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First_rows tips

Oracle Tips by Don Burleson

February 19, 2015

The first_rows_n optimizer mode is used in online transaction systems to gather data with an index, to quickly start returning rows back to the application. 

The all_rows optimizer mode (the default) is NOT deigned to maximize response time!  Rather, it is designed to minimize computing resources and it favors full-table scans. Index access (first_rows) adds additional I/O overhead, but they return rows faster, back to the originating query:

Oracle full-table scan Illustration

Oracle Index access illustration

In sum, the first_rows optimization algorithm did not limit the scope of the row retrieval, and assume that the query would desire all of the returned rows. The first-N rows optimization instructs the query optimizer to choose a query execution plan that minimizes the response time to produce the first N rows of query results. Starting in Oracle9i, we see the introduction of several new optimizer directives.

  • FIRST_ROWS_1
  • FIRST_ROWS_10
  • FIRST_ROWS_100
  • FIRST_ROWS_1000

Note: Staring in Oracle9i release 2, the Oracle performance tuning guide says that the first_rows optimizer mode has been deprecated and to use first_rows_n instead.

This new Oracle9i cost-based optimizer (CBO) mode can be set at several levels in our database, either system-wide, at the session level, or at the query level:

alter system set optimizer_mode = first_rows_100;
alter session set optimizer_mode = first_rows_100;
select /*+ first_rows(100) */ from student;

Oracle Corporation states that with first_rows_n optimization, Oracle queries give the best possible response time for the first rows of a result set. Quicker response with the first N rows enhances user satisfaction in many applications, since users receive their initial set of data more rapidly.

 

However, in the Oracle Performance Tuning documentation, starting in Oracle 9i release 2, we see this note that says that first_rows optimizer_mode is obsolete:

"Using heuristics sometimes leads the query optimizer to generate a plan with a cost that is significantly larger than the cost of a plan without applying the heuristic.

FIRST_ROWS is available for backward compatibility and plan stability; use FIRST_ROWS_n instead."

 


 
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