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Oracle SQL hints

Oracle SQL Hints

/*+ hint */
/*+ hint(argument) */
/*+ hint(argument-1 argument-2) */

All hints except /*+ rule */ cause the CBO to be used. Therefore, it is good practise to analyze the underlying tables if hints are used (or the query is fully hinted.

There should be no schema names in hints. Hints must use aliases if alias names are used for table names. So the following is wrong:

select /*+ index(scott.emp ix_emp) */ from scott.emp emp_alias

better:

select /*+ index(emp_alias ix_emp) */ ... from scott.emp emp_alias

Why using hints

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of the data in the database are changing rapidly, so that the optimizer (or more accuratly, its statistics) are out of date.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.

It must also be noted, that Oracle allows to lock the statistics when they look ideal which should make the hints meaningless again.

Hint categories

Hints can be categorized as follows:

Hints for Optimization Approaches and Goals,
Hints for Access Paths, Hints for Query Transformations,
Hints for Join Orders,
Hints for Join Operations,
Hints for Parallel Execution,
Additional Hints

Documented Hints

Hints for Optimization Approaches and Goals

ALL_ROWS
One of the hints that 'invokes' the Cost based optimizer
ALL_ROWS is usually used for batch processing or data warehousing systems.
FIRST_ROWS
One of the hints that 'invokes' the Cost based optimizer
FIRST_ROWS is usually used for OLTP systems.
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.
RULE
The RULE hint should be considered deprecated as it is dropped from Oracle9i2.

See also the following initialization parameters: optimizer_mode, optimizer_max_permutations, optimizer_index_cost_adj, optimizer_index_caching and

Hints for Access Paths

CLUSTER
Performs a nested loop by the cluster index of one of the tables.
FULL
Performs full table scan.
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.
ROWID
Retrieves the row by rowid
INDEX
Specifying that index index_name should be used on table tab_name: /*+ index (tab_name index_name) */
Specifying that the index should be used the the CBO thinks is most suitable. (Not always a good choice).
Starting with Oracle 10g, the index hint can be described: /*+ index(my_tab my_tab(col_1, col_2)) */. Using the index on my_tab that starts with the columns col_1 and col_2.
INDEX_ASC
INDEX_COMBINE
INDEX_DESC
INDEX_FFS
INDEX_JOIN
NO_INDEX
AND_EQUAL
The AND_EQUAL hint explicitly chooses an execution plan that uses an access path that merges the scans on several single-column indexes

Hints for Query Transformations

FACT
The FACT hint is used in the context of the star transformation to indicate to the transformation that the hinted table should be considered as a fact table.
MERGE
NO_EXPAND
NO_EXPAND_GSET_TO_UNION
NO_FACT
NO_MERGE
NOREWRITE
REWRITE
STAR_TRANSFORMATION
USE_CONCAT

Hints for Join Operations

DRIVING_SITE
HASH_AJ
HASH_SJ
LEADING
MERGE_AJ
MERGE_SJ
NL_AJ
NL_SJ
USE_HASH
USE_MERGE
USE_NL

Hints for Parallel Execution

NOPARALLEL
PARALLEL
NOPARALLEL_INDEX
PARALLEL_INDEX
PQ_DISTRIBUTE

Additional Hints

ANTIJOIN
APPEND
If a table or an index is specified with nologging, this hint applied with an insert statement produces a direct path insert which reduces generation of redo.
BITMAP
BUFFER
CACHE
CARDINALITY
CPU_COSTING
DYNAMIC_SAMPLING
INLINE
MATERIALIZE
NO_ACCESS
NO_BUFFER
NO_MONITORING
NO_PUSH_PRED
NO_PUSH_SUBQ
NO_QKN_BUFF
NO_SEMIJOIN
NOAPPEND
NOCACHE
OR_EXPAND
ORDERED
ORDERED_PREDICATES
PUSH_PRED
PUSH_SUBQ
QB_NAME
RESULT_CACHE (Oracle 11g)
SELECTIVITY
SEMIJOIN
SEMIJOIN_DRIVER
STAR
The STAR hint forces a star query plan to be used, if possible. A star plan has the largest table in the query last in the join order and joins it with a nested loops join on a concatenated index. The STAR hint applies when there are at least three tables, the large table's concatenated index has at least three columns, and there are no conflicting access or join method hints. The optimizer also considers different permutations of the small tables.
SWAP_JOIN_INPUTS
USE_ANTI
USE_SEMI