dynamic - How to use a parameter column name value in postgresql? -

i have issue need calculate variable on basis of different start , end date can change per client.

currently, have implemented solution below:-

for example:

created static table : ref_table
columns : training_start_date, training_end_date
values : isb_start_dt , isb_end_dt
i.e. 1 row. table created if client wants use dates, values can changed to
values : cs_start_dt , cs_end_dt
or anything.
note: table contain 1 row @ time.
query used:

   create table ref_table(training_start_date varchar(30), training_end_date     varchar(30);     insert ref_table values('isb_start_dt','isb_end_dt') 

now, there sql query use table , calculate features using that, there lot of columns , features being used in query giving sample code.

sample code:     create or replace view prm_vw      select s2.*,           case            when newvalue < 0 null           else newvalue end newvalue1          (                select                       s1.*,                      case                           when training_start_date  = 'isb_start_dt' ,                           training_start_date = 'isb_end_dt'                           isb_start_dt - isb_end_dt                           when training_start_date  = 'cs_start_dt' ,                           training_start_date = 'cs_end_dt'                           cs_start_dt - cs_end_dt                       end newvalue                 (                      select                       avg(date_to_calculate) on (partition country)                       isb_start_dt,                       avg(date_to_calculate) on (partition region)                       cs_start_dt,                      isb_end_dt,                      cs_end_dt                      ref.*                      prm_cmop                      cross join                       ref_table ref                  )s1             )s2  

i have implemented in above way feel not optimized way still hard coding , checking if training_start_date = 'isb_start_dt' can't directly right code as
isb_start_dt - isb_end_dt newvalue training_start_date - training_end_date because training_start_date contains column name isb_start_dt , interested in column value of isb_start_dt. also, if in future client wants use column, have change view script again include in case when lines.

please can me in explaining best way achieve this.