BUG #14675: Perfomance Issue after Upgrading from 9.5.3 to 9.6.3

The following bug has been logged on the website:

Bug reference: 14675
Logged by: Martin Walpen
Email address: walpino(at)gmx(dot)ch
PostgreSQL version: 9.6.3
Operating system: CentOS Linux release 7.3.1611
Description:

Hello

we wanna upgrade our Installations from Version 9.5.3 to 9.6.3

everything works fine with the pg_upgrade

after the Upgrade process we encountered an performance issue

we have a join in a select statement

simply said, it looks like

select....
from
where column_a = coalesce(col_b, coalesce(col_c, col_b))

this was fast on 9.5.3
but now on 9.6.3 is is slow really really slow

the explain plan looks similar and not big difference

when I changed this query to

select....
from
where column_a = coalesce(col_b, col_c, col_b)

which should be the same

it was as fast as on 9.5.3

do you have any explanation for that?

thanks