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总结SQL语句中的优化提示

原创 Linux操作系统 作者:tolywang 时间:2005-02-25 00:00:00 0 删除 编辑
会话级:
ALTER SESSION SET optimizer_goal= rule | first_rows | all_rows | choose ;  

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)

  - Use ALIASES for the tablenames in the hints.

  - Ensure tables are analyzed.   


语法: /*+ HINT  HINT  ... */ 

  (PLSQL中, '+'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

Access tables in the order of the FROM clause

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)               

        Use FTS on tab

CACHE(tab)

If table within treat as if it had the CACHE option set. See . 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)

Use hash scan to access 'tab'

INDEX( tab index )     

Use 'index' to access 'tab'

INDEX_ASC( tab index ) 

Use 'index' to access 'tab' for range scan.

INDEX_DESC( tab index )

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 , 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, [, ] )

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

 

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/35489/viewspace-84299/,如需转载,请注明出处,否则将追究法律责任。

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