我有一个表“项目”，其中有一列“位置”。position 具有唯一且非空的约束。为了在位置 x 插入新行，我首先尝试增加后续项目的位置：
UPDATE items SET position = position + 1 WHERE position >= x;
这会导致违反唯一约束：
ERROR: duplicate key value violates unique constraint
问题似乎是 PostgreSQL 执行更新的顺序。PostgreSQL < 9.0 中的唯一约束不可延迟，不幸的是，目前不能选择使用 9.0。此外，UPDATE 语句不支持 ORDER BY 子句，以下也不起作用（仍然重复键违规）：
UPDATE items SET position = position + 1 WHERE id IN (
SELECT id FROM items WHERE position >= x ORDER BY position DESC)
有人知道不涉及迭代代码中所有项目的解决方案吗？
另一个表，具有多个唯一索引：
create table utest(id integer, position integer not null, unique(id, position));
test=# \d utest
Table "public.utest"
Column | Type | Modifiers
----------+---------+-----------
id | integer |
position | integer | not null
Indexes:
"utest_id_key" UNIQUE, btree (id, "position")
一些数据：
insert into utest(id, position) select generate_series(1,3), 1;
insert into utest(id, position) select generate_series(1,3), 2;
insert into utest(id, position) select generate_series(1,3), 3;
test=# select * from utest order by id, position;
id | position
----+----------
1 | 1
1 | 2
1 | 3
2 | 1
2 | 2
2 | 3
3 | 1
3 | 2
3 | 3
(9 rows)
我创建了一个以正确顺序更新位置值的过程：
create or replace function update_positions(i integer, p integer)
returns void as $$
declare
temprec record;
begin
for temprec in
select *
from utest u
where id = i and position >= p
order by position desc
loop
raise notice 'Id = [%], Moving % to %',
i,
temprec.position,
temprec.position+1;
update utest
set position = position+1
where position=temprec.position and id = i;
end loop;
end;
$$ language plpgsql;
一些测试：
test=# select * from update_positions(1, 2);
NOTICE: Id = [1], Moving 3 to 4
NOTICE: Id = [1], Moving 2 to 3
update_positions
------------------
(1 row)
test=# select * from utest order by id, position;
id | position
----+----------
1 | 1
1 | 3
1 | 4
2 | 1
2 | 2
2 | 3
3 | 1
3 | 2
3 | 3
(9 rows)
希望能帮助到你。
由于 PostgreSQL 支持全套事务性 DDL，您可以轻松地执行以下操作：
create table utest(id integer unique not null);
insert into utest(id) select generate_series(1,4);
该表现在看起来像这样：
test=# \d utest
Table "public.utest"
Column | Type | Modifiers
--------+---------+-----------
id | integer | not null
Indexes:
"utest_id_key" UNIQUE, btree (id)
test=# select * from utest;
id
----
1
2
3
4
(4 rows)
现在整个魔术：
begin;
alter table utest drop constraint utest_id_key;
update utest set id = id + 1;
alter table utest add constraint utest_id_key unique(id);
commit;
之后我们有：
test=# \d utest
Table "public.utest"
Column | Type | Modifiers
--------+---------+-----------
id | integer | not null
Indexes:
"utest_id_key" UNIQUE, btree (id)
test=# select * from utest;
id
----
2
3
4
5
(4 rows)
这个解决方案有一个缺点：它需要锁定整个表，但也许这不是问题。
不更改表和删除约束的变体：
UPDATE items t1
SET position = t2.position + 1
FROM (SELECT position
FROM items
ORDER BY position DESC) t2
WHERE t2.position >= x AND t1.position = t2.position
“更正”的解决方案可能是使约束DEFERRABLE
ALTER TABLE channels ADD CONSTRAINT
channels_position_unique unique("position")
DEFERRABLE INITIALLY IMMEDIATE
然后在递增时将该约束设置为 DEFERRED 并在完成后将其设置回 IMMEDIATE。
SET CONSTRAINTS channels_position_unique DEFERRED;
UPDATE channels SET position = position+1
WHERE position BETWEEN 1 AND 10;
SET CONSTRAINTS channels_position_unique IMMEDIATE;