sql - Choosing the view query at runtime (postgres database) -

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i want create view choose between 2 possible selects based on session variable (set_config) on runtime.
today "union all" between 2 selects in following manner: 

create view my_view ( select * x cast(current_setting('first_select') int)=1 , ...; union select * y cast(current_setting('first_select') int)=0 , ...; )

the problem postgres optimizer takes bad decisions when target union. when run example: 

select * my_view id in (select id z field='value')

it decides full scan on table x although has index on "id".
there way define such view without using "union" clause?

just or them where-clause. optimiser find invariant conditions.

create view my_view ( select * x          ( cast(current_setting('first_select') int)=1 , <condition1> )  or     ( cast(current_setting('first_select') int)=0 , <condition2> )         ...         )         ;

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