测试如下:不加的会走全表扫描要是数据量大的话有可能结果需要很长时间才出来

SQL*Plus: Release 9.2.0.4.0 - Production on Mon Mar 24 10:00:23 2008

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - 64bit Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

SQL> set autot trace exp
SQL> select count(*) from tb_TEST

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=22978 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     PARTITION RANGE (ALL)
   3    2       TABLE ACCESS (FULL) OF 'TB_TEST (Co
          st=22978 Card=39590341)

SQL> alter table TB_test

2 add constraint pk_tb_test1 primary key (ID);

Table altered.

SQL> select count(*) from TB_TEST;

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=3 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     INDEX (FAST FULL SCAN) OF 'PK_TB_TEST1' (UNIQUE) (Cost=3
           Card=39590341)

查询结果也很快

SQL> set timing on
SQL> select count(*) from tb_test;

COUNT(*)
----------
39590341

Elapsed: 00:00:02.08

SELECT COUNT(*) 索引会走 index fast full scan