数据库是mariadb10.5 
有这么一个sql 
SELECT db.no
    FROM db, dt
    WHERE db.dt = dt.no
      and EXISTS (SELECT 1 FROM rr WHERE rr.db = db.no AND rr.project = '1111')

执行时间大约30ms 
然后分页插件在预查询的时候 给外面套了一个count 
  SELECT count(0) FROM (
    SELECT db.no
        FROM db, dt
        WHERE db.dt = dt.no
             and EXISTS (SELECT 1 FROM rr WHERE rr.db = db.no AND rr.project = '1111')
) table_count 

这个查询时间...