Oracle 18c

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create or replace function rever1(x int)
     return number
     is
       y varchar2(30);
       c varchar2(30);
       v int;
     begin
       y:=to_char(x);

       
       for i in reverse 1.. length(y)
       loop 
         c:= c|| substr(y,i,1); 
       end loop; 
       v:=to_number(c);
       return v;
     end rever1;
     /
create or replace function rever2(x int)
     return number
     is
       y varchar2(30);
       c varchar2(30);
       v int;
     begin
       y:=to_char(x);

       select reverse(y) into c from dual;

       v:=to_number(c);
       return v;
     end rever2;
     /
declare
  timestart timestamp;
  timeEnd timestamp;
  result number;
begin
  timestart := SYSTIMESTAMP;
  result:= rever1(123456789);
  timeEnd := SYSTIMESTAMP;
  dbms_output.put_line(((extract(hour from timeEnd)*3600)+(extract(minute from timeEnd)*60)+extract(second from timeEnd))-((extract(hour from timeStart)*3600)+(extract(minute from timeStart)*60)+extract(second from timeStart)));
  
  timestart := SYSTIMESTAMP;
  result:= rever2(123456789);
  timeEnd := SYSTIMESTAMP;
  dbms_output.put_line(((extract(hour from timeEnd)*3600)+(extract(minute from timeEnd)*60)+extract(second from timeEnd))-((extract(hour from timeStart)*3600)+(extract(minute from timeStart)*60)+extract(second from timeStart)));
end;
/
1 rows affected

dbms_output:
.003953
.031724

declare
  timestart timestamp;
  timeEnd timestamp;  
  result number;
begin
  timestart := SYSTIMESTAMP;
  result:= rever1(12345678912345678);
  timeEnd := SYSTIMESTAMP;
  dbms_output.put_line(((extract(hour from timeEnd)*3600)+(extract(minute from timeEnd)*60)+extract(second from timeEnd))-((extract(hour from timeStart)*3600)+(extract(minute from timeStart)*60)+extract(second from timeStart)));
  
  timestart := SYSTIMESTAMP;
  result:= rever2(12345678912345678);
  timeEnd := SYSTIMESTAMP;
  dbms_output.put_line(((extract(hour from timeEnd)*3600)+(extract(minute from timeEnd)*60)+extract(second from timeEnd))-((extract(hour from timeStart)*3600)+(extract(minute from timeStart)*60)+extract(second from timeStart)));
end;
/
1 rows affected

dbms_output:
.000037
.000196