Why hints Require ?

It is a perfect valid question to ask why hints should be used. Oracle comes with an optimizer that promises to optimize a query's execution plan. When this optimizer is really doing a good job, no hints should be required at all.

Sometimes, however, the characteristics of the data in the database are changing rapidly, so that the optimizer (or more accuratly, its statistics) are out of date. In this case, a hint could help.

You should first get the explain plan of your SQL and determine what changes can be done to make the code operate without using hints if possible. However, hints such as ORDERED, LEADING, INDEX, FULL, and the various AJ and SJ hints can take a wild optimizer and give you optimal performance Tables analyze and update Analyze Statement.

The ANALYZE statement can be used to gather statistics for a specific table, index or cluster. The statistics can be computed exactly, or estimated based on a specific number of rows, or a percentage of rows:

ANALYZE TABLE employees COMPUTE STATISTICS;

ANALYZE TABLE employees ESTIMATE STATISTICS SAMPLE 15 PERCENT;

EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');

Automatic Optimizer Statistics Collection

By default Oracle 10g automatically gathers optimizer statistics using a scheduled job called GATHER_STATS_JOB. By default this job runs within maintenance windows between 10 P.M. to 6 A.M. week nights and all day on weekends. The job calls the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC internal procedure which gathers statistics for tables with either empty or stale statistics, similar to the DBMS_STATS.GATHER_DATABASE_STATS procedure using the GATHER AUTO option. The main difference is that the internal job prioritizes the work such that tables most urgently requiring statistics updates are processed first.

Hint categories:

Hints can be categorized as follows:

• ALL_ROWS
  One of the hints that 'invokes' the Cost based optimizer
  ALL_ROWS is usually used for batch processing or data warehousing systems.

(/*+ ALL_ROWS */) 

• FIRST_ROWS
  One of the hints that 'invokes' the Cost based optimizer
  FIRST_ROWS is usually used for OLTP systems.

(/*+ FIRST_ROWS */) 

• CHOOSE
  One of the hints that 'invokes' the Cost based optimizer
  This hint lets the server choose (between ALL_ROWS and FIRST_ROWS, based on statistics gathered.

• Hints for Join Orders,
• Hints for Join Operations,
• Hints for Parallel Execution, (/*+ parallel(a,4) */)  specify degree either 2 or 4 or 16
• Additional Hints
• HASH
  Hashes one table (full scan) and creates a hash index for that table. Then hashes other table and uses hash index to find corresponding records. Therefore not suitable for < or > join conditions.

/*+ use_hash */

Use Hint to force using index

SELECT /*+INDEX (TABLE_NAME INDEX_NAME) */ COL1,COL2 FROM TABLE_NAME

 Select  ( /*+ hash  */ ) empno from

ORDERED-à This hint forces tables to be joined in the order specified. If you know table X has fewer rows, then ordering it first may speed execution in a join.

PARALLEL (table, instances)àThis specifies the operation is to be done in parallel.

If index is not able to create then will go for  /*+ parallel(table, 8)*/-----For select and update example---in where clase like st,not in ,>,< ,<> then we will use.