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Hints give specific information that we know about our data and application. A way to override the default query optimization in the DBMS Influence the execution plan of query
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Oracle optimizer may not always choose the best execution plan Using hints may improve the performance by changing the execution plan oracle takes.
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Hints can be used in the Select, Delete, and Update clauses. In each statement, the hint goes directly after the Select, Delete, or Update keyword. A few hints use Insert. Hints are placed in the /*+ */ tag, where the hint goes after the + sign Ex: SELECT /*+ ALL_ROWS */ From… www.orienit.com
Approach hints Access hints Join hints Misc. hints
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ALL_ROWS: Minimizes total resource consumption. Results will be returned only after all processing has been completed FIRST_ROWS(n): Minimized response time, or minimal resource usage to return the first n rows.
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FULL(table): Chooses a full table scan for the table, even if there is an index available. INDEX(table [index [index]...]): Chooses an Index scan for the table.
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CIS Department has far more males than females SELECT /*+ FULL(s) */ id, name FROM Student s WHERE sex = ‘m’; SELECT /*+ Index(s sex_index) */ id, name FROM Student s WHERE sex = ‘f’; www.orienit.com
ORDERED: tables are joined in the order in which they appear in the FROM clause. LEADING(table): specified table is the first table used in the join order. USE_HASH(table [table] …): Tables are joined using a hash join. Smaller table is used to make a hash table on join key. The larger table is scanned using hash table to find joined rows.
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USE_NL(table [table]…): Joins tables using nested loops join, using specified table as inner join. For every row in outer table, oracle accesses every row in inner table. USE_MERGE(table [table]…): Joins tables using a sort-merge join. Sorted list are made and then merged together. Best if tables are already sorted.
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APPEND: Data is appended to the end of table, rather then using existing space. CACHE(table): Blocks received are placed at the most recently used end of the LRU list in the buffer cache when a full table scan is preformed. NOCACHE(table): Blocks received are placed at the least recently used end of the LRU list in the buffer cache. www.orienit.com
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Oracle documentation at: http://oracle.cis.ksu.edu/oradocs/index.htm
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