infinite loop in an update statement

[Fabrice Chapuis]

Hi,

table a and b are empty, this query does not return. It seems we enter in
infinite loop.
why this update does not return instantly?

UPDATE table_a a
 SET col1 = (SELECT MIN(b.col1)
                    FROM table_b b
                    WHERE b.col2 = a.col2)

Regards,

Fabrice