Any general performance implications of a varchar vs a numeric field?

Question

I received an email that said, in effect, a client made their customer ID a varchar field, and this is causing a performance issue (yes, I know I need to dig deeper and find out why they think that is the cause but it's all I have for now). So my question is - generally speaking, are there performance concerns associated with using a varchar field vs numeric? I had never heard that before, and I can't seem to find any evidence online that it would, but figured I'd run it by the experts here.
Thanks all
Mike

Answer 1

The first thing that comes to mind is range expressions, since it will tend to look to the optimizer as though there are more possible values in any given range:
CREATE TABLE wr_test NOLOGGING
AS
SELECT ROWNUM numcol, CAST(ROWNUM AS VARCHAR2(10)) stringcol
FROM   dual CONNECT BY ROWNUM < 1e6;
CREATE INDEX wr_test_numcol_ix ON wr_test(numcol);
CREATE INDEX wr_test_stringcol_ix ON wr_test(stringcol);
BEGIN
       DBMS_STATS.GATHER_TABLE_STATS(USER,'WR_TEST',method_opt => 'for all columns size auto');
END;
SQL> select * from wr_test where numcol between 1200 and 1235
  2
SQL> @xplan
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Compatibility is set to 11.1.0.0.0
Plan hash value: 2758090675
| Id  | Operation                   | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |                   |    37 |   407 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| WR_TEST           |    37 |   407 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | WR_TEST_NUMCOL_IX |    37 |       |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("NUMCOL">=1200 AND "NUMCOL"<=1235)
SQL> select * from wr_test where stringcol between '1200' and '1235'
  2
SQL>
SQL> @xplan
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Compatibility is set to 11.1.0.0.0
Plan hash value: 1728591971
| Id  | Operation         | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |         |  4544 | 49984 |   658   (2)| 00:00:08 |
|*  1 |  TABLE ACCESS FULL| WR_TEST |  4544 | 49984 |   658   (2)| 00:00:08 |
Predicate Information (identified by operation id):
   1 - filter("STRINGCOL"<='1235' AND "STRINGCOL">='1200')