I ask for your help on this join problem!
Thank you in advance.

TABLE t1
*--*----*----*
|pk|col1|col2|
*--*----*----*
|1 |a |a...|
|2 |b |b...|
|3 |c |c...|
|4 |d |d...|
|5 |e |e...|
|6 |f |f...|
|.... |
*--*----*----*
pk pk
index col1

TABEL t2
*--*----*----*-----*
|pk|col1|col2|col3 |
*--*----*----*-----*
|1 |a |b |1,000|
|2 |c |a |2,000|
|3 |a |c |3,000|
|4 |d |e |4,000|
|5 |b |b |5,000|
|6 |c |e |6,000|
|7 |e |f |7,000|
|.... |
*--*----*----*-----*
pk pk
index col1
index col2

THE RESULT (should be):
*-------*-------*------------*------------*
|t1.col1|t1.col2|sum(t2.col3)|sum(t2.col3)|
*-------*-------*------------*------------*
|a |a... | 4,000| 2,000|
|b |b... | 5,000| 6,000|
|c |c... | 8,000| 3,000|
|d |d... | 4,000| 0|
|e |e... | 7,000| 10,000|
|f |f... | 0| 7,000|
*-------*-------*------------*------------*

I've try this:
SELECT
x.col1,
x.col2,
sum(y1.col3) as col3,
sum(y2.col3) as col4
FROM
t1 x
LEFT JOIN t2 y1 ON y1.col1 = x.col1
LEFT JOIN t2 y2 ON y2.col2 = x.col1
GROUP BY 1,2

but with a real big t1 and t2 tables it take forever...!?
</doru>