有没有办法使用SKIP LOCKED
使用PostgreSQL进行查询以避免已经锁定行,但同时如果所有内容都已锁定等待第一个未锁定的行?
在我的用例中,我希望在请求可用的电话号码时避免误报。但是在我将所有号码锁定的情况下,不太可能,但它仍然可能发生并应该解决。
SELECT plain_number FROM pool
ORDER BY RANDOM()
LIMIT 1
FOR UPDATE
SKIP LOCKED
答案 0 :(得分:2)
这似乎有效:
WITH sl AS (
SELECT plain_number FROM pool ORDER BY random() LIMIT 1 FOR UPDATE SKIP LOCKED
),
fu AS (
SELECT plain_number FROM pool WHERE NOT EXISTS(SELECT 1 FROM sl) ORDER BY random() LIMIT 1 FOR UPDATE
)
SELECT * FROM sl FULL JOIN fu USING (plain_number);
然而,它等待随机plain_number
释放其锁定。我不认为可以等待释放第一把锁。
设定:
create table pool(
plain_number text primary key
);
insert into pool(plain_number)
select generate_series(1, 9)::text;
这是我为测试编写的一个小node.js脚本:
const pg = require("pg");
const options = {
"user": "test",
"password": "test",
"database": "test"
};
function thread_ish() {
const client = new pg.Client(options);
const end = client.end.bind(client);
const rollback = function (client) {
client.query("ROLLBACK", end);
};
client.connect(function () {
client.query("BEGIN", function (err) {
if (err) {
console.error(err);
return rollback(client);
}
client.query(
"WITH sl AS ("
+ " SELECT plain_number FROM pool ORDER BY random() LIMIT 1 FOR UPDATE SKIP LOCKED"
+ "), fu AS ("
+ " SELECT plain_number FROM pool WHERE NOT EXISTS(SELECT 1 FROM sl) ORDER BY random() LIMIT 1 FOR UPDATE"
+ ")"
+ "SELECT * FROM sl FULL JOIN fu USING (plain_number)",
function (err, result) {
if (err) {
console.error(err);
return rollback(client);
}
console.log("Selected number is", result.rows);
setTimeout(function () {
client.query("COMMIT", end);
}, 1000);
}
);
});
});
}
for (var i = 0; i < 13; ++i) {
setTimeout(thread_ish, Math.random() * 100);
}