Retrieving Periodical data

Question

Hello ALL,
I have a requirement like to retrive past 6 months data and i have used below query.
select count(1) from event where trunc(start_datae) between trunc(sydate)-180 and trunc(sysdate);it is giving the results but am not sure whether it is giving correct data or not.If ran the above query it is taking more time to execute.
Is the above apporach is correct?
Is there any differece between trunc(sysdate)-180 and trunc(sysdate-180) ?
Please suggest me.
Thanks a lot for your help.

Answer 1

Rahul India wrote:
>
2. You have used TRUNC function on START_DATE. By doing so you have made any index on START_DATE unusable. Karthich how the index on start_date is made unusable?Index is nothing but a skinny version of the table. That is it stores the column data in a specific structure (BTree for example).
When you apply a function like TRUNC on a column you change the data, and hence there is no point in looking into the index.
consider this test case
SQL> create table t
  2  as
  3  select sysdate+ level dt, 'xx' val
  4    from dual
  5  connect by level <= 10000;
Table created.
SQL> create index t_idx on t(dt);
Index created.
SQL> exec dbms_stats.gather_table_stats(user, 'T', cascade=>true)
PL/SQL procedure successfully completed.I want to get row from tomorrow where dt is tomorrow (time part can varie).
First using TRUNC, You can see Index is not used
SQL> select *
  2    from t
  3   where trunc(dt) = trunc(sysdate+1);
DT        VA
08-MAR-13 xx
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
SQL_ID  82a8d4d4fpm3g, child number 0
select *   from t  where trunc(dt) = trunc(sysdate+1)
Plan hash value: 2153619298
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |       |       |    12 (100)|          |
|*  1 |  TABLE ACCESS FULL| T    |   100 |  1100 |    12  (34)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(TRUNC(INTERNAL_FUNCTION("DT"))=TRUNC(SYSDATE@!+1))
18 rows selected.Now without using any function on DT column
SQL> select *
  2    from t
  3   where dt between trunc(sysdate+1) and trunc(sysdate+2) - interval '1' second;
DT        VA
08-MAR-13 xx
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
SQL_ID  0m4knj1jsabx6, child number 0
select *   from t  where dt between trunc(sysdate+1) and trunc(sysdate+2) -
interval '1' second
Plan hash value: 258909497
| Id  | Operation                    | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |       |       |       |     1 (100)|          |
|*  1 |  FILTER                      |       |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| T     |     2 |    22 |     1   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | T_IDX |     2 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(TRUNC(SYSDATE@!+1)<=TRUNC(SYSDATE@!+2)-INTERVAL'+00
              00:00:01.000000' DAY(2) TO SECOND(6))
   3 - access("DT">=TRUNC(SYSDATE@!+1) AND
              "DT"<=TRUNC(SYSDATE@!+2)-INTERVAL'+00 00:00:01.000000' DAY(2) TO SECOND(6))
24 rows selected.
SQL>