如何编写Postgres动态SQL语句来计算存储在行中的表达式？

-- 创建一个示例表
CREATE TABLE example_table (
    id SERIAL PRIMARY KEY,
    value1 INTEGER,
    value2 INTEGER,
    operator VARCHAR(10),
    result INTEGER
);

-- 插入示例数据
INSERT INTO example_table (value1, value2, operator) VALUES (5, 10, '+');
INSERT INTO example_table (value1, value2, operator) VALUES (8, 3, '*');
INSERT INTO example_table (value1, value2, operator) VALUES (15, 5, '/');

-- 更新表中的结果列，计算存储在行中的表达式
UPDATE example_table
SET result = CASE
    WHEN operator = '+' THEN value1 + value2
    WHEN operator = '*' THEN value1 * value2
    WHEN operator = '/' THEN value1 / value2
    ELSE NULL
END;

-- 查询结果
SELECT * FROM example_table;