Oracle Tuning

The optimizer mode can be set at the system-wide level, for an individual session, or for a specific SQL statement:

alter system set optimizer_mode=first_rows_10;
alter session set optimizer_goal = all_rows;
select /*+ first_rows(100) */ from student;

We need to start by defining what is the "best" execution plan for a SQL statement. Is the best execution plan the one that begins to return rows the fastest, or is the best execution plan the one that executes with the smallest amount of computing resources? Of course, the answer depends on the processing needs of your database.


The choices of executions plans made by the CBO are only as good as the statistics available to it. The old-fashioned analyze table and dbms_utility methods for generating CBO statistics are obsolete and somewhat dangerous to SQL performance. As we may know, the CBO uses object statistics to choose the best execution plan for all SQL statements.


The dbms_stats utility does a far better job in estimating statistics, especially for large partitioned tables, and the better statistics result in faster SQL execution plans.


Ddbms_stats example that creates histograms on all indexes columns:
BEGIN
dbms_stats.gather_schema_stats(
ownname=>'TPCC',
METHOD_OPT=>'FOR ALL INDEXED COLUMNS SIZE SKEWONLY',
CASCADE=>TRUE,
ESTIMATE_PERCENT=>100);
END;
/

There are several values for the OPTIONS parameter that we need to know about:

GATHER_ reanalyzes the whole schema

GATHER EMPTY_ only analyzes tables that have no existing statistics

GATHER STALE_ only reanalyzes tables with more than 10 percent modifications (inserts, updates, deletes)

GATHER AUTO_ will reanalyze objects that currently have no statistics and objects with stale statistics. Using GATHER AUTO is like combining GATHER STALE and GATHER EMPTY.
Note that both GATHER STALE and GATHER AUTO require monitoring. If you issue the ALTER TABLE XXX MONITORING command, Oracle tracks changed tables with the dba_tab_modifications view.



SQL> desc dba_tab_modifications;



Automating sample size with dbms_stats.The better the quality of the statistics, the better the job that the CBO will do when determining your execution plans. Unfortunately, doing a complete analysis on a large database could take days, and most shops must sample your database to get CBO statistics. The goal is to take a large enough sample of the database to provide top-quality data for the CBO.


According to the Oracle documentation, the I/O and CPU costs are evaluated as follows:



Cost = (#SRds * sreadtim +
#MRds * mreadtim +
#CPUCycles / cpuspeed ) / sreadtim

where:

#SRDs - number of single block reads
#MRDs - number of multi block reads
#CPUCycles - number of CPU Cycles *)

sreadtim - single block read time
mreadtim - multi block read time
cpuspeed - CPU cycles per second

Note that the costs are a function of the number of reads and the relative read times, plus the CPU cost estimate for the query. Also note the external costing does not consider the number of data blocks that reside in the RAM data buffers, but a future release of the optimizer is likely to consider this factor.

Here we see that Oracle uses the both the CPU and I/O cost estimations in evaluating the execution plans. This equation becomes even more complex when we factor-in parallel query where many concurrent processes are servicing the query.

The best benefit for using CPU costing is for all_rows execution plans where costs is more important than with first_rows optimization.

v$sql_plan view can help us locate SQL tuning opportunities

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's execution plan is the sequence of operations Oracle performs to execute the statement.


You can specify a statement Id when using the INTO clause.

EXPLAIN PLAN
INTO my_plan_table
SET STATEMENT_ID = 'bad1' FOR
SELECT last_name FROM employees;




SELECT lpad(' ',level-1)||operation||' '||options||' '||
object_name "Plan"
FROM plan_table
CONNECT BY prior id = parent_id
AND prior statement_id = statement_id
START WITH id = 0 AND statement_id = '&1'
ORDER BY id;


CREATE TABLE emp_range
PARTITION BY RANGE(hire_date)
(
PARTITION emp_p1 VALUES LESS THAN (TO_DATE('1-JAN-1991','DD-MON-YYYY')),
PARTITION emp_p2 VALUES LESS THAN (TO_DATE('1-JAN-1993','DD-MON-YYYY')),
PARTITION emp_p3 VALUES LESS THAN (TO_DATE('1-JAN-1995','DD-MON-YYYY')),
PARTITION emp_p4 VALUES LESS THAN (TO_DATE('1-JAN-1997','DD-MON-YYYY')),
PARTITION emp_p5 VALUES LESS THAN (TO_DATE('1-JAN-1999','DD-MON-YYYY'))

) AS SELECT * FROM employees;

For the first example, consider the following statement:

EXPLAIN PLAN FOR SELECT * FROM emp_range;

EXPLAIN PLAN FOR SELECT * FROM emp_range
WHERE hire_date >= TO_DATE('1-JAN-1995','DD-MON-YYYY');



CREATE TABLE emp_comp PARTITION BY RANGE(hire_date) SUBPARTITION BY
HASH(department_id)
SUBPARTITIONS 3
(
PARTITION emp_p1 VALUES LESS THAN (TO_DATE('1-JAN-1991','DD-MON-YYYY')),
PARTITION emp_p2 VALUES LESS THAN (TO_DATE('1-JAN-1993','DD-MON-YYYY')),
PARTITION emp_p3 VALUES LESS THAN (TO_DATE('1-JAN-1995','DD-MON-YYYY')),
PARTITION emp_p4 VALUES LESS THAN (TO_DATE('1-JAN-1997','DD-MON-YYYY')),
PARTITION emp_p5 VALUES LESS THAN (TO_DATE('1-JAN-1999','DD-MON-YYYY'))

) AS SELECT * FROM employees;

EXPLAIN PLAN FOR SELECT * FROM emp_comp
WHERE hire_date = TO_DATE('15-FEB-1997', 'DD-MON-YYYY');

How to obtain explain plans

Explain plan for
Main advantage is that it does not actually run the query - just parses the sql. This means that it executes quickly. In the early stages of tuning explain plan gives you an idea of the potential performance of your query without actually running it. You can then make a judgement as to any modifications you may choose to make.

Autotrace
Autotrace can be configured to run the sql & gives a plan and statistics afterwards or just give you an explain plan without executing the query.

Tkprof
Analyzes trace file


Using Oracle hints can be very complicated and Oracle developers only use hints as a last resort, preferring to alter the statistics to change the execution plan. Oracle contains more than 124 hints, and many of them are not found in the Oracle documentation


Optimizer hint is an optimizer directive placed inside comments inside your SQL statement and used in those rare cases where the optimizer makes an incorrect decision about the execution plan. Because hints are inside comments, it is important to ensure that the hint name is spelled correctly and that the hint is appropriate to the query.