Oracle Database 11g The Complete Reference (Osborne ORACLE ...

Chapter 46: The Hitchhiker’s Guide to Tuning Applications and SQL 805
hint to force a NESTED LOOPS join to be used, then you may also use an INDEX hint to specify
which index should be used during the NESTED LOOPS join and which table should be accessed
via a full table scan.
Hints About Goals
You can specify a hint that directs the optimizer to execute a query with a specific goal in mind.
The available goals related to joins are the following:
■
■
ALL_ROWS Execute the query so that all of the rows are returned as quickly as possible.
FIRST_ROWS Execute the query so that the first row will be returned as quickly as
possible.
By default, the optimizer will execute a query using an execution path that is selected to
minimize the total time needed to resolve the query. Thus, the default is to use ALL_ROWS as the
goal. If the optimizer is only concerned about the total time needed to return all rows for the
query, then set-based operations such as sorts and MERGE JOIN can be used. However, the ALL_
ROWS goal may not always be appropriate. For example, online users tend to judge the
performance of a system based on the time it takes for a query to return the first row of data. The
users thus have FIRST_ROWS as their primary goal, with the time it takes to return all of the rows
as a secondary goal.
The available hints mimic the goals: the ALL_ROWS hint allows the optimizer to choose from
all available operations to minimize the overall processing time for the query, and the FIRST_
ROWS hint tells the optimizer to select an execution path that minimizes the time required to
return the first row to the user.
If you use the FIRST_ROWS hint, the optimizer will be less likely to use MERGE JOIN and
more likely to use NESTED LOOPS.
Hints About Methods
In addition to specifying the goals the optimizer should use when evaluating join method
alternatives, you can list the specific operations to use and the tables to use them on. If a query
involves only two tables, you do not need to specify the tables to join when providing a hint
for a join method to use.
The USE_NL hint tells the optimizer to use a NESTED LOOPS operation to join tables. In the
following example, the USE_NL hint is specified for the join query example. Within the hint, the
BOOKSHELF table is listed as the inner table for the join.
select /*+ USE_NL(bookshelf) */
BOOKSHELF.Publisher,
BOOKSHELF_AUTHOR.AuthorName
from BOOKSHELF, BOOKSHELF_AUTHOR
where BOOKSHELF.Title = BOOKSHELF_AUTHOR.Title;
If you want all of the joins in a many-table query to use NESTED LOOPS operations, you
could just specify the USE_NL hint with no table references. In general, you should specify table
names whenever you use a hint to specify a join method, because you do not know how the
query may be used in the future. You may not even know how the database objects are currently
set up—for example, one of the objects in your from clause may be a view that has been tuned to
use MERGE JOIN operations.