Oracle SQL: Optimizer Hint

It is often easy to forget this, but in many ways it is after we hit the execute button that the really exciting stuff starts with our code. A number of engines silently spring into action; including the optimizer. The optimizer analyses your SQL statement and decides the most efficient way to execute it based on the objects involved in the statement and the conditions you’re subjecting them to. Your database automatically gathers stats about your objects – stuff like the number of rows, number of distinct values, of nulls, data distribution – and the optimizer uses this information in its decision-making. (You can study the explain plan to see what decisions the optimizer has taken.)  The optimizer arrives at its conclusions, often in barely a whisper of time.

And when the SQL statement is executed, you sit back and you feel like a genius.

And that’s it, right? THE END.

Optimizer Hints

Well, not necessarily. The optimizer is the hero of our story; let me introduce the potential villains, Optimizer Hints. An optimizer hint is a code snippet within an SQL statement controlling the decisions of the optimizer. Hints give us the opportunity, in cases where we have superior knowledge about the database, to influence the optimizer. In fact, the very name is a misnomer – they are not hints; they are commands that override the optimizer (as long as the hint is valid and the _OPTIMIZER_IGNORE_HINTS initialization parameter is not TRUE).

Hints are injected into DML statements within the bounds of a comment. The syntax is as follows:

{DELETE|INSERT|SELECT|UPDATE} /*+ hint [text] [hint [text]] */

Also valid is the less fashionable

{DELETE|INSERT|SELECT|UPDATE} --+ hint [text] [hint [text]]

The + tells Oracle that this isn’t an ordinary comment, that it is in fact a hint. No spaces are allowed between the comment delimiter and the plus sign.

Here’s an example instructing that a full table scan should be carried out on the emp table:

Select /*+ FULL(emp) */ name
From emp

Where job = 'SALESMAN';

That’s pretty straightforward. So why am I painting optimizer hints as the baddies here? 

The rules

1. Don’t.
2. If you must use hints, assume you’ve used them incorrectly. So don’t.
3. With every Oracle patch or upgrade, assume every hint is going to go wrong. So don’t.
4. With every DDL, assume every hint applied to that object is going to go wrong. So don’t.

The reason to be wary of hinting is that by embedding hints in your SQL, you are overriding the optimizer and saying that you know more than it does – not just now, but every time in the future that your SQL will be run, irrespective of any other changes that may happen to your database. The likely consequence of this is that your SQL will possibly run sub-optimally now and almost certainly in the future.

Hints in Detail

Well, not necessarily. Cos telling you about these enchanting things called hints and then telling you to immediately forget everything you’ve just heard would be like telling you there’s a tree of knowledge in the middle of the garden but that you must never, ever eat its apple. And we know how that story ends.

So, more details. There are many dozens of different hints (have a look in the v$sql_hint view), however close to half of them are undocumented. So The First Rule of Hinting must really be borne in mind if you decide to use them. Here are a select few.

FIRST_ROWS(n): This hint instructs the optimizer to select a plan that returns the first n rows most efficiently.

SELECT /*+ FIRST_ROWS(10) */ empno, ename
FROM emp

WHERE deptno = 10;

You may also want to read up about FIRST_ROWS_1, FIRST_ROWS_10 and FIRST_ROWS_100. Of interest, also, is ALL_ROWS which details the optimizer to choose the plan that most effectively returns the resultset at the minimum cost.

NO_INDEX(<table_name> < index_name>): Instructs the optimizer to specifically not use the named index in determining a plan.

SELECT /*+ NO_INDEX(emp emp_ix) */ empno, ename
FROM emp, dept

WHERE emp.deptno = dept.deptno;

See also: the INDEX hint. You may also want to investigate INDEX_COMBINE, INDEX_JOIN, INDEX_ASC and INDEX_FFS.

LEADING(table_name): This hint tells Oracle to use the parameterised table as the first in the join order. The optimizer will consequently select a join chain that starts with this table.

SELECT /*+ LEADING (dept) */ empno, ename
FROM emp, dept

WHERE emp.deptno = dept.deptno;

Related to the LEADING hint is the ORDERED hint.  This hint instructs Oracle to join tables in the exact order in which they are listed in the FROM clause.

CACHE(table): This hint tells Oracle to add the blocks retrieved for the table to the head of the most recently used list. This might be useful with regularly-used lookup tables, for example.

SELECT /*+ CACHE (d) */ deptno, dname

FROM dept d;

Oracle caches small tables by default, making this hint redundant in many cases. Also often redundant is the NOCACHE hint, since this places blocks at the tail of the LRU list, which is also Oracle’s default behaviour with the majority of blocks.

CARDINALITY(table n): This hint instructs Oracle to use n as the table, rather than rely on its own stats. You may need to use this hint with a global temporary table, for instance.

SELECT /*+ CARDINALITY (gtt, 1000) */ gtt.gtt_id, dname
FROM dept d, global_temp_tab gtt

WHERE d.deptno = gtt.deptno;

REWRITE: This hint instructs Oracle to rewrite the query using a materialized view, irrespective of cost. 

PARALLEL (table n): This hint tells the optimizer to use n concurrent servers for a parallel operation.

APPEND: This hint instructs the optimizer to carry out a direct-path insert.  This may make INSERT … SELECT statements faster because inserted data is simply appended to the end of the table and any referential constraints are ignored.

RULE: This hint basically turns off the optimizer.  This hint has been deprecated and should not be used. Never ever. 

Conclusion

Well, not necessarily the end. I was holidaying in Serbia recently and, when you visit a foreign country, it is always advisable to arm yourself with a few handy local words: Good day (dobar dan), thank you (hvala) – and, of course, beer (pivo). Take the examples above as your first handy words as you discover optimizer hints. There is more to learn, much more (indeed Jonathan Lewis has written dozens of articles on the subject, as have others). Hopefully, you are now ready to tackle those articles.

How to use hints in Oracle sql for performance

With hints one can influence the optimizer. The usage of hints (with exception of the RULE-hint) causes Oracle to use the Cost Based optimizer.

The following syntax is used for hints:

select /*+ HINT */ name
from emp
where id =1;

Where HINT is replaced by the hint text.
When the syntax of the hint text is incorrect, the hint text is ignored and will not be used.

Hints for Optimization Approaches and Goals
ALL_ROWS	The ALL_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best throughput (that is, minimum total resource consumption).
FIRST_ROWS	The FIRST_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best response time (minimum resource usage to return first row). In newer Oracle version you should give a parameter with this hint: FIRST_ROWS(n) means that the optimizer will determine an execution plan to give a fast response for returning the first n rows.
CHOOSE	The CHOOSE hint causes the optimizer to choose between the rule-based approach and the cost-based approach for a SQL statement based on the presence of statistics for the tables accessed by the statement
RULE	The RULE hint explicitly chooses rule-based optimization for a statement block. This hint also causes the optimizer to ignore any other hints specified for the statement block. The RULE hint does not work any more in Oracle 10g.
Hints for Access Paths
FULL	The FULL hint explicitly chooses a full table scan for the specified table. The syntax of the FULL hint is FULL(table) where table specifies the alias of the table (or table name if alias does not exist) on which the full table scan is to be performed.
ROWID	The ROWID hint explicitly chooses a table scan by ROWID for the specified table. The syntax of the ROWID hint is ROWID(table) where table specifies the name or alias of the table on which the table access by ROWID is to be performed. (This hint deprecated in Oracle 10g)
CLUSTER	The CLUSTER hint explicitly chooses a cluster scan to access the specified table. The syntax of the CLUSTER hint is CLUSTER(table) where table specifies the name or alias of the table to be accessed by a cluster scan.
HASH	The HASH hint explicitly chooses a hash scan to access the specified table. The syntax of the HASH hint is HASH(table) where table specifies the name or alias of the table to be accessed by a hash scan.
HASH_AJ	The HASH_AJ hint transforms a NOT IN subquery into a hash anti-join to access the specified table. The syntax of the HASH_AJ hint is HASH_AJ(table) where table specifies the name or alias of the table to be accessed.(deprecated in Oracle 10g)
INDEX	The INDEX hint explicitly chooses an index scan for the specified table. The syntax of the INDEX hint is INDEX(table index) where:table specifies the name or alias of the table associated with the index to be scanned and index specifies an index on which an index scan is to be performed. This hint may optionally specify one or more indexes:
NO_INDEX	The NO_INDEX hint explicitly disallows a set of indexes for the specified table. The syntax of the NO_INDEX hint is NO_INDEX(table index)
INDEX_ASC	The INDEX_ASC hint explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in ascending order of their indexed values.
INDEX_COMBINE	If no indexes are given as arguments for the INDEX_COMBINE hint, the optimizer will use on the table whatever Boolean combination of bitmap indexes has the best cost estimate. If certain indexes are given as arguments, the optimizer will try to use some Boolean combination of those particular bitmap indexes. The syntax of INDEX_COMBINE is INDEX_COMBINE(table index).
INDEX_JOIN	Explicitly instructs the optimizer to use an index join as an access path. For the hint to have a positive effect, a sufficiently small number of indexes must exist that contain all the columns required to resolve the query.
INDEX_DESC	The INDEX_DESC hint explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in descending order of their indexed values.
INDEX_FFS	This hint causes a fast full index scan to be performed rather than a full table.
NO_INDEX_FFS	Do not use fast full index scan (from Oracle 10g)
INDEX_SS	Exclude range scan from query plan (from Oracle 10g)
INDEX_SS_ASC	Exclude range scan from query plan (from Oracle 10g)
INDEX_SS_DESC	Exclude range scan from query plan (from Oracle 10g)
NO_INDEX_SS	The NO_INDEX_SS hint causes the optimizer to exclude a skip scan of the specified indexes on the specified table. (from Oracle 10g)
Hints for Query Transformations
NO_QUERY_TRANSFORMATION	Prevents the optimizer performing query transformations. (from Oracle 10g)
USE_CONCAT	The USE_CONCAT hint forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. Normally, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them.
NO_EXPAND	The NO_EXPAND hint prevents the optimizer from considering OR-expansion for queries having OR conditions or IN-lists in the WHERE clause. Usually, the optimizer considers using OR expansion and uses this method if it decides that the cost is lower than not using it.
REWRITE	The REWRITE hint forces the optimizer to rewrite a query in terms of materialized views, when possible, without cost consideration. Use the REWRITE hint with or without a view list. If you use REWRITE with a view list and the list contains an eligible materialized view, then Oracle uses that view regardless of its cost.
NOREWRITE / NO_REWRITE	In Oracle 10g renamed to NO_REWRITE. The NOREWRITE/NO_REWRITE hint disables query rewrite for the query block, overriding the setting of the parameter QUERY_REWRITE_ENABLED.
MERGE	The MERGE hint lets you merge views in a query.
NO_MERGE	The NO_MERGE hint causes Oracle not to merge merge-able views. This hint is most often used to reduce the number of possible permutations for a query and make optimization faster.
FACT	The FACT hint indicated that the table should be considered as a fact table. This is used in the context of the star transformation.
NO_FACT	The NO_FACT hint is used in the context of the star transformation to indicate to the transformation that the hinted table should not be considered as a fact table.
STAR_TRANSFORMATION	The STAR_TRANSFORMATION hint makes the optimizer use the best plan in which the transformation has been used. Without the hint, the optimizer could make a query optimization decision to use the best plan generated without the transformation, instead of the best plan for the transformed query.
NO_STAR_TRANSFORMATION	Do not use star transformation (from Oracle 10g)
UNNEST	The UNNEST hint specifies subquery unnesting.
NO_UNNEST	Use of the NO_UNNEST hint turns off unnesting for specific subquery blocks.
Hints for Join Orders
LEADING	Give this hint to indicate the leading table in a join. This will indicate only 1 table. If you want to specify the whole order of tables, you can use the ORDERED hint. Syntax: LEADING(table)
ORDERED	The ORDERED hint causes Oracle to join tables in the order in which they appear in the FROM clause. If you omit the ORDERED hint from a SQL statement performing a join , the optimizer chooses the order in which to join the tables. You may want to use the ORDERED hint to specify a join order if you know something about the number of rows selected from each table that the optimizer does not. Such information would allow you to choose an inner and outer table better than the optimizer could.
Hints for Join Operations
USE_NL	The USE_NL hint causes Oracle to join each specified table to another row source with a nested loops join using the specified table as the inner table. The syntax of the USE_NL hint is USE_NL(table table) where table is the name or alias of a table to be used as the inner table of a nested loops join.
NO_USE_NL	Do not use nested loop (from Oracle 10g)
USE_NL_WITH_INDEX	Specifies a nested loops join. (from Oracle 10g)
USE_MERGE	The USE_MERGE hint causes Oracle to join each specified table with another row source with a sort-merge join. The syntax of the USE_MERGE hint is USE_MERGE(table table) where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a sort-merge join.
NO_USE_MERGE	Do not use merge (from Oracle 10g)
USE_HASH	The USE_HASH hint causes Oracle to join each specified table with another row source with a hash join. The syntax of the USE_HASH hint is USE_HASH(table table) where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a hash join.
NO_USE_HASH	Do not use hash (from Oracle 10g)
Hints for Parallel Execution
PARALLEL	The PARALLEL hint allows you to specify the desired number of concurrent query servers that can be used for the query. The syntax is PARALLEL(table number number). The PARALLEL hint must use the table alias if an alias is specified in the query. The PARALLEL hint can then take two values separated by commas after the table name. The first value specifies the degree of parallelism for the given table, the second value specifies how the table is to be split among the instances of a parallel server. Specifying DEFAULT or no value signifies the query coordinator should examine the settings of the initialization parameters (described in a later section) to determine the default degree of parallelism.
NOPARALLEL / NO_PARALLEL	The NOPARALLEL hint allows you to disable parallel scanning of a table, even if the table was created with a PARALLEL clause. In Oracle 10g this hint was renamed to NO_PARALLEL.
PQ_DISTRIBUTE	The PQ_DISTRIBUTE hint improves the performance of parallel join operations. Do this by specifying how rows of joined tables should be distributed among producer and consumer query servers. Using this hint overrides decisions the optimizer would normally make.
NO_PARALLEL_INDEX	The NO_PARALLEL_INDEX hint overrides a PARALLEL attribute setting on an index to avoid a parallel index scan operation.
Additional Hints
APPEND	When the APPEND hint is used with the INSERT statement, data is appended to the table. Existing free space in the block is not used. 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.
NOAPPEND	Overrides the append mode.
CACHE	The CACHE hint specifies that the blocks retrieved for the table in the hint are placed at the most recently used end of the LRU list in the buffer cache when a full table scan is performed. This option is useful for small lookup tables. In the following example, the CACHE hint overrides the table default caching specification.
NOCACHE	The NOCACHE hint specifies that the blocks retrieved for this table are placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. This is the normal behavior of blocks in the buffer cache.
PUSH_PRED	The PUSH_PRED hint forces pushing of a join predicate into the view.
NO_PUSH_PRED	The NO_PUSH_PRED hint prevents pushing of a join predicate into the view.
PUSH_SUBQ	The PUSH_SUBQ hint causes non-merged subqueries to be evaluated at the earliest possible place in the execution plan.
NO_PUSH_SUBQ	The NO_PUSH_SUBQ hint causes non-merged subqueries to be evaluated as the last step in the execution plan.
QB_NAME	Specifies a name for a query block. (from Oracle 10g)
CURSOR_SHARING_EXACT	Oracle can replace literals in SQL statements with bind variables, if it is safe to do so. This is controlled with the CURSOR_SHARING startup parameter. The CURSOR_SHARING_EXACT hint causes this behavior to be switched off. In other words, Oracle executes the SQL statement without any attempt to replace literals by bind variables.
DRIVING_SITE	The DRIVING_SITE hint forces query execution to be done for the table at a different site than that selected by Oracle
DYNAMIC_SAMPLING	The DYNAMIC_SAMPLING hint lets you control dynamic sampling to improve server performance by determining more accurate predicate selectivity and statistics for tables and indexes. You can set the value of DYNAMIC_SAMPLING to a value from 0 to 10. The higher the level, the more effort the compiler puts into dynamic sampling and the more broadly it is applied. Sampling defaults to cursor level unless you specify a table.
SPREAD_MIN_ANALYSIS	This hint omits some of the compile time optimizations of the rules, mainly detailed dependency graph analysis, on spreadsheets. Some optimizations such as creating filters to selectively populate spreadsheet access structures and limited rule pruning are still used. (from Oracle 10g)
Hints with unknown status
MERGE_AJ	The MERGE_AJ hint transforms a NOT IN subquery into a merge anti-join to access the specified table. The syntax of the MERGE_AJ hint is MERGE_AJ(table) where table specifies the name or alias of the table to be accessed.(deprecated in Oracle 10g)
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. The syntax of the AND_EQUAL hint is AND_EQUAL(table index index) where table specifies the name or alias of the table associated with the indexes to be merged. and index specifies an index on which an index scan is to be performed. You must specify at least two indexes. You cannot specify more than five. (depricated in Oracle 10g)
STAR	The STAR hint forces the large table to be joined last using a nested loops join on the index. The optimizer will consider different permutations of the small tables. (deprecated in Oracle 10g)
BITMAP	Usage: BITMAP(table_name index_name) Uses a bitmap index to access the table. (deprecated ?)
HASH_SJ	Use a Hash Anti-Join to evaluate a NOT IN sub-query. Use this hint in the sub-query, not in the main query. Use this when your high volume NOT IN sub-query is using a FILTER or NESTED LOOPS join. Try MERGE_AJ if HASH_AJ refuses to work.(deprecated in Oracle 10g)
NL_SJ	Use a Nested Loop in a sub-query. (deprecated in Oracle 10g)
NL_AJ	Use an anti-join in a sub-query. (deprecated in Oracle 10g)
ORDERED_PREDICATES	(deprecated in Oracle 10g)
EXPAND_GSET_TO_UNION	(deprecated in Oracle 10g)