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oracle rowid and postgresql ctid

oracle rowid and postgresql ctid
 
首先介绍一下oracle rowid,数据库表行中的物理标识
SQL> select rowid from book_info where rownum<=1;
 
ROWID
------------------
AAAQTJAAaAAAAoaAAA
 
ROWID的格式如下:
数据对象编号        文件编号        块编号           行编号
OOOOOO             FFF             BBBBBB          RRR
由此看出,AAAQTJ是数据对象编号,AAa是文件编号,AAAAoa是块编号,AAA是行编号
select rowid ,
substr(rowid,1,6) "OBJECT",
substr(rowid,7,3) "FILE",
substr(rowid,10,6) "BLOCK",
substr(rowid,16,3) "ROW"
from book_info where rownum<=5;
查出64位编码的值:
ROWID              OBJECT     FILE       BLOCK      ROW
------------------ ---------- ---------- ---------- ------------------------------------------------------
AAAQTJAAaAAAAoCAAB AAAQTJ     AAa        AAAAoC     AAB
AAAQTJAAaAAAAoCAAC AAAQTJ     AAa        AAAAoC     AAC
AAAQTJAAaAAAAoCAAD AAAQTJ     AAa        AAAAoC     AAD
AAAQTJAAaAAAAoCAAE AAAQTJ     AAa        AAAAoC     AAE
AAAQTJAAaAAAAoCAAF AAAQTJ     AAa        AAAAoC     AAF
查出10进制的值:
SQL> select dbms_rowid.rowid_object(rowid)  object_id, dbms_rowid.rowid_relative_fno(rowid) file_id,
        dbms_rowid.rowid_block_number(rowid)  block_id ,dbms_rowid.rowid_row_number(rowid)  num from book_info where             rownum<=5;
 
 OBJECT_ID    FILE_ID   BLOCK_ID        NUM
---------- ---------- ---------- ----------
     66761         26       2568          0
     66761         26       2568          1
     66761         26       2568          2
     66761         26       2568          3
     66761         26       2568          4
获取rowid函数:
create or replace function get_rowid
(l_rowid in varchar2)
return varchar2
is
ls_my_rowid     varchar2(200);          
rowid_type     number;          
object_number     number;          
relative_fno     number;          
block_number     number;          
row_number     number; 
begin
 dbms_rowid.rowid_info(l_rowid,rowid_type,object_number,relative_fno, block_number, row_number);          
 ls_my_rowid := 'Object# is      :'||to_char(object_number)||chr(10)||
        'Relative_fno is :'||to_char(relative_fno)||chr(10)||
        'Block number is :'||to_char(block_number)||chr(10)||
        'Row number is   :'||to_char(row_number);
 return ls_my_rowid ;
end;         
/
 
select get_rowid(rowid) from book_info where rownum<=1;
Object# is      :66761
Relative_fno is :26
Block number is :2586
Row number is   :0
当然最常用的是用rowid去除重复:
查出重复数据:
select a.rowid,a.* from 表名 a 
where a.rowid != 
(
   select max(b.rowid) from 表名 b 
   where a.字段1 = b.字段1 and 
   a.字段2 = b.字段2 
)
删除重复数据:
delete from 表名 a 
where a.rowid != 
(
   select max(b.rowid) from 表名 b 
   where a.字段1 = b.字段1 and 
   a.字段2 = b.字段2 
)
对于整行都重复的那么,可以使用distinct函数。
 
以下介绍下postgresql的ctid
testuser=# select ctid,* from t1 limit 1;
 ctid  |     a     
-------+-----------
 (0,1) | 100000000
和oracle rowid类似也是一个物理字段,自动生成,不过结构和oracle rowid不一样,可以看到是(blockid,itemid)
ctid在数据更改后也会变化。
 
利用ctid去除重复数据:
建立测试表,插入数据:
testuser=# create table t2 (id int,name varchar(20));
CREATE TABLE
testuser=# insert into t2 values (1,'apple');
INSERT 0 1
testuser=# insert into t2 values (1,'apple');
INSERT 0 1
testuser=# insert into t2 values (1,'apple');
INSERT 0 1
testuser=# insert into t2 values (2,'orange');
INSERT 0 1
testuser=# insert into t2 values (2,'orange');
INSERT 0 1
testuser=# insert into t2 values (2,'orange');
INSERT 0 1
testuser=# insert into t2 values (2,'orange');
INSERT 0 1
testuser=# insert into t2 values (3,'banana');
INSERT 0 1
testuser=# insert into t2 values (3,'banana');
INSERT 0 1
 
testuser=# select * from t2;
 id |  name  
----+--------
  1 | apple
  1 | apple
  1 | apple
  2 | orange
  2 | orange
  2 | orange
  2 | orange
  3 | banana
  3 | banana
查询重复的数据:
testuser=# select ctid,* from t2 where ctid in (select min(ctid) from t2 group by id);
 ctid  | id |  name  
-------+----+--------
 (0,1) |  1 | apple
 (0,4) |  2 | orange
 (0,8) |  3 | banana
删除重复数据并查看结果:
testuser=# delete from t2 where  ctid not in (select min(ctid) from t2 group by id);
DELETE 6
testuser=# select * from t2;
 id |  name  
----+--------
  1 | apple
  2 | orange
  3 | banana
(3 rows)
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