Internals - what are the stages of partition creation?

[Shlomo Priymak]

Hi,

I'm running an ALTER TABLE on a very large table which looks like this:

CREATE TABLE t(id bigint, data text).

The alter looks like this:

ALTER TABLE t PARTITION BY RANGE (id) (
PARTITION p0 VALUES LESS THAN (10000000),
PARTITION p1 VALUES LESS THAN (20000000),
PARTITION p2 VALUES LESS THAN (30000000),
PARTITION p3 VALUES LESS THAN (40000000),
PARTITION p4 VALUES LESS THAN (50000000),
PARTITION pother VALUES LESS THAN MAXVALUE;

I ran this overnight, and assumed it would be done, since it created 6 files for the partitions, and filled them up one after another.
When I came back in the morning, to my surprise the query was still working with a state of "copy to tmp table" in the process list. It created another 6 files, and filled them up again.
The original file t.ibd was deleted though, so i didn't need x3 the space to perform this, only x2.

The question is, What is it doing now? What is this extra stage? Shouldn't going over the original table and spilling the data into the partition buckets be enough? Is there a way to avoid it?

Thanks in advance!
Shlomo