Oracle測(cè)試：truncate作用與DBA_OBJECTS實(shí)驗(yàn)驗(yàn)證

0)| 00:00:01 |
| 2 | UNION-ALL | | | | | |
|* 3 | FILTER | | | | | |
| 4 | NESTED LOOPS | | 2 | 162 | 17 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| OBJ$ | 2 | 156 | 16 (0)| 00:00:01 |
|* 6 | INDEX SKIP SCAN | I_OBJ2 | 2 | | 14 (0)| 00:00:01 |
| 7 | TABLE ACCESS CLUSTER | USER$ | 1 | 3 | 1 (0)| 00:00:01 |
|* 8 | INDEX UNIQUE SCAN | I_USER# | 1 | | 0 (0)| 00:00:01 |
|* 9 | TABLE ACCESS BY INDEX ROWID | IND$ | 1 | 8 | 2 (0)| 00:00:01 |
|* 10 | INDEX UNIQUE SCAN | I_IND1 | 1 | | 1 (0)| 00:00:01 |
| 11 | NESTED LOOPS | | 1 | 82 | 1 (0)| 00:00:01 |
|* 12 | INDEX SKIP SCAN | I_LINK1 | 1 | 79 | 0 (0)| 00:00:01 |
| 13 | TABLE ACCESS CLUSTER | USER$ | 1 | 3 | 1 (0)| 00:00:01 |
|* 14 | INDEX UNIQUE SCAN | I_USER# | 1 | | 0 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
SQL> truncate table xtdwtest;
Table truncated.
SQL> select object_id,data_object_id from dba_objects where object_name='XTDWTEST';
OBJECT_ID DATA_OBJECT_ID
---------- --------------
59436 59437

其中OBJ#/OBJECT_ID可以被看作是對(duì)象的邏輯號(hào)（類(lèi)似序列號(hào)一樣分配），應(yīng)序號(hào)一旦分配之后就不會(huì)發(fā)生改變其中DATAOBJ#/DATA_OBJECT_ID則是和物理存儲(chǔ)關(guān)聯(lián)的編號(hào)，通常認(rèn)為是對(duì)象的物理號(hào)，這個(gè)編號(hào)會(huì)隨著對(duì)象物理存儲(chǔ)結(jié)構(gòu)的改變而發(fā)生改變。

obj#與dataobj#說(shuō)明：
obj# number not null, /* object number */
/* DO NOT CREATE INDEX ON DATAOBJ# AS IT WILL BE UPDATED IN A SPACE
* TRANSACTION DURING TRUNCATE */
dataobj# number, /* data layer object number */

以上實(shí)際上也暗示了TRUNCATE作為DDL可以快速回收空間的本質(zhì)，在執(zhí)行TRUNCATE操作時(shí)，數(shù)據(jù)庫(kù)只簡(jiǎn)單地回收空間，將空間標(biāo)記為可用（并不會(huì)在數(shù)據(jù)塊上真正刪除數(shù)據(jù)），同時(shí)將對(duì)象的數(shù)據(jù)對(duì)象重新定位，完成空間回收。

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