1 ROWID的特性

(1)rowid是一個(gè)偽列，是用來(lái)確保表中行的唯一性，它并不能指示出行的物理位置，但可以用來(lái)定位行。

(2)rowid是存儲(chǔ)在索引中的一組既定的值(當(dāng)行確定后)。我們可以像表中普通的列一樣將它選出來(lái)。(3)利用rowid是訪問(wèn)表中一行的最快方式。(4)rowid需要10個(gè)字節(jié)來(lái)存儲(chǔ)，顯示為18位的字符串。

2 ROWID的結(jié)構(gòu)

rowid的組成結(jié)構(gòu)為：

data object number(6位字符串) relative file number(3位字符串) block number(6位字符串) row number(3位字符串)，如：AAAADeAABAAAAZSAAA

64編碼的物理地址,編碼字符是A-Z, a-z, 0-9, +,and/.

由4部分組成OOOOOOFFFBBBBBBRRR (obj#file#block#row#)

OOOOOO -–data object number

FFF –-表空間相對(duì)的數(shù)據(jù)文件號(hào)

BBBBBB –-塊號(hào)

RRR ---行號(hào)

我們可以借助oracle提供的包dbms_rowid，來(lái)對(duì)rowid進(jìn)行解析從而獲取關(guān)于行的相關(guān)信息，這里你可以選擇任何一個(gè)數(shù)據(jù)庫(kù)中的表，如下所示：

SQL> get c:\rowid.sql

1 select

2 rowid,

3 dbms_rowid.rowid_object(rowid) obj_id,

4 dbms_rowid.rowid_relative_fno(rowid) df#,

5 dbms_rowid.rowid_block_number(rowid) blknum,

6 dbms_rowid.rowid_row_number(rowid) rowno

7* from scott.emp

SQL> r

1 select

2 rowid,

3 dbms_rowid.rowid_object(rowid) obj_id,

4 dbms_rowid.rowid_relative_fno(rowid) df#,

5 dbms_rowid.rowid_block_number(rowid) blknum,

6 dbms_rowid.rowid_row_number(rowid) rowno

7* from scott.emp

ROWID????????????????? OBJ_ID??????? DF#???? BLKNUM????? ROWNO

------------------ ---------- ---------- ---------- ----------

AAAL+ZAAEAAAAAdAAA????? 49049????????? 4???????? 29????????? 0

AAAL+ZAAEAAAAAdAAB????? 49049????????? 4???????? 29????????? 1

AAAL+ZAAEAAAAAdAAC????? 49049????????? 4???????? 29????????? 2

AAAL+ZAAEAAAAAdAAD????? 49049????????? 4???????? 29????????? 3

AAAL+ZAAEAAAAAdAAE????? 49049????????? 4???????? 29????????? 4

AAAL+ZAAEAAAAAdAAF????? 49049????????? 4???????? 29????????? 5

AAAL+ZAAEAAAAAdAAG????? 49049????????? 4???????? 29????????? 6

AAAL+ZAAEAAAAAdAAH????? 49049????????? 4???????? 29????????? 7

AAAL+ZAAEAAAAAdAAI????? 49049????????? 4???????? 29????????? 8

AAAL+ZAAEAAAAAdAAJ????? 49049????????? 4???????? 29????????? 9

AAAL+ZAAEAAAAAdAAK????? 49049????????? 4???????? 29???????? 10

AAAL+ZAAEAAAAAdAAL????? 49049????????? 4???????? 29???????? 11

AAAL+ZAAEAAAAAdAAM????? 49049????????? 4???????? 29???????? 12

AAAL+ZAAEAAAAAdAAN????? 49049????????? 4???????? 29???????? 13

14 rows selected.

當(dāng)如下情況發(fā)生時(shí)，rowid將發(fā)生改變：

(1)對(duì)一個(gè)表做表空間的移動(dòng)后

(2)對(duì)一個(gè)表進(jìn)行了EXP/IMP

總結(jié)

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