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2. 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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3. 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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4. 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…
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6. 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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7. 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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8. 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’;
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9. 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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10. 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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11. 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.
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