索引负面影响测试

什么是索引

索引是为了提供更好的查询性能，通过更少的IO查找同样的数据，索引的性能参照就是全表扫描，oracle按照执行计划在表中查找数据只有全表扫描和索引这两种方式，这两种方式没有好坏之分，只有适不适合

索引负面影响

1，索引要额外占据存储空间，这个额外空间并不小

SQL> create table test as select * from dba_objects;

Table created.

SQL> create index idx_test_name on test(object_name);

Index created.

SQL> select segment_name,segment_type,bytes,blocks from user_segments where segment_name in('T
EST','IDX_TEST_NAME');
SEGMENT_NAME		  SEGMENT_TYPE		  BYTES     BLOCKS
------------------------- ------------------ ---------- ----------
IDX_TEST_NAME		  INDEX 		3145728        384
TEST			  TABLE 		9437184       1152

如果test是压缩表，那索引占据的空间跟表就差不多了

2，对DML操作有影响，DML操作如果涉及到索引列，在表中数据更新的同时，索引也会产生维护操作

SQL> truncate table test;

Table truncated.

Elapsed: 00:00:00.42
SQL> insert into test select * from dba_objects;    //无索引 插入

72040 rows created.

Elapsed: 00:00:00.39

Statistics
----------------------------------------------------------
781  recursive calls
9987  db block gets
3840  consistent gets
2  physical reads
8421032  redo size
843  bytes sent via SQL*Net to client
798  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
2  sorts (memory)
0  sorts (disk)
72040  rows processed

SQL> commit;

Commit complete.

Elapsed: 00:00:00.02
SQL> create index idx_test_name on test(object_name);   //创建索引

Index created.

Elapsed: 00:00:00.15
SQL> truncate table test;

Table truncated.

Elapsed: 00:00:00.46
SQL> insert into test select * from dba_objects;    //有索引 插入

72041 rows created.

Elapsed: 00:00:01.36

Statistics
----------------------------------------------------------
1497  recursive calls
87825  db block gets
6116  consistent gets
5  physical reads
29216072  redo size
843  bytes sent via SQL*Net to client
798  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
1  sorts (memory)
0  sorts (disk)
72041  rows processed

SQL> commit;

Commit complete.

Elapsed: 00:00:00.00

这个表上只有一个索引，性能差距如此明显，如果再加一个索引，性能耗费成倍上升

使用索引不一定总是能提升查询性能，在索引的使用上不能只考虑个别查询性能上的提升，毕竟数据库性能的提升是多方面综合的结果。