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23.读书笔记收获不止Oracle之 组合索引

发布时间：2020-12-12 14:16:51 所属栏目：百科 来源：网络整理
导读：23.读书笔记收获不止Oracle之 组合索引 索引在适当的场合能避免回表。考虑多列删上的索引，就是联合索引。 回表的动作叫做：TABLEACCESS BY INDEX ROWID。非常直白。 1. 组合列返回越少越高效 如果a字段上查询返回的记录比较多，b字段上查询返回的字段也比较

23.读书笔记收获不止Oracle之 组合索引

索引在适当的场合能避免回表。考虑多列删上的索引，就是联合索引。

回表的动作叫做：TABLEACCESS BY INDEX ROWID。非常直白。

1. 组合列返回越少越高效

如果a字段上查询返回的记录比较多，b字段上查询返回的字段也比较多，如果a和b同时查询，返回的记录比较少，那就适合建联合索引了。

查询select * fromt where a=1 and b=2，在a和b字段建联合索引是不可能消除回表的，因为返回的是所有字段。但是如果 a=某值返回较多， b=某值，返回也较多，组合起来返回比较少，适合建联合索引。

过多字段建联合索引往往是不可取的，因为这样的索引必然过大，影响定位数据，影响更新性能，一般不宜超过3个字段组合。

2. 组合列谁在前

索引两列，谁适合放钱？举个例子如下：

SQL>drop tablet purge;

Table dropped.

SQL> createtable t as select * from dba_objects;

建立两个组合索引

Create index idx1_object_idon t(object_id,object_type);

Create index idx2_object_idon t(object_type,object_id);

进行查询对比：

2.1等值查询

SQL> set autotracetraceonly

SQL> select/*+index(t,idx1_object_id)*/ * from t where object_id=20 andobject_type='TABLE';

ExecutionPlan

----------------------------------------------------------

Planhash value: 2919362295

--------------------------------------------------------------------------------

----------------------

|Id | Operation | Name | Rows| Bytes | C

ost (%CPU)|Time |

--------------------------------------------------------------------------------

----------------------

| 0 | SELECT STATEMENT | | 1 | 115 |

2(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 115 |

2(0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 1 | |

1(0)| 00:00:01 |

--------------------------------------------------------------------------------

----------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_ID"=20 AND"OBJECT_TYPE"='TABLE')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

4 consistent gets

5 physical reads

0 redo size

1863bytes sent via SQL*Net to client

551bytes received via SQL*Net from client

2 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> select/*+index(t,idx2_object_id)*/ * from t where object_id=20 andobject_type='TABLE';

ExecutionPlan

----------------------------------------------------------

Planhash value: 957115215

--------------------------------------------------------------------------------

----------------------

|Id | Operation | Name | Rows| Bytes | C

ost(%CPU)| Time |

--------------------------------------------------------------------------------

----------------------

| 0 | SELECT STATEMENT | | 1 | 115 |

2(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 115 |

2(0)| 00:00:01 |

|* 2 |INDEX RANGE SCAN | IDX2_OBJECT_ID | 1 | |

1(0)| 00:00:01 |

--------------------------------------------------------------------------------

----------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_TYPE"='TABLE'AND "OBJECT_ID"=20)

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

4 consistent gets

1 physical reads

0 redo size

1863bytes sent via SQL*Net to client

551bytes received via SQL*Net from client

2 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

1 rows processed

在等值查询情况下，组合索引的列无论哪一列在前，性能都一样。

2.2范围查询

SQL> select/*+index(t,idx1_object_id)*/ * from t where object_id>20 andobject_id<2000 and object_type='TABLE';

589 rowsselected.

ExecutionPlan

----------------------------------------------------------

Planhash value: 2919362295

--------------------------------------------------------------------------------

----------------------

|Id | Operation | Name | Rows | Bytes | C

ost(%CPU)| Time |

--------------------------------------------------------------------------------

----------------------

| 0 | SELECT STATEMENT | | 42 | 4830|

10(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 42 | 4830|

10(0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IDX1_OBJECT_ID | 42 | |

8(0)| 00:00:01 |

--------------------------------------------------------------------------------

----------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_ID">20 AND"OBJECT_TYPE"='TABLE' AND "OBJECT_ID"<2000)

filter("OBJECT_TYPE"='TABLE')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

108 consistentgets

23physical reads

0 redo size

72752bytes sent via SQL*Net to client

980bytes received via SQL*Net from client

41 SQL*Netroundtrips to/from client

0 sorts (memory)

0 sorts (disk)

589rows processed

SQL> select/*+index(t,idx2_object_id)*/ * from t where object_id>20 andobject_id<2000 and object_type='TABLE';

589 rowsselected.

ExecutionPlan

----------------------------------------------------------

Planhash value: 957115215

--------------------------------------------------------------------------------

----------------------

|Id | Operation | Name | Rows| Bytes | C

ost(%CPU)| Time |

--------------------------------------------------------------------------------

----------------------

| 0 | SELECT STATEMENT | | 42 | 4830|

4(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 42 | 4830|

4(0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IDX2_OBJECT_ID | 42 | |

2(0)| 00:00:01 |

--------------------------------------------------------------------------------

----------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_TYPE"='TABLE'AND "OBJECT_ID">20 AND "OBJECT_ID"<2000)

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

105 consistentgets

4 physical reads

0 redo size

72752bytes sent via SQL*Net to client

980bytes received via SQL*Net from client

41SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

589rows processed

组合索引的两列，当一列是范围查询，一列是等值查询的情况下，等值查询列在前，范围查询列在后，这样索引才高效。

2.3原理解释

某t表，AREA_CODE表示地区号，大量重复。另一个字段ID为序列号，重复度很少。

2.3.1等值查询性能一样

针对SELECT * FROM TWHERE AREA_CODE=591 AND ID=101查询，

索引是AREA_CODE+ID高效还是ID+AREA_CODE高效。

先看下AREA_CODE+ID组合如下图1

AREA_CODE+ID这个索引，按AREA_CODE进行排序，在AREA_CODE相同的情况下，按照ID进行二次排序。

在步骤一中根据索引定位的原理，定位到AREA_CODE=591，ID=101，找到ROWID，然后是ID=102，因为ID列也是有序的，因此查询完毕。

关于ID+AREA_CODE组合，如下图2：

ID+AREA_CODE这个索引同样具有明显的特征。

先找到ID=101和AREA_CODE=591这一行。然后AREA_CODE=592了，因为AREA_CODE也是有序的，说明不会再出现了，查询完毕。

2.3.2范围查询性能不一样

比如：SELECT * FROM TWHERE AREA_CODE=591 AND ID>=98 AND COL2<=105

先来看下AREA_CODE+ID的组合情况：

快速定位到AREA_CODE=591和ID=99这一行记录，然后在满足AREA_CODE=591的前提下，定位到满足条件的AREA_CODE=591和ID=100，AREA_CODE=591和ID=101等记录。当查询到AREA+591，ID=106后，发现没必要走下去了，因为ID=105不可能再出现了。

如下图3:

再讨论ID+AREA_CODE这个组合

先定位到ID=99和AREA_CODE=591，由于ID列不是等值而是范围，在ID>=98和ANDCOL2<=105的条件下，AREA_CODE定位到AREA_CODE=592后，前进的步伐无法停止。

因为在一下个区域内，比如ID=101后，完全可能再出现591，如下图4：

2.4组合查询改写

Droptable t purge;

SQL>Create table t as select * from dba_objects where rownum<1000;

Tablecreated.

SQL>update t set object_id=rownum;

999 rowsupdated.

SQL>update t set object_id=20 where rownum<=600;

600 rowsupdated.

SQL>Update t set object_id=21 where object_id<>20;

399 rowsupdated.

SQL>Commit;

SQL>Createindex idx1_object_id on t(object_id,object_type);

SQL>setautotrace traceonly

SQL>Setlinesize 1000

SQL> Select/*+index(t,idx1_object_id)*/ * from t where object_type='TABLE' andOBJECT_ID>=20 and OBJECT_ID<=21;

389 rowsselected.

ExecutionPlan

----------------------------------------------------------

Planhash value: 2919362295

------------------------------------------------------------------------------------------------------

|Id | Operation | Name | Rows| Bytes | Cost (%CPU)| Time |

------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 97 | 3(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 97 | 3 (0)| 00:00:01 |

|* 2 |INDEX RANGE SCAN | IDX1_OBJECT_ID | 1 | | 2 (0)| 00:00:01 |

------------------------------------------------------------------------------------------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_ID">=20 AND"OBJECT_TYPE"='TABLE' AND "OBJECT_ID"<=21)

filter("OBJECT_TYPE"='TABLE')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

70consistent gets

4 physical reads

0 redo size

47736bytes sent via SQL*Net to client

826bytes received via SQL*Net from client

27SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

389rows processed

SQL> Select/*+index(t,idx1_object_id)*/ * from t where object_type='TABLE' and object_idin (20,21);

389 rowsselected.

ExecutionPlan

----------------------------------------------------------

Planhash value: 547203187

-------------------------------------------------------------------------------------------------------

|Id | Operation | Name | Rows| Bytes | Cost (%CPU)| Time |

-------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | |167 | 16199 | 13 (0)| 00:00:01 |

| 1 |INLIST ITERATOR | || | | |

| 2 |TABLE ACCESS BY INDEX ROWID BATCHED| T |167 | 16199 | 13 (0)| 00:00:01 |

|* 3 |INDEX RANGE SCAN | IDX1_OBJECT_ID | 167 || 2 (0)| 00:00:01 |

-------------------------------------------------------------------------------------------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

3 - access(("OBJECT_ID"=20 OR"OBJECT_ID"=21) AND "OBJECT_TYPE"='TABLE')

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

71 consistentgets

0 physical reads

0 redo size

47736bytes sent via SQL*Net to client

826bytes received via SQL*Net from client

27SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

389rows processed

这边行数太少，看不到明显效果，大家可以把行数设置的足够大进行尝试。

使用IN改写的性能。

2.5设计考虑单列的查询

当两个索引列都是等值查询时，无论哪一列在前都无所谓，性能都一样。如果涉及两索引列中的一列是等值查询而另一列是范围查询时，等值查询列在前的组合索引更高效。

单列查询，例如select * from t where object_id=2 and object_type=’TABLE’是两列查询，而select* from t where object_id=2就是单列的查询。

2.5.1试验1

Drop tablet purge;

Createtable t as select * from dba_objects;

Createindex idx_object_id on t (object_id,object_type);

Setautotrace traceonly;

Setlinesize 1000

SQL> Select * from t whereobject_id=19;

ExecutionPlan

----------------------------------------------------------

Planhash value: 1296629646

-----------------------------------------------------------------------------------------------------

|Id | Operation | Name | Rows| Bytes | Cost (%CPU)| Time |

-----------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 115| 3(0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 115| 3(0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IDX_OBJECT_ID | 1 | | 2(0)| 00:00:01 |

-----------------------------------------------------------------------------------------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_ID"=19)

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

4 consistent gets

4 physical reads

0 redo size

1862bytes sent via SQL*Net to client

551bytes received via SQL*Net from client

2 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

1 rows processed

2.5.2试验2

Dropindex idx_object_id;

Createindex idx_object_id on t(object_type,object_id);

SQL> Select * from t whereobject_id=19;

ExecutionPlan

----------------------------------------------------------

Planhash value: 974686798

-----------------------------------------------------------------------------------------------------

|Id | Operation | Name | Rows| Bytes | Cost (%CPU)| Time |

-----------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 115| 46 (0)| 00:00:01 |

| 1 |TABLE ACCESS BY INDEX ROWID BATCHED| T | 1 | 115| 46 (0)| 00:00:01 |

|* 2 | INDEX SKIP SCAN | IDX_OBJECT_ID | 1 | | 45 (0)| 00:00:01 |

-----------------------------------------------------------------------------------------------------

PredicateInformation (identified by operation id):

---------------------------------------------------

2 - access("OBJECT_ID"=19)

filter("OBJECT_ID"=19)

Statistics

----------------------------------------------------------

1 recursive calls

0 db block gets

26consistent gets

49physical reads

0 redo size

1862bytes sent via SQL*Net to client

551bytes received via SQL*Net from client

2 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

1 rows processed

如果单列的查询列和联合索引的前置列一样，那单列就可以不建索引，直接利用联合索引来进行检索数据。试验2使用了跳跃索引INDEX SKIP SCAN

如果是偶尔执行又不是非常重要的查询，不会考虑为这些SQL语句的查询性能提升而建特定的索引的。

（编辑：李大同）

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