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Question and Answer

Connor McDonald

Thanks for the question, Ionut.

Asked: March 20, 2018 - 3:38 pm UTC

Last updated: March 21, 2018 - 11:36 am UTC

Version: 12c

Viewed 10K+ times! This question is

You Asked

Recently, I observed a usage of FIRST_ROWS hint written as 
FIRST_ROWS EXPMA 851.120.01
I know the general syntax of FIRST_ROWS hint which should have a 
(n)
value which means the number of rows to be ordered.

Can anyone tell me if the syntax FIRST_ROWS EXPMA 851.120.01 is correct and what might be the meaning of "EXPMA" and "851.120.01"?

Thank you,

and Connor said...

FIRST_ROWS is the historical version of FIRST_ROWS(n). And generally it should not be used.

FIRST_ROWS(n) is "optimize for the first n rows using all the normal costing algorithms", whereas

FIRST_ROWS is "optimizer for the first rows using a pre-defined set of rules that will force index use, nested loops, etc."

The extra text is just comments and this does not impact hints that occur before it.

SQL> create table t as select * from dba_Objects;

Table created.

SQL> create index ix on t ( owner );

Index created.

SQL> exec dbms_stats.gather_table_stats('','T',method_opt=>'for all columns size 200');

PL/SQL procedure successfully completed.

SQL>
SQL> set autotrace traceonly explain
SQL> select * from t where owner = 'SYS';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 51685 |  6662K|   427   (1)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    | 51685 |  6662K|   427   (1)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter("OWNER"='SYS')

SQL> select /*+ first_rows(1) */ * from t where owner = 'SYS';

Execution Plan
----------------------------------------------------------
Plan hash value: 3947747388

--------------------------------------------------------------------------------------------
| Id  | Operation                           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |      |     1 |   132 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| T    |     1 |   132 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IX   |       |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

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

   2 - access("OWNER"='SYS')

SQL> select /*+ first_rows */ * from t where owner = 'SYS';

Execution Plan
----------------------------------------------------------
Plan hash value: 3947747388

--------------------------------------------------------------------------------------------
| Id  | Operation                           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |      | 51685 |  6662K|  1522   (1)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| T    | 51685 |  6662K|  1522   (1)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IX   | 51685 |       |   119   (1)| 00:00:01 |
--------------------------------------------------------------------------------------------

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

   2 - access("OWNER"='SYS')

SQL> select /*+ first_rows other comments */ * from t where owner = 'SYS';

Execution Plan
----------------------------------------------------------
Plan hash value: 3947747388

--------------------------------------------------------------------------------------------
| Id  | Operation                           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |      | 51685 |  6662K|  1522   (1)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| T    | 51685 |  6662K|  1522   (1)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IX   | 51685 |       |   119   (1)| 00:00:01 |
--------------------------------------------------------------------------------------------

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

   2 - access("OWNER"='SYS')

SQL>
SQL>
SQL>



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