mysql全文索引排序问题

请问我用以下命令：

select * from search where match(tissue) against ('lung*' in boolean mode) limit 0,2;

所用时间是0.01

但是我进行排序后慢了很多：

select * from search where match(tissue) against ('lung*' in boolean mode) order by  gene asc limit 0,2;

我该怎么修改或者加个什么索引让速度变快呢？谢谢！

1、可以explain select * from search where match(tissue) against ('lung*' in boolean mode) order by gene asc limit 0,2;看下执行计划，
2、tissue和gene列加索引