Oracle 10.2.0.4 Index on the time stamp column not used when SYSTIMESTAMP is used.

Hello
I have a table with a timestamp (6) column B. About 300000 rows in the tables...
I created index 'idx' on column B.

When I compare the column 'B' with systimestamp, it does not use the index, whereas if I compare 'B' with sysdate it uses the index.

For example:
Select count (*) in a b where clause < = sysdate;

The above used the subscript "idx" and executed in 1 second

Select count (*) in a b where clause < = systimestamp;

The foregoing does not use the index and performed in 19 seconds.

Any clue?

Thanks in advance

Oracle uses the internal functions when you use SYSTIMESTAMP:

Work around is to use TO_TIMESTAMP as shown below... Or define a function based index...

You can check the performance querying against a column "TIMESTAMP WITH time ZONE SCHEDULE" issue link too

SQL> create table a(b timestamp(6));

Table created.

SQL> insert into a
  2  select systimestamp+(level/24) from dual connect by level <= 30000;

30000 rows created.

SQL> commit;

Commit complete.

SQL> create index ndx on a(b);

Index created.

SQL> explain plan for
  2  select count(*) from a where b <= sysdate;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3858831102

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    13 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE   |      |     1 |    13 |            |          |
|*  2 |   INDEX RANGE SCAN| NDX  |     1 |    13 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("B"<=SYSDATE@!)

Note
-----
   - dynamic sampling used for this statement

18 rows selected.

SQL> explain plan for
  2  select count(*) from a where b <= systimestamp;

Explained.

SQL>  select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3918351354

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    13 |    20  (15)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |    13 |            |          |
|*  2 |   TABLE ACCESS FULL| A    |     1 |    13 |    20  (15)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(SYS_EXTRACT_UTC(INTERNAL_FUNCTION("B"))<=SYS_EXTRACT_UTC(S
              YSTIMESTAMP(6)))

Note
-----
   - dynamic sampling used for this statement

19 rows selected.

--"Just tried using TO_TIMESTAMP"

SQL> explain plan for
  2  select count(*) from a where b <= to_timestamp(systimestamp);

Explained.

SQL>
SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3858831102

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    13 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE   |      |     1 |    13 |            |          |
|*  2 |   INDEX RANGE SCAN| NDX  |     4 |    52 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("B"<=TO_TIMESTAMP(TO_CHAR(SYSTIMESTAMP(6))))

14 rows selected.

Published by: JAC on October 23, 2012 11:18

Tags: Database

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