script to find all bigint that can be converted to int fields in postgresql performance
Following script should help to do the job:

create or replace procedure cbi ()
language plpgsql
as $$
--
declare 
s text;
max_value bigint;
c cursor for
 select 
  pg_namespace.nspname, 
  pg_class.relname, 
  pg_attribute.attname, 
  pg_type.typname 
 from pg_class
 join pg_namespace
 on pg_class.relnamespace = pg_namespace.oid
 and pg_namespace.nspname &lt;&gt; 'pg_catalog'
 join pg_attribute
 on pg_class.oid = pg_attribute.attrelid
 join pg_type
 on pg_attribute.atttypid = pg_type.oid
 and pg_type.typname='int8'
 where pg_class.relkind in ('r','v');
begin
 for r in c
 loop
  s := 'select  max(' || r.attname || ') from ' || r.nspname || '.' || r.relname; 
  execute s into max_value;
  if max_value &lt;= +2147483647 and max_value &gt;= -2147483648 
  then
   raise notice '%.%.% max=% can be replaced with int.', r.nspname, r.relname, r.attname, max_value;
  else
   raise notice '%.%.% max=% CANNOT be replaced with int.', r.nspname, r.relname, r.attname, max_value;
  end if;
 end loop;
end;
--
$$;


Example:

postgres=# call cbi();
NOTICE:  public.pg_stat_statements.queryid max=9148650842799727408 CANNOT be replaced with int.
NOTICE:  public.pg_stat_statements.calls max=118 can be replaced with int.
NOTICE:  public.pg_stat_statements.rows max=4060311 can be replaced with int.
NOTICE:  public.pg_stat_statements.shared_blks_hit max=577699 can be replaced with int.
NOTICE:  public.pg_stat_statements.shared_blks_read max=637 can be replaced with int.
NOTICE:  public.pg_stat_statements.shared_blks_dirtied max=991 can be replaced with int.
NOTICE:  public.pg_stat_statements.shared_blks_written max=991 can be replaced with int.
NOTICE:  public.pg_stat_statements.local_blks_hit max=0 can be replaced with int.
NOTICE:  public.pg_stat_statements.local_blks_read max=0 can be replaced with int.
NOTICE:  public.pg_stat_statements.local_blks_dirtied max=0 can be replaced with int.
NOTICE:  public.pg_stat_statements.local_blks_written max=0 can be replaced with int.
NOTICE:  public.pg_stat_statements.temp_blks_read max=171 can be replaced with int.
NOTICE:  public.pg_stat_statements.temp_blks_written max=171 can be replaced with int.
NOTICE:  public.t.x max=1 can be replaced with int.
CALL
postgres=#