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.
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