postgresql – 涉及外键约束的死锁

假设树可以有苹果(通过苹果桌上的外键).似乎在选择树时进行更新锁定是在苹果上获得的.但是,即使其他人已经锁定了这个苹果,操作也不会被阻止.

为什么会这样？

附：请不要建议删除“选择更新”.

脚本

Transaction 1      Transaction 2
BEGIN              .
update apple;      .
.                  BEGIN
.                  select tree for update;
.                  update apple;
.                  --halts because of the other transaction locking an apple
update apple;      .
-- deadlock        .
                   COMMIT
                   --transaction succeeds

码

如果你想在你的postgres中尝试 – 这是一个你可以复制/粘贴的代码.

我有一个以下的数据库架构

CREATE TABLE trees (
    id       integer primary key
);

create table apples (
    id       integer primary key,tree_id  integer references trees(id)
);

而且非常简单的数据

insert into trees values(1);
insert into apples values(1,1);

有两个简单的交易.一个是更新苹果,第二个是锁定树并更新苹果.

BEGIN;
    UPDATE apples SET id = id WHERE id = 1;
    -- run second transaction in paralell
    UPDATE apples SET id = id WHERE id = 1;
COMMIT;

BEGIN;
    SELECT id FROM trees WHERE id = 1 FOR UPDATE;
    UPDATE apples SET id = id WHERE id = 1;
COMMIT;

当我运行它们时 – 第一次事务的第二次更新发生死锁.

ERROR:  deadlock detected
DETAIL:  Process 81122 waits for ShareLock on transaction 227154; blocked by process 81100.
Process 81100 waits for ShareLock on transaction 227153; blocked by process 81122.
CONTEXT:  SQL statement "SELECT 1 FROM ONLY "public"."trees" x WHERE "id" OPERATOR(pg_catalog.=) $1 FOR SHARE OF x"