I create a new DB just as same as the previous one. I use this new DB for my business (new data inserting to this new DB), after running for a while (about 1 hour), I import my history data from the old DB into the new one. When all this been finished, I find a phenomena very schange. When I query data for last 1 days, it's rather slow; while query data for rencent 30 days, it is quick as before. When querying data 2 days before for any period (1 day or n days), it is quick as before. I don't know why. Can any one give me an answer. Thanks. I use cluster index at date column and a replication at the tables queried on.