列的顺序应该与顺序相匹配稍后查询列或MySQL不会使用它们。
这是你应该考虑的问题。

The order of the columns should match the order in which the columns are queried later or MySQL will not use them. This is the question you should really think about.

阅读更多 here 。

UPDATE：

关于基数的问题，请阅读这个。
这与你的问题相似吗？它回答了吗？

For your question about cardinality maybe read this. Is this similar to your question? Does it answer it?