Reg: query plan -

Question

Hi Experts,
I was playing/experimenting with query plans but got stuck up with the below. I guess, I'm overlooking something.
ranit@XE11GR2>> create table t
  2  as
  3  select * from all_objects;
Table created.
ranit@XE11GR2>> exec dbms_stats.gather_table_stats('rb1','t');
PL/SQL procedure successfully completed.
ranit@XE11GR2>> create index t_uidx
  2  on
  3  t(owner, object_type, object_name);
Index created.
ranit@XE11GR2>> select * from t
  2  where owner = 'SYS';
Elapsed: 00:00:00.00
Execution Plan
Plan hash value: 1601196873
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |  1642 |   144K|    67   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |  1642 |   144K|    67   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("OWNER"='SYS')
ranit@XE11GR2>> select count(*) from t
  2  where owner = 'SYS';
Elapsed: 00:00:00.00
Execution Plan
Plan hash value: 1979783846
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |        |     1 |     7 |    11   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE   |        |     1 |     7 |            |          |
|*  2 |   INDEX RANGE SCAN| T_UIDX |  1642 | 11494 |    11   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("OWNER"='SYS')
Doubt: Why does the first query have a FTS while the second query (with COUNT(*)) does an Index scan?
ranit@XE11GR2>> select owner, object_name, namespace from t
  2  where owner = 'SYS';
Elapsed: 00:00:00.05
Execution Plan
Plan hash value: 1601196873
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |  1642 | 47618 |    67   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |  1642 | 47618 |    67   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("OWNER"='SYS')
Also, in the above I'm seeing that if I add a non-indexed column in SELECT clause (like column 'namespace') it goes for a FTS. How does column in SELECT clause affect the use of Indexes?
Any pointers if highly appreciated.
Thanks,
Ranit

Answer 1

The second query doesn't have to visit the table - all the required data is in the index- and Oracle thinks that a range scan will only have to go through a small number of leaf blocks to find all the SYS entries.  The first query needs data which can only be found in the table, and Oracle thinks the 1642 rows it need to find will be spread across so many blocks that it's quicker to do the tablescan than jump to each one separately after doing an index range scan.
If you want to see how much work Oracle thinks it would take to use an indexed access path into the table, you could test by adding a hint to the query.
Regards
Jonathan Lewis
Now on twitter: @jloracle