To_date function in procedure which returns a cursor type

Question

Hi All
When i execute the following query ,it runs fine and i get required result also
select *from a where adate>to_date('31-dec-2001')
But When i use the same query in a procedure which returns a cursor i get a error
"non numeric character was found where a numeric value was expected".
Then i changed the query to
select *from a where adate>to_date('31-dec-2001','dd-mon-yyyy') and it worked fine.
As i know default date format is 'dd-mon-yyyy' and no need to give the date format
explicitly in to_date function if the date(string) is in default format.
Can any body tell me the reason behind it?
Thanks
Satya

Answer 1

Given a comparison between a date and a string Oracle will convert the string to a date. This means that if you have an index on a string column and compare it to a date the index will not be available, but if you have an index ona date column and compare it to a string column the index will be available.
create table t (string varchar2(30), d  date);
insert into t values (to_char(sysdate), sysdate);
commit;
create index t_n1 on t(string);
create index t_n2 on t(d);
SQL> set autotrace traceonly
SQL> select /*+ index t_n1 */
  2         *
  3    from t
  4   where string = sysdate;
no rows selected
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   TABLE ACCESS (FULL) OF 'T'
SQL> select /*+ index t_n2 */
  2         *
  3    from t
  4   where d = to_char(sysdate);
no rows selected
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'T'
   2    1     INDEX (RANGE SCAN) OF 'T_N2' (NON-UNIQUE)
SQL>