan 1 (returns $0)
     ->  Limit  (cost=0.15..4.17 rows=1 width=4) (actual time=0.026..0.026 rows=1 loops=1)
           ->  Index Only Scan using i1 on usercomments  (cost=0.15..24.27 rows=6 width=4) (actual time=0.021..0.021 rows=1 loops=1)
                 Index Cond: ((userid = 1) AND (commentnr IS NOT NULL))
                 Heap Fetches: 1
 Planning time: 0.238 ms
 Execution time: 0.095 ms

---

主键（id），

：：表中没有列

id

。请发布真实代码。你是对的-本例不需要主键。

                                                             QUERY PLAN                                                              
-------------------------------------------------------------------------------------------------------------------------------------
 Result  (cost=4.17..4.18 rows=1 width=4) (actual time=0.032..0.032 rows=1 loops=1)
   InitPlan 1 (returns $0)
     ->  Limit  (cost=0.15..4.17 rows=1 width=4) (actual time=0.026..0.026 rows=1 loops=1)
           ->  Index Only Scan using i1 on usercomments  (cost=0.15..24.27 rows=6 width=4) (actual time=0.021..0.021 rows=1 loops=1)
                 Index Cond: ((userid = 1) AND (commentnr IS NOT NULL))
                 Heap Fetches: 1
 Planning time: 0.238 ms
 Execution time: 0.095 ms