sql - MySQL remove duplicates from big database quick

Question

I've got big (>Mil rows) MySQL database messed up by duplicates. I think it could be from 1/4 to 1/2 of the whole db filled with them. I need to get rid of them quick (i mean query execution time). Here's how it looks:
id (index) | text1 | text2 | text3
text1 & text2 combination should be unique, if there are any duplicates, only one combination with text3 NOT NULL should remain. Example:

1 | abc | def | NULL  
2 | abc | def | ghi  
3 | abc | def | jkl  
4 | aaa | bbb | NULL  
5 | aaa | bbb | NULL  

...becomes:

1 | abc | def | ghi   #(doesn't realy matter id:2 or id:3 survives)   
2 | aaa | bbb | NULL  #(if there's no NOT NULL text3, NULL will do)

New ids cold be anything, they do not depend on old table ids.
I've tried things like:

CREATE TABLE tmp SELECT text1, text2, text3
FROM my_tbl;
GROUP BY text1, text2;
DROP TABLE my_tbl;
ALTER TABLE tmp RENAME TO my_tbl;

Or SELECT DISTINCT and other variations.
While they work on small databases, query execution time on mine is just huge (never got to the end, actually; > 20 min)

Is there any faster way to do that? Please help me solve this problem.

Answer 1

I believe this will do it, using on duplicate key + ifnull():

create table tmp like yourtable;

alter table tmp add unique (text1, text2);

insert into tmp select * from yourtable 
    on duplicate key update text3=ifnull(text3, values(text3));

rename table yourtable to deleteme, tmp to yourtable;

drop table deleteme;

Should be much faster than anything that requires group by or distinct or a subquery, or even order by. This doesn't even require a filesort, which is going to kill performance on a large temporary table. Will still require a full scan over the original table, but there's no avoiding that.