Hi Zhigang, On Feb 11, 2014, at 8:48 PM, Zhigang Zhang <zzgang2...@gmail.com> wrote:
> I want to know the reason, in my opinion, to scan the smaller index data has > better performance than to scan the whole table data. I think I understand the question - you are asking why MySQL will not index scan, find matching records, and then look them up rather than table scan? I believe the answer is that there is no way of knowing if 1 row matches, or all rows match. In the worst case (where all rows match), it is much more expensive to traverse between index and data rows for-each-record. So a table scan is a “safe choice" / has less variance. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql
