概述

今天主要介绍下Oracle索引的常见执行计划：

INDEX FULL SCAN：索引的全扫描，单块读，有序
INDEX RANGE SCAN：索引的范围扫描
INDEX FAST FULL SCAN：索引的快速全扫描，多块读，无序
INDEX FULL SCAN(MIN/MAX)：针对MAX(),MIN()函数的查询
INDEX SKIP SCAN：查询条件没有用到组合索引的第一列，而组合索引的第一列重复度较高时，可能用到。

这里用简单的测试案例，体会下索引使用这些执行计划的场景。

1、准备测试环境

--创建测试表和索引
drop table test_objects;
create table test_objects as select * from all_objects;
create index idx_test_objects_1 on test_objects(owner, object_name, subobject_name);
create index idx_test_objects_2 on test_objects(object_id);
desc test_objects;
--查看测试表上的索引信息
select index_name, column_name, column_position from user_ind_columns where table_name = 'TEST_OBJECTS';
--分析表并清空测试环境的shared_pool和buffer_cache：
analyze table test_objects compute statistics;
alter system flush shared_pool;
alter system flush buffer_cache;

2、准备SQL语句

根据不同执行计划的场景，编写SQL语句：

--INDEX RANGE SCAN（索引的范围扫描）
SELECT owner, object_name FROM test_objects WHERE owner = 'SYS' AND object_name = 'DBMS_OUTPUT';

--INDEX SKIP SCAN（针对MAX(),MIN()函数的查询）
SELECT owner, object_name FROM test_objects WHERE object_name = 'DBMS_OUTPUT';

--INDEX FAST FULL SCAN（索引的快速全扫描，多块读，无序）
SELECT owner, object_name FROM test_objects;

--INDEX FULL SCAN（索引的全扫描，单块读，有序）
SELECT owner, object_name FROM test_objects order by 1, 2;

--INDEX FULL SCAN (MIN/MAX)（针对MAX(),MIN()函数的查询）
SELECT max(object_id) FROM test_objects;

3、实验论证

结果如下：

3.1. INDEX RANGE SCAN

set autotrace traceonly
SELECT owner, object_name FROM test_objects WHERE owner = 'SYS' AND object_name = 'DBMS_OUTPUT';

3.2. INDEX SKIP SCAN

SELECT owner, object_name FROM test_objects WHERE object_name = 'DBMS_OUTPUT';

3.3. INDEX FAST FULL SCAN

SELECT owner, object_name FROM test_objects;

3.4. INDEX FULL SCAN

SELECT owner, object_name FROM test_objects order by 1, 2;

3.5. INDEX FULL SCAN (MIN/MAX)

SELECT max(object_id) FROM test_objects;

觉得有用的朋友多帮忙转发哦！后面会分享更多devops和DBA方面的内容，感兴趣的朋友可以关注下~