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

测试如下:不加主键的会走全表扫描 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
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.

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