Why full index scan is faster than full table scan?

Hi friends,
In the where clause of a query,if we give a column that contains index on it,then oracle uses index to search data rather than a TABLE ACCESS FULL Operation.
Why index searching is faster?

Sometimes it is faster to use index and sometimes it is faster to use full table scan. If your statistics are up to date Oracle is far more likely to get it right. If the query can be satisfied entirely from the index, then an index scan will almost always be faster as there are fewer blocks to read in the index than there would be if the table itself were scanned. However if the query must extract data from the table when that data is not in te index, then the index scan will be faster only if a small percentage of the rows are to be returned. Consiter the case of an index where 40% of the rows are returned. Assume the index values are distributed evenly among the data blocks. Assume 10 rows will fit in each data block thus 4 of the 10 rows will match the condition. Then the average datablock will be fetched 4 times since most of the time adjacent index entries will not be in the same block. The number of single datablock fetches will be about 4 times the number of datablocks. Compare this to a full table scan that does multiblock reads. Far fewer reads are required to read the entire table. Though it depends on the number of rows per block, a general rule is any query returning more than about 10% of a table is faster NOT using an index.

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I have explation guys..why apple dual qure is faster than other quad qure devices?and appl 1g ram .is faster 2g of other devises?...i wish to get the reason

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steffi2 wrote:
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Ed MacDonald wrote:
          > I have a stateless session bean that exposes a business method which calls another stateless session bean's worker method many times. Both beans use container-managed transactions and are deployed in a WL8.1 server.
          >
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          >
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          >
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hi all
There is a table called "si_sync_operation" that have 171040 number of rows.
I partitioned that table into a new table called "si_sync_operation_par" with 7 partitoins.
I issued the following statements
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SQL> drop table part purge;
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SQL>
SQL> CREATE TABLE non_part
2        (id NUMBER(5),
3         dt    DATE);
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SQL>
SQL> CREATE TABLE part
2        (id NUMBER(5),
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4         PARTITION BY RANGE(dt)
5         (
6         PARTITION part1_jan2008 VALUES LESS THAN(TO_DATE('01/02/2008','DD/MM/YYYY')),
7         PARTITION part2_feb2008 VALUES LESS THAN(TO_DATE('01/03/2008','DD/MM/YYYY')),
8         PARTITION part3_mar2008 VALUES LESS THAN(TO_DATE('01/04/2008','DD/MM/YYYY')),
9         PARTITION part4_apr2008 VALUES LESS THAN(TO_DATE('01/05/2008','DD/MM/YYYY')),
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11       );
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SQL>
SQL>
SQL> insert into non_part select rownum, trunc(sysdate) - rownum from dual connect by level <= 140;
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Execution Plan
Plan hash value: 1731520519
| Id | Operation                     | Name | Rows | Cost (%CPU)| Time     |
|   0 | INSERT STATEMENT              |      |     1 |     2   (0)| 00:00:01 |
|   1 | COUNT                        |      |       |            |          |
|* 2 |   CONNECT BY WITHOUT FILTERING|      |       |            |          |
|   3 |    FAST DUAL                  |      |     1 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
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SQL>
SQL> insert into part select * from non_part;
140 rows created.
Execution Plan
Plan hash value: 1654070669
| Id | Operation         | Name     | Rows | Bytes | Cost (%CPU)| Time     |
|   0 | INSERT STATEMENT |          |   140 | 3080 |     3   (0)| 00:00:01 |
|   1 | TABLE ACCESS FULL| NON_PART |   140 | 3080 |     3   (0)| 00:00:01 |
Note
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SQL>
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|   0 | SELECT STATEMENT |          |   140 | 3080 |     3   (0)| 00:00:01 |
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Plan hash value: 3392317243
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|   0 | SELECT STATEMENT    |      |   140 | 3080 |     9   (0)| 00:00:01 |       |       |
|   1 | PARTITION RANGE ALL|      |   140 | 3080 |     9   (0)| 00:00:01 |     1 |     5 |
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SQL>
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Plan hash value: 1654070669
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|   0 | SELECT STATEMENT    |      |   140 | 1540 |     9   (0)| 00:00:01 |       |       |
|   1 | PARTITION RANGE ALL|      |   140 | 1540 |     9   (0)| 00:00:01 |     1 |     5 |
|   2 |   TABLE ACCESS FULL | PART |   140 | 1540 |     9   (0)| 00:00:01 |     1 |     5 |
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Hi gurus -
Oracle 11gR2
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Hi everyone,
I'm encountering some strange problems with the CBO in Oracle 10.2.0.3 - it's telling me that I have more rows in the indexes than there are rows in the tables.
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Edited by: user630084 on Mar 11, 2009 10:45 AM

Is your issue that you are having problems with the execution plans of queries referencing these objects? Or is your problem that you are observing more num_rows in the index than in the table when you query the data dictionary?
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Why full index scan is faster than full table scan?

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