Oracle DML操作時索引不會維護空值(NULL)

如果一個欄位有索引，以insert為例，插入時候欄位的值為NULL，不會維護該欄位的索引。

create table zkm.test (id int);

create index zkm.idx_id on zkm.test(id);

begin
for i in 1..1000000 loop
insert into zkm.test values (null);
end loop;
end;
/

begin
for i in 1..1000000 loop
insert into zkm.test values (1);
end loop;
end
 
;
/

實際測試實驗如下：

1.無索引，插入NULL值。

15:42:06 SYS@testdb(330)> create table zkm.test (id int);

Table created.

Elapsed: 00:00:00.01
15:42:09 SYS@testdb(330)> begin
15:42:14   2  for i in 1..1000000 loop
15:42:14   3  insert into zkm.test values
 
 (null);
15:42:14   4  end loop;
15:42:14   5  end;
15:42:14   6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:22.40

2.有索引，插入NULL值

15:42:37 SYS@testdb(330)> drop table zkm.test purge;

Table dropped.

Elapsed: 00:00:00.15
15:43:25 SYS@testdb(330)> create table zkm.test (id int);

Table created.

Elapsed: 
 
00:00:00.01
15:43:31 SYS@testdb(330)> create index zkm.idx_id on zkm.test(id);

Index created.

Elapsed: 00:00:00.00
15:43:33 SYS@testdb(330)> begin
15:43:37   2  for i in 1..1000000 loop
15:43:37   3  insert into zkm.test values (null);
15:43:37   4  end loop;
15:43:37   5  end;
15:43:37   6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:22.72

3.無索引，插入非NULL值

15:44:50 SYS@testdb(330)> drop table zkm.test purge;

Table dropped.

Elapsed: 00:00:00.14
15:44:51 SYS@testdb(330)> create table zkm.test (id int);

Table created.

Elapsed: 00:00:00.00
15:44:53 SYS@testdb(330)> begin
15:44:56   2  for i in 1..1000000 loop
15:44:56   3  insert into zkm.test values (1);
15:44:56   4  end loop;
15:44:56   5  end;
15:44:56   6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:21.59

4.有索引，插入非NULL值

15:45:49 SYS@testdb(330)> drop table zkm.test purge;

Table dropped.

Elapsed: 00:00:00.15
15:45:55 SYS@testdb(330)> create table zkm.test (id int);

Table created.

Elapsed: 00:00:00.01
15:45:57 SYS@testdb(330)> create index zkm.idx_id on zkm.test(id);

Index created.

Elapsed: 00:00:00.01
15:45:59 SYS@testdb(330)> begin
15:46:02   2  for i in 1..1000000 loop
15:46:02   3  insert into zkm.test values (1);
15:46:02   4  end loop;
15:46:02   5  end;
15:46:02   6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:34.73