INSERT SELECT LIMIT very very slow

[Laurent Quérel]

I used an INSERT ... SELECT ... LIMIT to copy data between 2 tables.

INSERT IGNORE INTO table2 (col1, col2)
SELECT t1.col1, t2.col2
FROM table1 t1 LEFT JOIN table2 t2 ON t1.col1=t2.col1
WHERE t1.last_update >= 0
AND t1.col1 IS NOT NULL AND t2.col1 IS NULL LIMIT 1000

Volume and indexes :
Table 1 : 2 000 000 rows (index : col1 and last_update)
Table 2 : 1000 rows (index : col1)

I used the IGNORE option to deduplicate rows in table2 (and avoid DISTINCT or GROUP BY clause).

This query takes severals minutes to run. Why ?
In fact, if I used two separate queries (one for select and one for insert) the response is much quicker !

Can anyone explain the problem to me.

Thanks

Laurent Quérel