Oracle Hints Types with Examples : In this section I would like to give you Oracle Hints types and Oracle Hints Examples. March 10, 2011. I would like to give the different examples which are useful for SQL query performance tuning in depth. SELECT /*+ result_cache */ number FROM main_table; Output - Oracle (11g and above) allows the SQL queries to be cached in the SGA and reused to improve performance. As time passed from Oracle 9i to 10g, we harnessed hints less, trusting the CBO and by Oracle 11g, it started to be frowned upon unless you had a very strong use case for hinting. For example: SELECT /*+ FIRST_ROWS(100) */ C1, C2 FROM T1 WHERE C1 BETWEEN 1 AND 1000 ORDER BY C1; No spaces are allowed between the comment delimiter and the plus sign. The + tells Oracle that this isn’t an ordinary comment, that it is in fact a hint. This is not the case; the hint in this form is simply a shorthand for a pair of single-table hints, viz: use_hash(table_1) use_hash(table_2). Subsequent execution of same query is faster because now the data is being pulled from cache. Index hints can also be placed inside subqueries. Single Table Hints : Single Table Hints are hints which can be Specified on one table or view. The Oracle materialize hint is used to ensure that the Oracle cost-based optimizer materializes the temporary tables that are created inside the "WITH" clause. 1. A common misunderstanding is that a hint of the form /*+ use_hash(table_1 table_2) */ is a directive to Oracle to do a hash join with table_1 as the build table and table_2 as the probe table. Oracle provides the index hint, the and_equal hint, the index_asc hint, the index_combine hint, the index_desc hint, and the index_ffs hint to redirect the optimizer's use of indexes to access table rows. Here’s an example instructing that a full table scan should be carried out on the emp table: A couple of days ago I saw an interesting question that asked what is the difference between using the FIRST_ROWS(100) hint in a SQL statement and using ROWNUM<=100 in the WHERE clause with the original SQL statement moved to an inline view. It queries the data from cache rather than database. This is not necessary in Oracle10g, but it helps ensure that the tables are only created one time.
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