PostgreSQL序列基于另一列

Question

可以说我有一张桌子:

Column   |     Type    |                        Notes
---------+------------ +----------------------------------------------------------
 id      | integer     | An ID that's FK to some other table
 seq     | integer     | Each ID gets its own seq number
 data    | text        | Just some text, totally irrelevant.

id+ seq是组合键.

我想看到的是:

ID  | SEQ   |                        DATA
----+------ +----------------------------------------------
 1  | 1     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 1  | 2     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 1  | 3     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 1  | 4     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 2  | 1     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 3  | 1     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 3  | 2     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 3  | 3     | Quick brown fox, lorem ipsum, lazy dog, etc etc.
 3  | 4     | Quick brown fox, lorem ipsum, lazy dog, etc etc.

如您所见,组合id和seq独特.

我不知道如何设置我的表(或插入语句？)来做到这一点.我想插入,id并data导致seq依赖于子序列id.

Answer 1

没问题!我们将让两个表,things和stuff.stuff将是您在问题中描述的表格,并且things是它所引用的表格:

CREATE TABLE things (
    id serial primary key,
    name text
);

CREATE TABLE stuff (
    id integer references things,
    seq integer NOT NULL,
    notes text,
    primary key (id, seq)
);

然后我们将设置things一个触发器,每次创建一行时都会创建一个新序列:

CREATE FUNCTION make_thing_seq() RETURNS trigger
    LANGUAGE plpgsql
    AS $$
begin
  execute format('create sequence thing_seq_%s', NEW.id);
  return NEW;
end
$$;

CREATE TRIGGER make_thing_seq AFTER INSERT ON things FOR EACH ROW EXECUTE PROCEDURE make_thing_seq();

现在,我们将结束与thing_seq_1,thing_seq_2等,等...

现在是另一个触发器,stuff以便每次使用正确的序列:

CREATE FUNCTION fill_in_stuff_seq() RETURNS trigger
    LANGUAGE plpgsql
    AS $$
begin
  NEW.seq := nextval('thing_seq_' || NEW.id);
  RETURN NEW;
end
$$;

CREATE TRIGGER fill_in_stuff_seq BEFORE INSERT ON stuff FOR EACH ROW EXECUTE PROCEDURE fill_in_stuff_seq();

这将确保当行进入时stuff,该id列用于查找要调用的正确序列nextval.

这是一个演示:

test=# insert into things (name) values ('Joe');
INSERT 0 1
test=# insert into things (name) values ('Bob');
INSERT 0 1
test=# select * from things;
 id | name
----+------
  1 | Joe
  2 | Bob
(2 rows)

test=# \d
              List of relations
 Schema |     Name      |   Type   |  Owner
--------+---------------+----------+----------
 public | stuff         | table    | jkominek
 public | thing_seq_1   | sequence | jkominek
 public | thing_seq_2   | sequence | jkominek
 public | things        | table    | jkominek
 public | things_id_seq | sequence | jkominek
(5 rows)

test=# insert into stuff (id, notes) values (1, 'Keychain');
INSERT 0 1
test=# insert into stuff (id, notes) values (1, 'Pet goat');
INSERT 0 1
test=# insert into stuff (id, notes) values (2, 'Family photo');
INSERT 0 1
test=# insert into stuff (id, notes) values (1, 'Redundant lawnmower');
INSERT 0 1
test=# select * from stuff;
 id | seq |        notes
----+-----+---------------------
  1 |   1 | Keychain
  1 |   2 | Pet goat
  2 |   1 | Family photo
  1 |   3 | Redundant lawnmower
(4 rows)

test=#

Answer 2

您可以使用窗口函数来分配SEQ值,例如:

INSERT INTO YourTable
    (ID, SEQ, DATA)
    SELECT ID, ROW_NUMBER() OVER(PARTITION BY ID ORDER BY DATA), DATA
        FROM YourSource

Answer 3

如果seq反映（或应该反映）插入行的顺序，我宁愿使用timestamp自动填充并在使用row_number()以下方法选择行时动态生成序列号：

create table some_table
( 
  id          integer   not null,
  inserted_at timestamp not null default current_timestamp,
  data text
);

要获取seq列，您可以执行以下操作：

select id,  
       row_number() over (partition by id order by inserted_at) as seq,
       data
from some_table
order by id, seq;

然而，与使用持久seq列（尤其是在 上有索引的情况下id, seq）相比，选择会慢一点。

如果这成为一个问题，您可以考虑使用物化视图，或者添加seq列然后定期更新它（出于性能原因，我不会在触发器中这样做）。

SQLFiddle 示例：http ://sqlfiddle.com/#!15/db69b/1