Question

If i understand the documentation correctly; the full hint should force a full table scan. In the below scenario it is not performing the same;

Num as in index created on it.

SQL> desc test;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 NUM                                       NOT NULL NUMBER
 NUM2                                               NUMBER(10)
 NUM3                                               NUMBER

Query:

select num from test;

Result:

       NUM
----------
         1
         2

Execution Plan

Plan hash value: 410557223

-------------------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     2 |     4 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | ID   |     2 |     4 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------

Query:

select /* +full(test) */ num from test;

Result:

       NUM
----------
         1
         2

Execution Plan

Plan hash value: 410557223

-------------------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     2 |     4 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | ID   |     2 |     4 |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------

I understand i am selecting a value which is stored in the index. Addition of any other column makes the scan as full. Hence i have to ask the obvious. Is hint a request or is it a command to the optimizer ?

On a side note , what does computation of statistics have to do with optimization. Are the statistics of an index updated automatically or is this an explicit operation?

Answer 1

I haven’t tested this, but the correct syntax for using a hint is:

select /*+ full(test) */ num from test;

It’s slash-star-plus-space.