Oracle10g的current_scn是如何计算的?

SQL*Plus: Release 10.2.0.1.0 - Production on Thu Jun 21 10:15:08 2007

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Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options

SQL> @scn
SQL> col current_scn for 99999999999999999
SQL> select current_scn from v$database;

CURRENT_SCN
------------------
8910961765228

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

CURRENT_SCN
------------------
8910961765228

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

CURRENT_SCN
------------------
8910961765228

SQL> select current_scn from v$database;

CURRENT_SCN
------------------
8910961765229

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

CURRENT_SCN
------------------
8910961765229

SQL> select dbms_flashback.get_system_change_number current_scn from dual;

CURRENT_SCN
------------------
8910961765229

SQL> select current_scn from v$database;

CURRENT_SCN
------------------
8910961765230