BUG #14607: session variables are non-deterministic

The following bug has been logged on the website:

Bug reference: 14607
Logged by: Jan Karaffa
Email address: jankaraffa(at)gmail(dot)com
PostgreSQL version: 9.4.11
Operating system: Red Hat 4.4.7-17, 64bit
Description:

let's have myPartTable as partitioned table with check constraint on
part_key::text column

SET myvar.partition_key='ABC';
select count(*) from myPartTable
where part_key = current_setting('myvar.partition_key')::text;

Execution plan shows that all inherited (partitioned) tables are scanned. It
behaves like current_setting function is non-deterministic and therefore sql
optimalizator is not considered output as constant. For me it looks like bug
as session variable can not change during parsing/execution.

If current_setting function would behave deterministic it would enable us to
pass session parameters into the view and perform limitation inside of view
where it makes sense. Current "workaround" is to move logic from view to
function, which is not as easy to use in hibernate as the standard views.