##第四. 索引
###4.1 定义
索引是与表关联的可选结构。通过创建索引可提高数据更新和检索的性能。Oracle 索引提供到数据行的直接访问路径。
可以对表的一个或多个列创建索引。创建索引后，Oracle 服务器会自动维护和使用索引。表数据的更新（如添加新行、更新行或删除行）会自动传播到所有相关的索引，这些对用户来说是完全透明的。
索引还可以提高实施主键和唯一键约束条件时的性能。如果没有索引，则每次对表执行DML 操作时都会扫描整个表（全表扫描）。
###4.2 类型
有多种类型的索引结构，可以根据需要使用。最常用的两种 类型是：
####4.2.1 B 树索引
默认的索引类型； 采用平衡树的形式. B 树索引的键值存储在平衡树（B 树）中，这样可以快速执行二进制搜索。
B 树索引的结构
索引的顶层为根，它包含指向索引中下一层次的条目。下一层次为分支块，它又指向位于索引中下一层次的块。最底层是叶节点，它包含指向表行的索引条目。叶块是双向关联的，这便于按键值升序或降序扫描索引。
索引叶条目的格式
- 条目头：存储列数和锁定信息
- 键列长度/值对：用于定义键中的列大小，后面跟随列值（此类长度/值对的数目就是索引中的最大列数）。
- ROWID：包含键值的行的行ID
索引叶条目的特性
在非分区表的B 树索引中：
- 当多个行具有相同的键值时，如果不压缩索引，键值会出现重复
- 当某行包含的所有键列为NULL 时，该行没有对应的索引条目。因此，当WHERE 子句指定了NULL 时，将始终执行全表扫描
- 因为所有行属于同一个段，所以要使用受限的ROWID 指向表行
对索引执行DML 操作的效果
对表执行DML 操作时，Oracle 服务器会维护所有索引。下面说明对索引执行DML 命令产生的效果：
- 执行插入操作导致在相应块中插入索引条目。
- 删除一行只导致对索引条目进行逻辑删除。已删除行所占用的空间不可供后面新的叶条目使用。
- 更新键列导致对索引进行逻辑删除和插入。PCTFREE设置对索引没有影响，但创建时除外。即使索引块的空间少于PCTFREE指定的空间，也可以向索引块添加新条目。
###4.3 创建
#创建索引 create index emp3_name_ix on emp3(emp3_name); #查看索引的信息 select index_name, index_type, table_name, table_type, uniqueness, status from user_indexes where table_name = 'EMP3'; INDEX_NAME INDEX_TYPE TABLE_NAME TABLE_TYPE UNIQUENES STATUS --------------- --------------- --------------- ----------- --------- -------- EMP3_ID_PK NORMAL EMP3 TABLE UNIQUE VALID EMP3_NAME_IX NORMAL EMP3 TABLE NONUNIQUE VALID #查看索引对应的列 SQL> select * from user_ind_columns where table_name = 'EMP3'; INDEX_NAME TABLE_NAME COLUMN_NAME COLUMN_POSITION COLUMN_LENGTH CHAR_LENGTH DESC --------------- --------------- --------------- --------------- ------------- ----------- ---- EMP3_ID_PK EMP3 EMP3_ID 1 22 0 ASC EMP3_NAME_IX EMP3 EMP3_NAME 1 10 10 ASC SQL> select * from emp3 where emp3_name = 'qa1'; EMP3_ID EMP3_NAME DEP_ID ---------- ---------- ---------- 2 qa1 2 Execution Plan ---------------------------------------------------------- Plan hash value: 215206995 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 33 | 1 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| EMP3 | 1 | 33 | 1 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | EMP3_NAME_IX | 1 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMP3_NAME"='qa1') Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 675 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed ###4.4 决定使用全表扫描还是使用索引
在大多数情况下，全表扫描可能会导致更多的物理磁盘输入输出，但是全表扫描有时又可能会因为高度并行化的存在而执行的更快。
索引范围扫描的总体原则是：
- 对于原始排序的表仅读取少于表记录数
40％的查询应该使用索引范围扫描。 反之，读取记录数目多于表记录数的40％的查询应该使用全表扫描。
- 对于未排序的表仅读取少于表记录数
7％的查询应该使用索引范围扫描。 反之，读取记录数目多于表记录数的7％的查询应该使用全表扫描。
###决定使用全表扫描还是使用索引 SQL> select index_name, index_type, table_name, uniqueness, status from user_indexes where table_name = 'EMP3'; INDEX_NAME INDEX_TYPE TABLE_NAME UNIQUENESS STATUS --------------- ---------- --------------- ---------- -------- EMP3_ID_PK NORMAL EMP3 UNIQUE VALID EMP3_NAME_IX NORMAL EMP3 NONUNIQUE VALID SQL> select count(*) from emp3; COUNT(*) ---------- 19 #虽然有索引，但是此时是全表扫描 SQL> select * from emp3 where emp3_name = 'qa8'; EMP3_ID EMP3_NAME DEP_ID ---------- ---------- ---------- 16 qa8 2 Execution Plan ---------------------------------------------------------- Plan hash value: 2425169977 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 11 | 2 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| EMP3 | 1 | 11 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("EMP3_NAME"='qa8') Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 671 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed #往emp3表添加数据 [root@hzvscmdb sql]# more insert_data.sql #!/bin/bash i=$1; while [ $i -le $2 ] do sqlplus hr/CCM%lab123@tony1521 <<EOF insert into emp3 values($i,'$3',$4); commit; quit; EOF let i=i+1 done echo "inset into emp3 table" [root@hzvscmdb sql]# ./insert_data.sql 90 100 dev 1 SQL> select max(emp3_id) from emp3; MAX(EMP3_ID) ------------ 100000 SQL> analyze table emp3 estimate statistics; Table analyzed. SQL> select blocks, empty_blocks, num_rows from user_tables where table_name = 'EMP3'; BLOCKS EMPTY_BLOCKS NUM_ROWS ---------- ------------ ---------- 374 10 101081 #查看一条数据 SQL> select * from emp3 where emp3_name = 'qa33333'; EMP3_ID EMP3_NAME DEP_ID ---------- ---------- ---------- 33333 qa33333 2 Execution Plan ---------------------------------------------------------- Plan hash value: 215206995 -------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 14 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| EMP3 | 1 | 14 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | EMP3_NAME_IX | 1 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMP3_NAME"='qa33333') Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 681 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed #不加条件的查询，索引扫描 SQL> select count(*) from emp3; COUNT(*) ---------- 100000 Execution Plan ---------------------------------------------------------- Plan hash value: 2418373429 ---------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | ---------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 70 (2)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| EMP3_ID_PK | 101K| 70 (2)| 00:00:01 | ---------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 2 recursive calls 2 db block gets 262 consistent gets 37 physical reads 176 redo size 526 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed #没有创建索引的，查询全表扫描 SQL> select count(*) from emp3 where dep_id = 2; COUNT(*) ---------- 85726 Execution Plan ---------------------------------------------------------- Plan hash value: 1396384608 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 2 | 104 (1)| 00:00:02 | | 1 | SORT AGGREGATE | | 1 | 2 | | | |* 2 | TABLE ACCESS FULL| EMP3 | 50541 | 98K| 104 (1)| 00:00:02 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("DEP_ID"=2) Statistics ---------------------------------------------------------- 1 recursive calls 0 db block gets 373 consistent gets 0 physical reads 0 redo size 528 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> select count(*) from emp3 where emp3_name like 'qa%'; COUNT(*) ---------- 85726 Execution Plan ---------------------------------------------------------- Plan hash value: 3884997069 ---------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 8 | 2 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 8 | | | |* 2 | INDEX RANGE SCAN| EMP3_NAME_IX | 7 | 56 | 2 (0)| 00:00:01 | ---------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMP3_NAME" LIKE 'qa%') filter("EMP3_NAME" LIKE 'qa%') Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 395 consistent gets 0 physical reads 0 redo size 528 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed 来源：oschina
链接：https://my.oschina.net/u/2010611/blog/475681