High CPU usage after partitioning

I'm running postgresl 9.0. After partitioning a big table, CPU usage raised from average 5-10% to average 70-80%. 

- the table contains about 20.000.000 rows

- partitions are selected using a trigger, based on an indexed field, a date (IF date_taken >= x AND date_taken < y)

- I created 5 partitions, the 2012 one now contains most of the rows. The 2013 partition is the "live" partition, mostly insert, a few select based on the above indexed field. The 2013, 2014, 2015 partitions are empty

- constraint execution is on.

 

I have 2 weeks CPU usage reports and the pattern definately changed after I made the partitions. Any idea?  

thanks,
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rd

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