mysql多表join中，为什么子查询会那么慢，怎么解决

下面的sql执行后cpu100%，但是这三张join的表数据量都在2W左右，不应该啊，是不是数据库服务器配置出了问题啊。sql如下：

select a.excel_id, a.rpt_id, a.acc_std, a.prd_year, a.prd_qtr, a.accumulate, a.biz_cd, a.acc_cd, a.row_num, 
a.col_num,a.col_unit , a.col_cur, a.row_unit, a.row_cur, a.data_id, a.item_cd, a.data_val,     
a.data_comment, a.data_trans_comment, a.trans_flag, a.trace_flag, a.trace_edit,    
b.data_src, b.orig_unit, b.pre_unit, c.data_trace 
from (select * from cdp.listed_cpny_rpt_fin_style_6 where excel_id = 32) a 

inner join (select * from cdp.listed_cpny_fin_rpt_prd where excel_id = 32) b 

on (a.excel_id=b.excel_id and a.rpt_id=b.rpt_id and a.acc_std=b.acc_std and a.prd_year=b.prd_year 
and a.prd_qtr=b.prd_qtr and a.accumulate=b.accumulate and a.biz_cd=b.biz_cd and a.acc_cd=b.acc_cd )
left outer join (select * from cdp.listed_cpny_rpt_fin_trace_6 where excel_id = 32) c on 
(a.excel_id = c.excel_id and a.rpt_id = c.rpt_id and a.acc_std = c.acc_std and a.prd_year = c.prd_year  
and a.prd_qtr = c.prd_qtr and a.accumulate=c.accumulate and a.biz_cd = c.biz_cd 
and a.acc_cd = c.acc_cd and a.row_num = c.row_num)