4

我在 Postgres 中使用 ltree 扩展来管理树的路径。我想确保名称字段对于树上的任何给定路径都是唯一的。我可以使用约束来完成此操作，还是必须将其构建到查询中？

  demo table
------------------------------
| name (text) | path (ltree) |
------------------------------
| a           | 1            |
------------------------------
| b           | 1.2          |
------------------------------
| b           | 1.3          |
------------------------------
| b           | 1.2.4        | <-- this should fail on insert
------------------------------

如果我必须将它构建到查询中，如果我使用 READ COMMITTED，这是否可能存在竞争条件？

CREATE TABLE demo (
    name text NOT null,
    path ltree NOT null
);
CREATE INDEX path_gist_idx ON demo USING GIST (path);
INSERT INTO demo (name, path)
SELECT 'a', '1'
WHERE NOT EXISTS (
    SELECT * FROM demo WHERE name = 'a' AND (path <@ '1' OR path @> '1')
);
INSERT INTO demo (name, path)
SELECT 'b', '1.2'
WHERE NOT EXISTS (
    SELECT * FROM demo WHERE name = 'b' AND (path <@ '1.2' OR path @> '1.2')
);
INSERT INTO demo (name, path)
SELECT 'b', '1.3'
WHERE NOT EXISTS (
    SELECT * FROM demo WHERE name = 'b' AND (path <@ '1.3' OR path @> '1.3')
);
INSERT INTO demo (name, path)
SELECT 'b', '1.2.4'
WHERE NOT EXISTS (
    SELECT * FROM demo WHERE name = 'b' AND (path <@ '1.2.4' OR path @> '1.2.4')
);
SELECT * FROM demo;
4

3 回答 3

2

另一种解决方案可能是在触发函数或规则中测试新路径（更好的性能）：

触发功能：

CREATE OR REPLACE FUNCTION path_test ()
RETURNS trigger LANGUAGE plpgsql AS
$$
BEGIN
IF EXISTS (SELECT *
             FROM demo
            WHERE name = NEW.name
              AND (path <@ NEW.pah OR path @> NEW.path))
THEN RETURN NULL ;
ELSE RETURN NEW ;
END ;
$$

CREATE OR REPLACE TRIGGER before_insert_update_demo BEFORE INSERT OR UPDATE OF name, path ON demo
FOR EACH ROW EXECUTE FUNCTION path_test () ;

规则

CREATE OR REPLACE RULE path_test_insert AS ON INSERT TO demo
WHERE EXISTS (SELECT *
                 FROM demo
                WHERE name = NEW.name
                  AND (path <@ NEW.pah OR path @> NEW.path))
DO NOTHING ;

CREATE OR REPLACE RULE path_test_update AS ON UPDATE TO demo
WHERE EXISTS (SELECT *
                 FROM demo
                WHERE name = NEW.name
                  AND (path <@ NEW.pah OR path @> NEW.path))
DO NOTHING ;
于 2021-12-08T09:02:00.410 回答
2

您可以创建一个单独的表，其中存储名称和路径

-- this table is equivalent to the demo table
CREATE TABLE record_table (
    title text NOT null
);
-- this table holds the titles
CREATE TABLE title_table (
    name text NOT null,
    path ltree NOT null,
);

并创建一个并行安全函数，您可以在插入原始表时调用该函数

CREATE OR REPLACE FUNCTION verify_unique_title(title text, tree_path ltree) 
RETURNS TEXT AS '
  DECLARE
      duplicate_path ltree;
  BEGIN
      SELECT path 
       FROM title_table 
       INTO duplicate_path 
       WHERE name=title AND path <@> tree_path;
      IF duplicate_path IS NOT NULL 
       THEN raise exception ''title was not unique''; 
       END IF;
      INSERT INTO title_table (name, path) VALUES (title, tree_path);
      RETURN title;
  END;
' LANGUAGE plpgsql PARALLEl UNSAFE;

-- these inserts should work
INSERT INTO record_table (title) VALUES (verify_unique_title('a', '1'::ltree));
INSERT INTO record_table (title) VALUES (verify_unique_title('b', '1.2'::ltree));
INSERT INTO record_table (title) VALUES (verify_unique_title('b', '1.3'::ltree));

-- the following should fail
INSERT INTO record_table (title) VALUES (verify_unique_title('b', '1.2.3'::ltree));

如果要维护单独的索引，则必须实现类似的触发器来更新和删除项目。

于 2021-12-10T10:00:42.883 回答
1

如果您使用已提交的读取，则会出现竞争条件。

原则上，您可以使用 EXCLUDE 约束，但没有可以使用的单个自交换运算符。这意味着您必须发明一个新的运算符，可能称为 <@>，并将其添加到 ltree。我认为这是可能的，但不是你可能期待做的事情。

你可以只使用可序列化的吗？

于 2020-02-20T02:51:02.893 回答