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

Question

每个使用mysql的人都知道:

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

并在运行之后:

SELECT FOUND_ROWS();

我怎么在postrgesql中这样做？到目前为止,我发现只有两次我必须发送查询...

Answer 1

不,没有(至少截至2007年7月).我担心你不得不诉诸:

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;

该隔离级别必须SERIALIZABLE确保查询不会看到SELECT语句之间的并发更新.

但是,您拥有的另一个选择是使用触发器来计算INSERTed或DELETEd时的行数.假设您有以下表格:

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,执行以下操作以创建名为post_count的表,该表包含以下行数的运行计数posts:

-- 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;

请注意,这会保留所有行的运行计数posts.要保持某些行的运行计数,您必须调整它:

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;