v$sysstat——存储自数据库实例运行那刻起就开始累计全实例(instance-wide)的资源使用情况！

v$statname 视图记录了所有的统计指标：

SQL> select * from v$statname where rownum < 11;

STATISTIC# NAME                                                                  CLASS    STAT_ID
---------- ---------------------------------------------------------------- ---------- ----------
         0 OS CPU Qt wait time                                                       1  576270482
         1 logons cumulative                                                         1 2666645286
         2 logons current                                                            1 3080465522
         3 opened cursors cumulative                                                 1   85052502
         4 opened cursors current                                                    1 2301954928
         5 user commits                                                              1  582481098
         6 user rollbacks                                                            1 3671147913
         7 user calls                                                                1 2882015696
         8 recursive calls                                                           1 2656001462
         9 recursive cpu usage                                                       1 4009879262

已选择10行。

查看全表扫描的系统统计信息：

SQL> select name,value from v$sysstat where name like '%table scans%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
table scans (short tables)                                             3451
table scans (long tables)                                                 5
table scans (rowid ranges)                                                0
table scans (cache partitions)                                            0
table scans (direct read)                                                 0

table scans (short tables) ：小表全表扫描次数。
table scans (long tables) ：大表全表扫描次数。

下面最一张大表执行一次全表扫描：

SQL> select count(*) from t;

  COUNT(*)
----------
    402344

SQL> select * from t;

再次查看统计信息：

SQL> select name,value from v$sysstat where name like '%table scans%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
table scans (short tables)                                             3580
table scans (long tables)                                                 6
table scans (rowid ranges)                                                0
table scans (cache partitions)                                            0
table scans (direct read)                                                 0