Today, on irc, someone asked interesting question.
Basically she ran a query like:
SELECT a, b, c, d, e, f FROM TABLE ORDER BY a
then, she processed the query to get, for each a array of unique values of b, c, d, e, and f, and then he inserted it back to database, to some other table.
It was a problem, because the table had many rows (millions I would assume), and the whole process was slow.
So, how to make it faster?