mysql-Improve a join performance

I have a table PCUI_treat(CUI1,CUI2), and I want to combine each value from CUI1 with all values from CUI2. So I use the INNER JOIN like this:

select distinct t1.CUI1, t2.CUI2 
from PCUI_treat t1 
inner join PCUI_treat t2;

But when the number of rows in PCUI_treat is high, the performance is bad. The EXPLAIN like this: EXPLAIN OUTPUT

Indexes on CUI1 and CUI2 don't help here, right? How can I improve the performance?