首页 > Linux操作系统 > Linux操作系统 > 总结SQL语句中的优化提示
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 ... */
(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 | 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 |
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 |
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。