I was tried to partition one large table, and come with such Postgres behavior that I could not explain. Maybe you will have any suggestions?

I went quite straight forward way (i know that it is not most efficient, but still i had some time):

create table target_table  as select * from parent_table limit 0;
insert into target_table select * from parent_table where date_field >= '20120101' and date_field <='20120131' and tariff_id <> -1;
insert into target_table select * from parent_table where date_field >= '20120201' and date_field <='20120228' and tariff_id <> -1;
insert into target_table select * from parent_table where date_field >= '20120301' and date_field <='20120331' and tariff_id <> -1;
insert into target_table select * from parent_table where date_field >= '20120401' and date_field <='20120430' and tariff_id <> -1;
insert into target_table select * from parent_table where date_field >= '20120501' and date_field <='20120531' and tariff_id <> -1;
.....

And there is results how much time each command took to execute:

08:44:42  [INSERT - 8981055 row(s), 119.962 secs]  Command processed
08:46:36  [INSERT - 8222656 row(s), 114.057 secs]  Command processed
08:48:43  [INSERT - 8981454 row(s), 126.789 secs]  Command processed
08:51:02  [INSERT - 8929325 row(s), 139.815 secs]  Command processed
08:53:13  [INSERT - 9465383 row(s), 130.752 secs]  Command processed
08:55:25  [INSERT - 9302664 row(s), 131.911 secs]  Command processed
08:57:36  [INSERT - 9581798 row(s), 130.853 secs]  Command processed
08:59:58  [INSERT - 10080875 row(s), 142.071 secs]  Command processed
09:15:03  [INSERT - 9717698 row(s), 905.030 secs]  Command processed
10:24:22  [INSERT - 10169181 row(s), 4159.346 secs]  Command processed
11:33:11  [INSERT - 10000601 row(s), 4128.590 secs]  Command processed
12:38:36  [INSERT - 10576846 row(s), 3924.579 secs]  Command processed

I can't find the explanation why sometime inserts from the same table took 2 minutes and sometimes it took 1 hour! What could be reasons for such behavior?

Some info:

PG server running on amazon cloud xlarge EC2 instance.
I am the only one user and it was not overloaded with other tasks.
work_mem = '1GB'
parent_table is pretty large – 800 million of rows.