SQL语句中的优化提示Hints的总结

会话级：

ALTER SESSION SET optimizer_goal= rule | first_rows | all_rows | choose ;

修改会话级的优化提示。

Hints提示:

- 提示将使语句强制执行基于成本的优化器（除了提示RULE外）
- 在hints中使用表名的别名
- 确保表都被分析过了
语法: /*+ HINT HINT ... */
(在PLSQL中， '+'与hint的第一个字符之间的空格是很重要的
/*+ ALL_ROWS */ 是正确的，但/*+ALL_ROWS */ 是错误的)
1 Optimizer Mode优化模式:

FIRST_ROWS,ALL_ROWS	Force CBO first rows or all rows
RULE	Force Rule if possible
ORDERED	按照在FROM 子句中的先后顺序访问表
ORDERED_PREDICATES	Use in the WHERE clause to apply predicates in the order that they appear. Does not apply predicate evaluation on index keys

.
2 子查询或视图:

PUSH_SUBQ	Causes all subqueries in a query block to be executed at the earliest possible time. Normally subqueries are executed as the last is applied is outerjoined or remote or joined with a merge join. (>=7.2)
NO_MERGE(v)	Use this hint in a VIEW to PREVENT it being merged into the parent query. (>=7.2) or use NO_MERGE(v) in parent query block to prevent view V being merged
MERGE(v)	Do merge view V
MERGE_AJ(v) }	Put hint in a NOT IN subquery to perform (>=7.3)
HASH_AJ(v) }	SMJ anti-join or hash anti-join. (>=7.3) Eg: SELECT .. WHERE deptno is not null AND deptno NOT IN(SELECT /*+ HASH_AJ */ deptno ...)
HASH_SJ(v) }	Transform EXISTS subquery into HASH or MERGE
MERGE_SJ(v) }	semi-join to access "v"
PUSH_JOIN_PRED(v)	Push join predicates into view V
NO_PUSH_JOIN_PRED(v)	Do NOT push join predicates

3 读取方式:

FULL(tab)	对表实行全表扫描(FTS)
CACHE(tab)	If table within <Parameter:CACHE_SIZE_THRESHOLD> treat as if it had the CACHE option set. See <Parameter:CACHE_SIZE_THRESHOLD>. Only applies if FTS used.
NOCACHE(tab)	Do not cache table even if it has CACHE option set. Only relevant for FTS
ROWID(tab)	Access tab by ROWID directly SELECT /*+ ROWID( table ) */ ... FROM tab WHERE ROWID between '&1' and '&2';
CLUSTER(tab)	Use cluster scan to access 'tab'
HASH(tab)	使用hash scan来访问表
INDEX( tab ndex )	使用索引来访问表
INDEX_ASC( tab ndex )	Use 'index' to access 'tab' for range scan.
INDEX_DESC( tab ndex )	Use descending index range scan (Join problems pre 7.3)
INDEX_FFS( tab index)	Index fast full scan - rather than FTS.
INDEX_COMBINE( tab i1.. i5 )	Try to use some boolean combination of bitmap index/s i1,i2 etc
AND_EQUAL(tab i1.. i5 )	Merge scans of 2 to 5 single column indexes.
USE_CONCAT	Use concatenation (Union All) for OR (or IN) statements. (>=7.2). See(7.2 requires <Event:10078>, 7.3 no hint req)
NO_EXPAND	Do not perform OR-expansion (Ie: Do not use Concatenation).
DRIVING_SITE(table)	Forces query execution to be done at the site where "table" resides

4 连接:

USE_NL(tab)	Use table 'tab' as the driving table in a Nested Loops join. If the driving row source is a combination of tables name one of the tables in the inner join and the NL should drive off the entire row-source. Does not work unless accompanied by an ORDERED hint.
USE_MERGE(tab..)	Use 'tab' as the driving table in a sort-merge join. Does not work unless accompanied by an ORDERED hint.
USE_HASH(tab1 tab2)	Join each specified table with another row source with a hash join. 'tab1' is joined to previous row source using a hash join. (>=7.3)
STAR	Force a star query plan 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 3 tables and the large table's concatenated index has at least 3 columns and there are no conflicting access or join method hints. (>=7.3)
STAR_TRANSFORMATION	Use best plan containing a STAR transformation(if there is one)

5 并行查询选项:

PARALLEL ( table, <degree> [, <instances>] )	Use parallel degree / instances as specified
PARALLEL_INDEX(table, [ index, [ degree [,instances] ] ] )	Parallel range scan for partitioned index
PQ_DISTRIBUTE(tab,out,in)	How to distribute rows from tab in a PQ(out/in may be HASH/NONE/BROADCAST/PARTITION)
NOPARALLEL(table)	No parallel on "table"
NOPARALLEL_INDEX(table [,index])

6 Miscellaneous

APPEND	Only valid for INSERT .. SELECT. Allows INSERT to work like direct load or to perform parallel insert.
NOAPPEND	Do not use INSERT APPEND functionality
REWRITE(v1[,v2])	8.1+ With a view list use eligible materialized view Without view list use any eligible MV
NOREWRITE	8.1+ Do not rewrite the query