MySQL explain select stuck creating temporary table

[Eric Coll]

Why an explain select shows in the process list as "Copying to tmp table"? Shouldn't the explain just create the query plan and don't execute the query? It also takes a very long time and seems blocked, although nothing else is running on the server.

Here is the query:
explain select date(from_unixtime(timestamp)) as day, account, internal_az, max(count) as max
from
( select timestamp, account, zone as internal_az, sum(s_total) as count
from capacity.reservation
where timestamp between unix_timestamp('2012-03-10') and unix_timestamp('2012-04-10') and account in ('833008473584')
group by timestamp, account, internal_az
) sum
group by day, account, internal_az;