3.读书笔记收获不止Oracle之 物理体系
3.读书笔记收获不止Oracle之 物理体系
1. 体系结构
来看一张被看烂了的,但是可能没有完全理解的图1
2. 体系结构原理初探
当用户连接进来的时候,会在PGA中给用户创建私有的内存空间,只要改SESSION不断开连接,下次系统不用再去硬盘中读取数据,而直接从PGA内存区中获取。
如果用户执行一个SQL语句,
SQL会立即匹配成一条唯一的HASH值,然后进入SGA的共享池中。
在共享池内查看是否存储过这个SQL指令(通过HASH值),如果没有,先查询语句语法、语义的正确性,是否有权限,如果都没有问题,那么这个SQL的HASH值就存储下来。
然后进行解析,解析就是生成、选择执行计划。和之前的HASH值对应在一起。
然后就去数据缓存区取自己想要的数据了。
如果数据缓存区中没有,就只能去磁盘中找了。
2.1 实验
创建一个用户比如toad
create user toad identifiedby toad;
grant sysdba to toad;
create table t as select * from all_objects;
Create index idx_object_idon t(object_id);
Set autotrace on
Set linesize 10000
Set timing on
其中set autotrace on是开始跟踪SQL的执行计划和执行的统计信息。
Set timing on表示跟踪该语句执行完成的时间。
执行
SQL>Select object_namefrom t where object_id=29;
得到的执行计划如下:
OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
C_COBJ#
Elapsed: 00:00:00.05
Execution Plan
----------------------------------------------------------
Plan hash value:1296629646
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 79 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 79 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------
PredicateInformation (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=29)
Note
-----
- dynamic statistics used: dynamic sampling(level=2)
Statistics
----------------------------------------------------------
11 recursive calls
0 dbblock gets
96 consistent gets
1 physical reads
0 redo size
550 bytes sent via SQL*Net to client
551 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
1 rows processed
再执行一次得到如下图:
SQL> Select object_name from t whereobject_id=29;
OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
C_COBJ#
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 1296629646
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 79 | 2 (0)| 00:00:01 |
| 1| TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 79 | 2 (0)| 00:00:01 |
|* 2| INDEX RANGE SCAN | IDX_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------
Predicate Information (identified byoperation id):
---------------------------------------------------
2- access("OBJECT_ID"=29)
Note
-----
-dynamic statistics used: dynamic sampling (level=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redosize
550 bytes sent via SQL*Net to client
551 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
第一次执行时间更长,而且需要的成本也更高。
第一次需要 11 次递归低啊用,96次逻辑读和1次物理读。
而第二次,只需要 4次逻辑读就可以了。
差异有如下几点:
1、 用户执行该SQL指令时,从磁盘中获取用户连接信息和相关权限信息,并保存在PGA中。再次执行就可以再PGA直接获取,避免了物理读。
2、 首次执行SQL指令后,SGA的共享池里已经保存了该SQL唯一指令HASH值,并保留了语法语义检查及执行计划等相关解析动作。
3、 首次执行SQL时,数据一般不再SGA中,只能从磁盘获取,不可避免的产生了物理读。第二次执行的时候就不需要物理读了,数据全在缓存中,效率高得多。
2.2 关于ORACLE 的代价
使用HINT来做实验
HINT可以强行让某SQL语句不走索引,或者强行让某SQL语句走索引。
在原来的语句如下
Select object_name from t whereobject_id=29;
增加HINT后如下(可以先清除下sharepool : alter system flush SHARED_POOL;):
Select /*+full(t)*/object_name from t where object_id=29;
SQL> Select /*+full(t)*/ object_name from t whereobject_id=29;
OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
C_COBJ#
Elapsed:00:00:00.02
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 30 | 412 (1)| 00:00:01 |
|* 1| TABLE ACCESS FULL| T | 1 | 30 | 412 (1)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified byoperation id):
---------------------------------------------------
1- filter("OBJECT_ID"=29)
Statistics
----------------------------------------------------------
41 recursive calls
0 dbblock gets
1567 consistent gets
1 physical reads
0 redosize
550 bytes sent via SQL*Net to client
551 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
1 rows processed
再执行以下如下:
SQL> Select /*+full(t)*/ object_name from t where object_id=29;
OBJECT_NAME
--------------------------------------------------------------------------------------------------------------------------------
C_COBJ#
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 30 | 412 (1)| 00:00:01 |
|* 1| TABLE ACCESS FULL| T | 1 | 30 | 412 (1)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified byoperation id):
---------------------------------------------------
1- filter("OBJECT_ID"=29)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1516 consistent gets
0 physical reads
0 redosize
550 bytes sent via SQL*Net to client
551 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rowsprocessed
第二次执行后,消除了共享池解析、减少甚至消除物理读以及递归调用。
虽然不是特别明显的,但是时间上这个查询是走索引快一点的,
所以ORACLE是自动选择的索引方式,因为所以更快一点。