Many distinct values vs. few distinct values

Question

Hello,
If I have a table with many millions of records and I query this table with a where clause on a column with a few distinct values, would it perform faster than this column with many, many distinct values? If so, could someone please help me understand why? The where clause would be like this simply:
where column1 = 'xyz'
Could you please answer for the cases where the column IS and ISN'T indexed?
Thanks!

Answer 1

arizona9952 wrote:
Are you suggesting that the index might not be used if the column wasn't selective?
SQL> create table tbl1 as select 1 id from dba_source;
Table created.
SQL> create index tbl1_idx on tbl1(id);
Index created.
SQL> exec dbms_stats.gather_table_stats('SCOTT','TBL1');
PL/SQL procedure successfully completed.
SQL> explain plan for
  2  select * from tbl1 where id = 1;
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 312383637
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |   304K|   892K|   117  (10)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| TBL1 |   304K|   892K|   117  (10)| 00:00:02 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter("ID"=1)
13 rows selected.
SQL> update tbl1 set id = rownum;
303652 rows updated.
SQL> commit;
Commit complete.
SQL> exec dbms_stats.gather_table_stats('SCOTT','TBL1');
PL/SQL procedure successfully completed.
SQL> explain plan for
  2  select * from tbl1 where id = 1;
Explained.
SQL>  @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 2835566544
| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT |          |     1 |     4 |     3   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| TBL1_IDX |     1 |     4 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - access("ID"=1)
13 rows selected.
SQL> SY.