Oracle hints tips
Oracle Tips by Burleson Consulting
December 4, 2007
For a full treatment of tuning with Oracle hints see my book "Oracle
Tuning: The Definitive Reference", or our
training in advanced SQL
The use of Oracle hints allows you to change suboptimal SQL
execution plans. Some professionals misunderstand hints, believing that
they overcome shortcomings of the optimizer. This is not always true, as
Oracle hints are used to supply additional information to the optimizer, and
compensate to cardinality estimates for complex queries.
Oracle hints can be used to tune SQL as "optimizer directives", overrides that
change the execution plans of the SQL. There are times when Oracle hints
are legitimate for tuning.
Tuning with hints
many hints for
every possible step within execution plans:
Global hints ? rule,
first_rows, first_rows_n all_rows, driving_site
tune Oracle with hints by placing them after the SELECT statements, but Oracle
hints can also be used in subqueries:
Table join hints ? use_nl, use_hash
Index hints ? Specifies an index name
Table access hints ? parallel, full, cardinality
Table join hints - ordered
FROM author a
WHERE author_key in
(select /*+ FULL(a) FULL(b) */ b.author_key
from sales a, book_author b
Tips for tuning with Oracle hints
Carefully check the hint syntax. It is always a good idea to use the
full-comment syntax for a hint. For example, the /+* hint */ syntax is generally
preferred to the ? ?+ hint syntax.
select /*+ index(e,dept_idx) */ * from emp e;
Use the table alias Whenever you have a query that specifies an alias for a
table, you cannot use the table name. Instead, you must specify the table alias
name. For example, the following query will invoke the index hint because the
emp table is aliased with ?e?:
Never reference the schema name in a hint Hints will be ignored when the schema
owner is specified in the hint. For example, the following hint will be ignored:
select /*+ index(scott.emp,dept_idx) */ * from emp;
Inappropriate Oracle hints
First, please read this link to understand why
Oracle SQL can ignore hints.
Some Oracle hints are contradictory
and invalid, and a hint will be ignored if it assumes an access path that is not
available. For example,
You need to be especially careful with
validation of hints because it is not always obvious that a hint is
contradictory with the query. For example, consider the following query in the
emp table with no index on the ename column.
Specifying an index hint on a
table that has no indexes
Specifying a parallel hint for an
index range scan
select /*+ first_rows */ * from emp order by ename;
The following hint is invalid because first_rows access and parallel access are
mutually exclusive. That is because parallel access always assumes a full-table
scan and first_rows favors index access.
-- An invalid hint
select /*+ index (emp emp_idx) parallel(emp,8)*/
emp_type = 'SALARIED';
Some Oracle professionals will place hints
together to reinforce their wishes. For example, if there is a SMP server with
eight or more CPUs, one may want to use Oracle Parallel Query to speed up
legitimate full-table scans.
Spelling - If table name or index name
is spelled incorrectly, then the hint will not be used. Here we see a query with
a misspelled table name:
select /*+ index(erp, dept_idx) */ *
Table Name - The table name is mandatory in the hint. For example, the
following hint will be ignored because the table name is not specified in the
select /*+ index(dept_idx) */ * from
Other Oracle hints tips:
Here are guidelines for using Oracle hints for
- Because hints are placed inside comments,
they will be ignored if the hint is incompatible with the existing execution
plan or when the hint is formatted improperly.
- When using the RBO, hints can be used to
change specific queries to use the CBO. Always remember to analyze all table
and indexes that participate in the query.
- When using the CBO, you can start tuning a
suspect SQL statement by adding the rule or first_rows hint.
- Hints can be applied to subqueries, but a
hint in the outer query will not carry over into the subquery.
Tuning with the parallel hint
When using parallel query, one should seldom
turn on parallelism at the table level, alter table customer parallel 35,
because the setting of parallelism for a table influences the optimizer. This
causes the optimizer to see that the full-table scan is inexpensive. Hence, most
Oracle professionals specify parallel query on a query-by-query basis, combining
the full hint with the parallel hint to ensure a fast parallel full-table scan:
-- A valid hint
select /*+ full parallel(emp,35)*/
Lets take a look at one of the most important
hints for Oracle tuning.
The ordered hint determines the driving
table for the query execution and also specifies the order that tables are
joined together. The ordered hint requests that the tables should be joined in
the order that they are specified in the from clause, with the first table in
the from clause specifying the driving table.
ordered hint can save a
huge amount of parse time and speed SQL execution because the optimizer is given
the best order to join the tables.
Tuning with the
The use of the cardinality hint compensates for the
optimizers inability to estimate the inter-table join result set. This is
NOT a problem with the optimizer, as no amount of metadata will help when
joining tables with complex, multi-column where clauses.
The cardinality hint is used in two general cases, complex
joins and dynamically created tables like global temporary tables (and possibly
using the WITH clause):
select /*+ cardinality( gtt 500 )
The dynamic sampling hint
The use of the 10g dynamic_sampling hint is very
useful for highly volatile tables and global temporary tables.
In 10g, you can use the dynamic_sampling hint to
direct Oracle to estimate the cardinality of highly volatile tables.
The dynamic sampling hint is also useful for estimating the
size of dynamically created objects such as global temporary tables (GTT?s)
Here are my related
notes on Oracle hints:
select /*+ dynamic_sampling(customer 4) */ pol_no, sales_id, sum_assured,
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