mysql的SQL_CALC_FOUND_ROWS在postgresql中是否有类似的函数？

Question

大家都用mysql知道：

SELECT SQL_CALC_FOUND_ROWS ..... FROM table WHERE ... LIMIT 5, 10; 

，右运行此之后：

SELECT FOUND_ROWS(); 

我如何做到这一点的postrgesql？到目前为止，我发现只有我必须发送查询两次的方法...

Answer 1

不，没有（至少not as of July 2007）。恐怕你不得不诉诸：

BEGIN ISOLATION LEVEL SERIALIZABLE; 

SELECT id, username, title, date FROM posts ORDER BY date DESC LIMIT 20; 
SELECT count(id, username, title, date) AS total FROM posts; 

END; 

的isolation level需要为SERIALIZABLE，以确保查询没有看到SELECT语句之间的并发更新。

但是，您拥有的另一选项是使用触发器对行进行计数，因为它们是INSERT ed或DELETE d。假设你有如下表：

CREATE TABLE posts (
    id  SERIAL PRIMARY KEY, 
    poster TEXT, 
    title TEXT, 
    time TIMESTAMPTZ DEFAULT now() 
); 

INSERT INTO posts (poster, title) VALUES ('Alice', 'Post 1'); 
INSERT INTO posts (poster, title) VALUES ('Bob',  'Post 2'); 
INSERT INTO posts (poster, title) VALUES ('Charlie', 'Post 3'); 

然后，执行以下步骤来创建一个名为POST_COUNT表包含posts行数的运行计数：

-- Don't let any new posts be added while we're setting up the counter. 
BEGIN; 
LOCK TABLE posts; 

-- Create and initialize our post_count table. 
SELECT count(*) INTO TABLE post_count FROM posts; 

-- Create the trigger function. 
CREATE FUNCTION post_added_or_removed() RETURNS TRIGGER AS $$ 
    BEGIN 
     IF TG_OP = 'DELETE' THEN 
      UPDATE post_count SET count = count - 1; 
     ELSIF TG_OP = 'INSERT' THEN 
      UPDATE post_count SET count = count + 1; 
     END IF; 
     RETURN NULL; 
    END; 
$$ LANGUAGE plpgsql; 

-- Call the trigger function any time a row is inserted. 
CREATE TRIGGER post_added_or_removed_tgr 
    AFTER INSERT OR DELETE 
    ON posts 
    FOR EACH ROW 
    EXECUTE PROCEDURE post_added_or_removed(); 

COMMIT; 

注意这个保持运行计数为，全部为，行数为posts。为了保持一定行的运行计数，你必须调整它：

SELECT count(*) INTO TABLE post_count FROM posts WHERE poster <> 'Bob'; 

CREATE FUNCTION post_added_or_removed() RETURNS TRIGGER AS $$ 
    BEGIN 
     -- The IF statements are nested because OR does not short circuit. 
     IF TG_OP = 'DELETE' THEN 
      IF OLD.poster <> 'Bob' THEN 
       UPDATE post_count SET count = count - 1; 
      END IF; 
     ELSIF TG_OP = 'INSERT' THEN 
      IF NEW.poster <> 'Bob' THEN 
       UPDATE post_count SET count = count + 1; 
      END IF; 
     END IF; 
     RETURN NULL; 
    END; 
$$ LANGUAGE plpgsql; 

Answer 2

不，PostgreSQL并不试图计算所有相关结果时，你只需要10个结果。您需要一个单独的COUNT来计算所有结果。

Answer 3

有一个简单的方法，但请记住，以下COUNT（*）汇聚功能将应用于所有行回来后，在那里和限制之前/偏移量（可能是costy）

选择 ID， “计数”（*）OVER（）AS CNT FROM 对象 WHERE ID> 2 OFFSET 50 LIMIT 5