Oracle10g中current_scn的正確計(jì)算運(yùn)行
以下的文章主要向你解答Oracle10g中的current_scn是如何正確的計(jì)算運(yùn)行,我們大家都知道Oracle10g在v$database 視圖里引入了current_scn,其實(shí)這個(gè)SCN是來(lái)自底層表,代表當(dāng)前的SCN。
在Oracle9i中我們可以通過(guò)dbms_flashback.get_system_change_number來(lái)獲得系統(tǒng)的SCN。
但是注意current_scn還是有所不同的,我們看一下一個(gè)查詢:
復(fù)制代碼
- oracle@danaly ~]$ sqlplus '/ as sysdba'
- SQL*Plus: Release 10.2.0.1.0 - Production on Thu Jun 21 10:15:08 2007
- Copyright (c) 1982, 2005, Oracle. All rights reserved.
- 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$database;
- CURRENT_SCN
- 8910961765230
我們看到current_scn的查詢會(huì)直接導(dǎo)致SCN的增進(jìn),而其他方式并不會(huì)。也就是說(shuō)在這里的current_scn就像是一個(gè)Sequence一樣,查詢會(huì)導(dǎo)致增進(jìn)。這也很好理解,v$database只能通過(guò)增進(jìn)當(dāng)前的SCN才能保證獲得的SCN是Current的。可是如果不查詢呢?這個(gè)值肯定是不會(huì)增長(zhǎng)的。以上的相關(guān)內(nèi)容就是對(duì)Oracle10g的current_scn計(jì)算的介紹,望你能有所收獲。
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