如何在postgresql中编写关于最大行数的约束

卡西诺是对的；触发是实现这一目标的最佳方式.

以下是代码:

CREATE OR REPLACE FUNCTION enforce_photo_count() RETURNS trigger AS $$
DECLARE
    max_photo_count INTEGER := 10;
    photo_count INTEGER := 0;
    must_check BOOLEAN := false;
BEGIN
    IF TG_OP = 'INSERT' THEN
        must_check := true;
    END IF;

    IF TG_OP = 'UPDATE' THEN
        IF (NEW.owner != OLD.owner) THEN
            must_check := true;
        END IF;
    END IF;

    IF must_check THEN
        -- prevent concurrent inserts from multiple transactions
        LOCK TABLE photos IN EXCLUSIVE MODE;

        SELECT INTO photo_count COUNT(*) 
        FROM photos 
        WHERE owner = NEW.owner;

        IF photo_count >= max_photo_count THEN
            RAISE EXCEPTION 'Cannot insert more than % photos for each user.', max_photo_count;
        END IF;
    END IF;

    RETURN NEW;
END;
$$ LANGUAGE plpgsql;


CREATE TRIGGER enforce_photo_count 
    BEFORE INSERT OR UPDATE ON photos
    FOR EACH ROW EXECUTE PROCEDURE enforce_photo_count();

我加入了表锁定，以避免出现两个并发的tansaction会为一个用户计算照片数的情况，看到当前的计数低于限制1，然后两个都插入，这会导致您超过限制1.如果您不担心这一点，那么最好移除锁定，因为它可能会成为许多插入/更新的瓶颈.