BUG #13510: memory problem with mass-update statements

The following bug has been logged on the website:

Bug reference:      13510
Logged by:          Peter Slavov
Email address:      peter.slavov@sumup.com
PostgreSQL version: 9.4.4
Operating system:   Debian wheezy
Description:

Hi,

In our company we have a zabbix monitoring server which uses PostgreSQL
database to store data. Most of the time the servers run very fast and
stable. But after a zabbix server restart something strange happen - some
cache flush to database is activated and on the database there multiple
updates are executed. Updates are formed as a string with multiple queries
in it - like this:
------------
update trends_uint set num=12,value_min=1,value_avg=1,value_max=1 where
itemid=24134 and clock=1437400800;
update trends_uint set
num=6,value_min=7028177,value_avg=7029677,value_max=7031178 where
itemid=28942 and clock=1437400800;
update trends_uint set num=12,value_min=3,value_avg=3,value_max=3 where
itemid=38537 and clock=1437400800;
update trends_uint set num=12,value_min=1,value_avg=1,value_max=1 where
itemid=38538 and clock=1437400800;
update trends_uint set num=12,value_min=1,value_avg=1,value_max=1 where
itemid=38539 and clock=1437400800;
update trends_uint set
num=6,value_min=1434709932,value_avg=1434709932,value_max=1434709932 where
itemid=40349 and clock=1437400800;
update trends_uint set num=12,value_min=2,value_avg=2,value_max=2 where
itemid=44233 and
update trends_uint set num=12,value_min=1,value_avg=1,value_max=1 where
itemid=44234 and clock=1437400800;
-- more -- ~ 50 queries max
----------
when this happen this process on the PostgreSQL server starts to eat the
whole available memory (4GB RAM + 8GB swap), until it crashes with "out of
memory" error , and then it starts again.
When this happen zabbix server is not affected in any way.
Here is some more info:
- database size is ~ 50GB
- updated tables when this happen are partitioned daily for an year - ~365
partitions
- shared buffers are set to 900MB

Can you tell me where can be the problem - is it the partitions or something
else?

thanks