oracle优化器
oralce里优化器有RBO与CBO两种
RBO
RBO基于规则进行，根据固有的规则来选择执行sql的方式
CBO基于成本 选择执行成本最小的一条进行执行，依据是sql语句所涉及的表，索引，列等统计信息
在 10g版本以后rbo已经停止支持，单仍然保留源代码，故仍然可以使用该优化器，但调优手段相对cbo要少一些
RBO将执行计划划分为15个执行等级
1级为rowid访问，执行效率最高。15级为全表扫描，认为执行效率最低
注：即使修改了优化器，或者使用了rule hint oracle一般也不会使用rbo，而仍然强制使用cbo
在SQL中使用hint，则意味着启用CBO，也就是oracle会使用CBO来解析执行含hint的目标SQL
RBO调整手段
1.RBO一条可行的方法就是等价改写目标SQL,如：where条件中对number或者date类型的列假话是哪个0 ,varchar2则加上“|| ”
2.sql中有两条或以上执行路径的等级值相同情况，可以调整在数据字典缓存顺序来影响RBO执行计划的选择
3.对于sql出现的对象的先后顺序调整也可能引起执行计划的改变，关键在于决定随时驱动表，谁是被驱动表
例如：
使用alter session set optimizer_mode=’RULE’ 修改执行优化器为RULE，启用RBO
SQL> alter session set optimizer_mode='RULE';
Session altered先介绍查看执行计划的两种方法：
1.explain plan for + SQL语句
select * from table(dbms_xplay.display)查看刚刚解析的执行计划
2.set autotrace traceonly explain
SQL> explain plan for select * from nt where userid='10000000557765' and account='325989' ;
Explained
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2445682311
---------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| NT |
| 2 | AND-EQUAL | |
|* 3 | INDEX RANGE SCAN | INX_USERID |
|* 4 | INDEX RANGE SCAN | INX_ACCOUNT |
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("USERID"=10000000557765)
4 - access("ACCOUNT"='325989')
Note
-----
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
- rule based optimizer used (consider using cbo)
21 rows selected
可以看到使用了两个索引 INX_USERID，INX_ACCOUNT
如果我们此时不想让表走索引INX_ACCOUNT，那么可以对SQL进行等价修改：
select * from nt where userid='10000000557765' and account+0='325989' ;
SQL> explain plan for select * from nt where userid='10000000557765' and account+0='325989' ;
Explained
SQL> select * from table(dbms_xplan.display)
2 ;
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3772635577
--------------------------------------------------
| Id | Operation | Name |
--------------------------------------------------
| 0 | SELECT STATEMENT | |
|* 1 | TABLE ACCESS BY INDEX ROWID| NT |
|* 2 | INDEX RANGE SCAN | INX_USERID |
--------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("ACCOUNT")+0=325989)
2 - access("USERID"=10000000557765)
Note
-----
- rule based optimizer used (consider using cbo)
19 rows selected
可以查看到只是使用了inx_userid
调整对象的顺序，引起执行计划改变
SQL> explain plan for select * from nt where account='325989' and userid='10000000557765' ;
Explained
SQL> select * from table(dbms_xplan.display)
2 ;
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 17873495
---------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| NT |
| 2 | AND-EQUAL | |
|* 3 | INDEX RANGE SCAN | INX_ACCOUNT |
|* 4 | INDEX RANGE SCAN | INX_USERID |
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("ACCOUNT"='325989')
4 - access("USERID"=10000000557765)
Note
-----
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
- rule based optimizer used (consider using cbo)
21 rows selected
CBO
CBO基于成本的优化器
cbo生成执行计划会考虑到实际执行对象所涉及的数据量，数据分布情况，网络状态，IO，cpu等资源情况
网络资源一般消耗与使用了dblink的分布式SQL中，但是该种SQL查看执行计划略有特殊
CBO解析目标SQL，首先对sql进行查询转换，转换完成后得到等价改写sql，选择执行路径中成本最小的作为执行计划
cardinality 表示某个具体执行步骤执行结果所包含的记录数量的估算值
SQL> set autotrace traceonly
SQL> select * from nt;
690622 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3687718604
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 690K| 142M| 6132 (1)| 00:01:14 |
| 1 | TABLE ACCESS FULL| NT | 690K| 142M| 6132 (1)| 00:01:14 |
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
66775 consistent gets
22293 physical reads
0 redo size
167747687 bytes sent via SQL*Net to client
506971 bytes received via SQL*Net from client
46043 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
690622 rows processed
查看到数据量690K
逻辑读66775 consistent gets
物理读22293 physical reads
SQL> select * from nt where userid='10000000557765';
Execution Plan
----------------------------------------------------------
Plan hash value: 3772635577
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 217 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| NT | 1 | 217 | 4 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | INX_USERID | 1 | | 3 (0)| 00:00:01 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("USERID"=10000000557765)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
3834 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
使用where条件筛选，走索引
INX_USERID
行数1
逻辑读5 consistent gets
cbo对sql的可传递性改写
SQL> select * from nt where userid in ('10000000557780','10000000557765');
Execution Plan
----------------------------------------------------------
Plan hash value: 2503616604
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 434 | 5 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| NT | 2 | 434 | 5 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | INX_USERID | 2 | | 4 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("USERID"=10000000557765 OR "USERID"=10000000557780)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
4129 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> 即根据提示，
3 - access(“USERID”=10000000557765 OR “USERID”=10000000557780)
原来sql可以修改为
select * from nt where "USERID"=10000000557765 OR "USERID"=10000000557780SQL> select * from nt where "USERID"=10000000557765 OR "USERID"=10000000557780;
Execution Plan
----------------------------------------------------------
Plan hash value: 2503616604
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 434 | 5 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| NT | 2 | 434 | 5 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | INX_USERID | 2 | | 4 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("USERID"=10000000557765 OR "USERID"=10000000557780)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
4129 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> 注：
CBO在多表关联中，有可能漏选最优执行计划，因为N个表关联，则关联的可能性为N!，CBO在对目标SQL优化调整的时候，有可能会漏选
优化器的模式：
由参数optimizer_mode 设置
修改方法：
1.修改session会话级别的优化器设置 alter session set optimizer_mode=’RULE’;
2.修改全局系统优化器设置 alter system set optimizer_mode=rule scope=both;
有5种值：
1.RULE rbo解析
2.CHOOSE 意味着若该表有统计信息，则选用CBO，若涉及的对象都有没有统计信息，则使用RBO
3.FIRST_ROWS_n（n=1,10,100,1000） 以最快响应速度返回前n行记录
（ps：由于最快响应速度有可能违反CBO最小成本的原则，因此CBO会将该执行成本值修改为较小的值，这样就不违背CBO原则了，但这样选择出来的执行计划并不是最优，对于IO，cpu等消耗巨大）
4.FIRST_ROWS
5.ALL_ROWS 默认值，返回所有的行
SQL>
SQL> show parameter optimizer_mode;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode string ALL_ROWS
SQL> 优化器得到执行计划，将会决定db访问数据的方式
oracle访问数据的方式有两种：
1.直接访问表
2.先访问索引，再进行回表读取数据
直接访问表：
1.全表扫描
如：
SQL> select * from test;
Execution Plan
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2354 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| TEST | 1 | 2354 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
50 recursive calls
0 db block gets
142 consistent gets
1 physical reads
0 redo size
3834 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> 2.rowid访问，是最快的方式
rowid是oracle存储数据的物理地址，每个表默认都会有rowid列
select rowid from test;
AAAkeVAAEAAAW17AAA通过dbms_rowid包，可以直接得到具体的rowid包含的地址信息：
SQL> select dbms_rowid.rowid_object(rowid) object_id,dbms_rowid.rowid_relative_fno(rowid) file_id,
dbms_rowid.rowid_block_number(rowid) block_id,dbms_rowid.rowid_row_number(rowid) row_number from test;
OBJECT_ID FILE_ID BLOCK_ID ROW_NUMBER
---------- ---------- ---------- ----------
149397 4 93563 0
SQL> 表示的是对象编号就是149397，在4号文件的93563块的第0行
SQL> select dbms_rowid.rowid_object('AAAkeVAAEAAAW17AAA') object_id from dual;
OBJECT_ID
----------
149397
SQL> 查看具体块的信息
SQL> select dump('AAAkeVAAEAAAW17AAA') from test;
DUMP('AAAKEVAAEAAAW17AAA')
----------------------------------------------------------------------
Typ=96 Len=18: 65,65,65,107,101,86,65,65,69,65,65,65,87,49,55,65,65,65
SQL>
（rowid后续进行扩展）
索引：
索引类型：
索引扫描类型：
1） 唯一索引扫描（Index Unique Scans）
2） 索引范围扫描（Index Range Scans）
3） 索引降序范围扫描（Index Range Scans Descending）
4） 跳跃式索引扫描（Index Skip Scans）
5） 全索引扫描（Full Index Scans）
6） 快速全索引扫描（Fast Full Index Scans）
7） 索引连接（Index Joins）
附：
查询数据对象segments的大小：查询dba_segments表
SQL> desc dba_segments;
Name Type Nullable Default Comments
---------------- ------------ -------- ------- --------------------------------------------------------------------------------------------------------------------------------------
OWNER VARCHAR2(30) Y Username of the segment owner
SEGMENT_NAME VARCHAR2(81) Y Name, if any, of the segment
PARTITION_NAME VARCHAR2(30) Y Partition/Subpartition Name, if any, of the segment
SEGMENT_TYPE VARCHAR2(18) Y Type of segment: "TABLE", "CLUSTER", "INDEX", "ROLLBACK","DEFERRED ROLLBACK", "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE"
SEGMENT_SUBTYPE VARCHAR2(10) Y SubType of Lob segment: "SECUREFILE", "ASSM", "MSSM", NULL
TABLESPACE_NAME VARCHAR2(30) Y Name of the tablespace containing the segment
HEADER_FILE NUMBER Y ID of the file containing the segment header
HEADER_BLOCK NUMBER Y ID of the block containing the segment header
BYTES NUMBER Y Size, in bytes, of the segment
BLOCKS NUMBER Y Size, in Oracle blocks, of the segment
EXTENTS NUMBER Y Number of extents allocated to the segment
INITIAL_EXTENT NUMBER Y Size, in bytes, of the initial extent of the segment
NEXT_EXTENT NUMBER Y Size, in bytes, of the next extent to be allocated to the segment
MIN_EXTENTS NUMBER Y Minimum number of extents allowed in the segment
MAX_EXTENTS NUMBER Y Maximum number of extents allowed in the segment
MAX_SIZE NUMBER Y Maximum number of blocks allowed in the segment
RETENTION VARCHAR2(7) Y Retention option for SECUREFILE segment
MINRETENTION NUMBER Y Minimum Retention Duration for SECUREFILE segment
PCT_INCREASE NUMBER Y Percent by which to increase the size of the next extent to be allocated
FREELISTS NUMBER Y Number of process freelists allocated in this segment
FREELIST_GROUPS NUMBER Y Number of freelist groups allocated in this segment
RELATIVE_FNO NUMBER Y Relative number of the file containing the segment header
BUFFER_POOL VARCHAR2(7) Y The default buffer pool to be used for segments blocks
FLASH_CACHE VARCHAR2(7) Y
CELL_FLASH_CACHE VARCHAR2(7) Y
SQL>
segment段对象有如下类型：
"TABLE", "CLUSTER", "INDEX", "ROLLBACK","DEFERRED ROLLBACK", "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE"
1 SYS I_USER1 INDEX MSSM SYSTEM 1 416 65536 8 1 65536 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT
2 SYS CON$ TABLE MSSM SYSTEM 1 288 655360 80 10 65536 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT
3 SYS UNDO$ TABLE MSSM SYSTEM 1 224 65536 8 1 65536 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT
4 SYS C_COBJ# CLUSTER MSSM SYSTEM 1 296 3145728 384 18 57344 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT
5 SYS I_OBJ# INDEX MSSM SYSTEM 1 168 327680 40 5 65536 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT
6 SYS PROXY_ROLE_DATA$ TABLE MSSM SYSTEM 1 264 65536 8 1 65536 1048576 1 2147483645 2147483645 1 1 1 DEFAULT DEFAULT DEFAULT查看某个用户下的表
select a.owner,a.segment_name,a.BYTES,a.blocks from dba_segments a where a.segment_type='TABLE' and a.owner='xxx' order by a.BYTES desc查看oracle对于表的统计信息，可以使用dba_tables
SQL> select TABLE_NAME,STATUS,NUM_ROWS,AVG_ROW_LEN,TABLE_LOCK,LAST_ANALYZED from dba_tables where owner='xxx'
2 ;
TABLE_NAME STATUS NUM_ROWS AVG_ROW_LEN TABLE_LO LAST_ANALYZED
------------------------------ -------- ---------- ----------- -------- -------------------
NT VALID 690622 217 ENABLED 2017-06-18 06:01:06
N_FILE VALID 1471988 264 ENABLED 2017-06-18 06:02:15
NTTS VALID 690622 51 ENABLED 2017-06-19 22:01:39
TEST VALID ENABLED