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MSQL优化基础(SQL执行过程，链接方式案例)

2015-06-27 21:14 549 查看

1.count(*)的效率

select count(*) from Tab

select count(1) from Tab

select count(pk) from Tab

select count(col) from Tab

实际上经过transform之后，都会变成 count(*) 。(注意: col非空时)

<pre name="code" class="sql">SQL> alter session set sql_trace=true;

SQL> select count(*), count(1), count(name) from a;

COUNT(*)   COUNT(1) COUNT(NAME)
---------- ---------- -----------
1          1           1
SQL> alter session set sql_trace=false;

/* 生成的trace 在  user_dump_dest   目录下*/
SQL> show parameter dump
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
background_core_dump                 string      partial
background_dump_dest                 string      /opt/ora10g/admin/RACDB/bdump
core_dump_dest                       string      /opt/ora10g/admin/RACDB/cdump
max_dump_file_size                   string      UNLIMITED
shadow_core_dump                     string      partial
user_dump_dest                       string      /opt/ora10g/admin/RACDB/udump

优化器是Oracle用来生成SQL执行计划的一个组件，优化器通过考虑以下几个因素确定一个最好的计划:

a.Where子句 b.统计信息 c.DB参数 d.Hints e.数据字典里的一些信息

SQL查询提交到Oracle服务器，要经过以下几个步骤：

1.Parse：检查语法，分析语义。在这个阶段，Oracle会把SQL拆分成Query Block集，交给优化器去优化。

2.Transformer: 交给transformer的是经过Parse的SQL语句——Query Block的集合。这个集合是Query Block互相关联的。Transformer的任务是试图改变这些Query Block的关系，以得到更好的plan。例如， View Merging, Predicate Pushing, Subquery Unesting ,Query Rewrite, OR-expansion.

讲了两种 CNT, filter predicate push down

3.评估：评估器产生三种类型的量： Selectivity(选择率)，Cardinality(基数)和Cost(成本)。这三个量是相互关联的，Cardinality从Selectivity而来，通常Cost是通过cardinality计算出来的。准确的统计信息会提高这些量的准确度。

4.计划生成：计划生成器尝试为每个query block生成不同的access path(访问路径)， join的方式与join顺序。然后评估出每个path的row source，最后生成执行计划。

5.Row Source运算

Row Source是指一个查询中，可以被访问的数据集。它可能是一个表，一个索引，一个没有合并的视图，或者是一个Join的结果集。

在SQL里边最慢的两个事情（排序，链接）

5.1 数据结构

I. Heap Table(堆表)

II. 通过Index访问表:

B_tree

Index Bitmap

III. 通过以下方式组织的表: 注意表一般修改少，表一起查询,单独查询情况少才用 index cluster

Cluster

Hash cluster （对于比较长的列，用hash cluster可能比较好，比如ename, 先hash一次，然后再去找数据）

IOT 索引+列名存储更新的时候可能行链接，只能单块读

热块的一般解决：减少pctfree, 如果是索引上，可以建反向索引

5.2 一元Row Source运算

Access Path

Access path是从数据库获取数据的路径，数据可以通过以下途径定位并取得数据，具体采用哪个path，取决于优化器的计算出的成本。

Access Path/ Full Table Scan

I.　执行多块读

II. 读High Water Mark以下的所有块（包括被删除的空块）

III. 过滤数据行

Full Table Scan是顺序地读表里所有的行，同时过滤掉不符合条件的的数据，在全表扫描期间，Oracle会读取high-water-mark以下的所有块，即使这些块已经被删除。每行数据，都要检查一下是否满足where条件。

FTS所读取的表块都是彼此相邻的块，这意味着，它一次可以读取多个块，一次读多少个块取决于初始化参数DB_FILE_MULTIBLOCK_READ_COUNT的设置，注意：这个设置只是个最高值，真被一次读多少个块，还是取决于数据与操作系统。

Rowid Scan

rowID包含三个方面的信息：数据文件号, 块号, 当前行在块中的行号。

ROWID是指向一个特定数据行的指针。

ROWID可以用于block dump。

ROWID Scan可以用于：

当数据被索引了，index块包含所有行的ROWID，以便快速访问。

当常规方法不可用时，可以直接用ROWID来访问数据。

Access Path/Index Scan

Index类型

B*-tree

普通索引

倒序索引

反向索引

键值压缩

函数索引

索引组织表

Bitmap

域索引

Index Scan(Unique)

Index Scan(Range) 一般单块读

Index Scan(Full) //多块读

Index Scan(Fast Full) 多块读，表很宽，而且而且访问的列是非空，有索引一般会走index scan fast full

Index Skip(Scan)

Index Scan(Index Join)

Using Bitmap Indexes

Combining Bitmap Indexes

以上列举的这些access path没有包含，cluster, IOT, partition的访问。

Cluster是表数据的一种存储方式，把一组表的数据存储在同一组cluster块里，因为他们有相同的列，并且经常一起使用。例如， EMP表和DEPT表共享相同的数据。当你把EMP表和DEPT表聚合在一起， Oracle会把EMP表和DEPT两个表中，所有具有相同DEPTNO的行数据，都存储在相同的cluster块中。

Hash Cluster是单表cluster，它会把具有相同hash-key的值存储在一起，Oracle会用hash值来定位cluster里的数据行。所以具有相同值的数据在物理上存储在一起。

DBMS_ROWID这个包可以将数据文件号，块号与行号分解出来。

其它Row Source运算符

Sort - Hash

Buffer Sort

InList Iterator

View

Count Stop Key

First Row/(Min/Max)

5.3 二元Row Source运算

Join Tables

Nested Loop(嵌套循环连接)

Sort Merge Join(排序合并连接)

Hash Join（哈希连接）

Cartesian product（笛卡尔连接）

Join Types (Anti - Semi - Star)

5.4 多元 Row Source运算

Filter(过滤)

Concatenation(串连)

Connect by(递归连接)

Union all / intersect / Minus (集合操作)

6.分析执行计划

7.预估cardinality

8.计算成本

开10046

alter session set events '10046 trace name context forever , level 12';
select * from t_iot where object_id<=1000;
alter session set events '10046 trace name context off';

查看执行计划

select * from table(dbms_xplan.display_cursor(null,null,'last'))

证明全表扫描fetch 比较耗时

drop table t5;
create table t5 as select object_id, object_name from all_objects;
alter session set events '10046 trace name context forever , level 12';
select * from t5 where object_id=1000;
alter session set events '10046 trace name context off';

tkprof racdb1_ora_17133.trc racdb1_ora_17133.txt

racdb1_ora_17133.txt 中部分内容
select * from t5 where object_id=1000
call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.05          0          1          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2      0.00       0.04         71        223          0           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4      0.01       0.10         71        224          0           1

建测试表

create table test_objects as select * from all_objects;
alter table test_objects  add constraint PK_test_objects primary key (OBJECT_ID);
create index index_SUBOBJECT_NAME on test_objects(SUBOBJECT_NAME);
exec dbms_stats.gather_table_stats(null,'test_objects');

Index Scan(Unique)案例

条件列是主键或者unique索引，并且是 = 操作符

explain plan for  select * from test_objects where OBJECT_ID=1000;
select * from table(dbms_xplan.display);
-----------------------------------------------------------------------------------------------
| Id  | Operation                   | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                 |     1 |    93 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST_OBJECTS    |     1 |    93 |     2   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | PK_TEST_OBJECTS |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Index Scan(Range)案例

当<,> between作用在索引列时候，一般会发生 index range Scan , 当然具体怎么走， 还要看ORACLE根据表的情况比较各种路径后作出决定，大表一般都遵循此规律，小表可能会走 table full scan

explain plan for  select OBJECT_ID  from test_objects where OBJECT_ID<1000;
select * from table(dbms_xplan.display);
------------------------------------------------------------------------------------
| Id  | Operation        | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                 |   962 |  4810 |     3   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| PK_TEST_OBJECTS |   962 |  4810 |     3   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Index Scan(Full)案例

将要排序，排序的字段就是索引，大表，排序的字段又是非空，一般都会走 Index Full Scan

explain plan for  select OBJECT_ID  from test_objects order by OBJECT_ID;
select * from table(dbms_xplan.display);
------------------------------------------------------------------------------------
| Id  | Operation        | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                 | 45949 |   224K|    97   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN | PK_TEST_OBJECTS | 45949 |   224K|    97   (2)| 00:00:02 |
------------------------------------------------------------------------------------

Index Scan(FAST Full)案例

大表，不排序， select 部分是索引，选择字段非空一般都会走 Index Fast Full Scan

explain plan for  select OBJECT_ID  from test_objects
select * from table(dbms_xplan.display);
----------------------------------------------------------------------------------------
| Id  | Operation            | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |                 | 45949 |   224K|    23   (5)| 00:00:01 |
|   1 |  INDEX FAST FULL SCAN| PK_TEST_OBJECTS | 45949 |   224K|    23   (5)| 00:00:01 |
----------------------------------------------------------------------------------------

注意：如果选择的字段定义可以是null的，不会走索引，因为B树索引里边木有null值

explain plan for  select SUBOBJECT_NAME  from test_objects
select * from table(dbms_xplan.display);
----------------------------------------------------------------------------------
| Id  | Operation         | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |              | 45949 | 91898 |   141   (1)| 00:00:02 |
|   1 |  TABLE ACCESS FULL| TEST_OBJECTS | 45949 | 91898 |   141   (1)| 00:00:02 |
----------------------------------------------------------------------------------

explain plan for  select SUBOBJECT_NAME  from test_objects  where  SUBOBJECT_NAME is not null
select * from table(dbms_xplan.display);
-----------------------------------------------------------------------------------------
| Id  | Operation        | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                      |   153 |   306 |     1   (0)| 00:00:01 |
|*  1 |  INDEX FULL SCAN | INDEX_SUBOBJECT_NAME |   153 |   306 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Index Skip(Scan) 案例

如果先导列的NDV (number of distinct value ) 太大，成本太高，一般还是会走 table access full

set autot on
drop table tc3;
create table tc3 as select * from all_objects;
exec dbms_stats.gather_table_stats(null,'TC3');
create index tc3_own_obj on tc3(owner,object_id);
select object_name from tc3 where object_id=3;
-------------------------------------------------------------------------------------------
| Id  | Operation                   | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |             |     1 |    30 |    14   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TC3         |     1 |    30 |    14   (0)| 00:00:01 |
|*  2 |   INDEX SKIP SCAN           | TC3_OWN_OBJ |     1 |       |    13   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------

drop index tc3_own_obj;
create index tc3_obj_own on tc3(object_id,owner);
select object_name from tc3 where owner='SCOTT';
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |  3829 |   115K|   141   (1)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| TC3  |  3829 |   115K|   141   (1)| 00:00:02 |
--------------------------------------------------------------------------
set autot off

Nested Loop 案例

两表关联，一张是小表，一张是大表，而且小表和大表的关联条件的列在大表中有索引，一般会走 Nested Loop

create table tmp_test_objects  as select * from test_objects where rownum<20;
create index index_tmp_SUBOBJECT_NAME on tmp_test_objects(SUBOBJECT_NAME);
exec dbms_stats.gather_table_stats(null,'tmp_test_objects');

explain plan for select  * from tmp_test_objects a , test_objects b where a.OBJECT_ID=b.OBJECT_ID ;
select * from table(dbms_xplan.display);
-------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name             | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                  |    19 |  4199 |    21   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |                  |    19 |  4199 |    21   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL          | TMP_TEST_OBJECTS |    19 |  2432 |     2   (0)| 00:00:01 |
|   3 |   TABLE ACCESS BY INDEX ROWID| TEST_OBJECTS     |     1 |    93 |     1   (0)| 00:00:01 |
|*  4 |    INDEX UNIQUE SCAN         | PK_TEST_OBJECTS  |     1 |       |     0   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------

Hash Join 案例

两张都是大表，不管连接条件有没有索引

有索引
explain plan for select  * from  test_objects a , test_objects b where a.SUBOBJECT_NAME=b.SUBOBJECT_NAME ;
select * from table(dbms_xplan.display);
-----------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                      |   520 | 96720 |     5  (20)| 00:00:01 |
|*  1 |  HASH JOIN                   |                      |   520 | 96720 |     5  (20)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| TEST_OBJECTS         |   153 | 14229 |     2   (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN           | INDEX_SUBOBJECT_NAME |     1 |       |     1   (0)| 00:00:01 |
|   4 |   TABLE ACCESS BY INDEX ROWID| TEST_OBJECTS         |   153 | 14229 |     2   (0)| 00:00:01 |
|*  5 |    INDEX FULL SCAN           | INDEX_SUBOBJECT_NAME |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

无索引
explain plan for select  * from  test_objects a , test_objects b where a.owner=b.owner ;
select * from table(dbms_xplan.display);
-------------------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |              |   211M|    36G|       |  2858  (75)| 00:00:35 |
|*  1 |  HASH JOIN         |              |   211M|    36G|  4712K|  2858  (75)| 00:00:35 |
|   2 |   TABLE ACCESS FULL| TEST_OBJECTS | 45949 |  4173K|       |   142   (2)| 00:00:02 |
|   3 |   TABLE ACCESS FULL| TEST_OBJECTS | 45949 |  4173K|       |   142   (2)| 00:00:02 |
-------------------------------------------------------------------------------------------

Join Types (Anti - Semi - Star)

Semi:exists 方式

Anti: not exists 方式

MERGE JOIN CARTESIAN 案例

当有一个表只有一行的时候才会出现 merge join cartesian 可以通过 alter session set "_optimizer_cartesian_enabled"=true; 来关闭或启用这种行为

drop table tmp1;
drop table tmp2;
drop table tmp3;
create table tmp1 as select * from all_objects where rownum<2;
create table tmp2 as select * from all_objects where rownum<3;
create table tmp3 as select * from all_objects;
exec dbms_stats.gather_table_stats(null,'tmp1');
exec dbms_stats.gather_table_stats(null,'tmp2');
exec dbms_stats.gather_table_stats(null,'tmp3');
commit;
select a.object_id,b.object_id,c.object_id from tmp1 a, tmp2 b, tmp3 c where a.object_id = c.object_id and  b.object_id = c.object_id;
------------------------------------------------------------------------------
| Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |     1 |    11 |   146   (2)| 00:00:02 |
|*  1 |  HASH JOIN            |      |     1 |    11 |   146   (2)| 00:00:02 |
|   2 |   MERGE JOIN CARTESIAN|      |     2 |    12 |     4   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL  | TMP1 |     1 |     3 |     2   (0)| 00:00:01 |
|   4 |    BUFFER SORT        |      |     2 |     6 |     2   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL | TMP2 |     2 |     6 |     2   (0)| 00:00:01 |
|   6 |   TABLE ACCESS FULL   | TMP3 | 45958 |   224K|   141   (1)| 00:00:02 |
------------------------------------------------------------------------------

concatenation (_b_tree_bitmap_plans) 案例

drop table tt2;
create table tt2 as select * from all_objects;
create index tt2_idx on tt2(owner);
create index tt2_idx1 on tt2(object_id);
exec dbms_stats.gather_table_stats(null,'TT2');

alter session set "_b_tree_bitmap_plans" = false;
select * from tt2 where owner ='SCOTT' or object_id = 1000;
-----------------------------------------------------------------------------------------
| Id  | Operation                    | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |          |  3536 |   321K|   104   (0)| 00:00:02 |
|   1 |  CONCATENATION               |          |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| TT2      |     1 |    93 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | TT2_IDX1 |     1 |       |     1   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS BY INDEX ROWID| TT2      |  3535 |   321K|   102   (0)| 00:00:02 |
|*  5 |    INDEX RANGE SCAN          | TT2_IDX  |  3535 |       |     9   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

--b*tree index bitmap
alter session set "_b_tree_bitmap_plans" = true;
select /*^^222*/ * from tt2 where owner ='SCOTT' or object_id = 1000;

--当两个or条件来自不同的表， CBO不会使用index bitmap, 而会用or-expansion

ORACLE 常用文档地址

http://docs.oracle.com/en/database/

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