Oracle教程：select操作产生的redo

数据库版本： Oracle Database 10g Enterprise Edition Release 10.1.0.3.0 创建测试表：SQLgt; create table a as select * f

数据库版本：
Oracle Database 10g Enterprise Edition Release 10.1.0.3.0


创建测试表：
SQL> create table a as select * from all_objects ;
Table created.

SQL> set autotrace on statistics ;

插入数据（ｈｉｎｔ　ａｐｐｅｎｄ）：

SQL> insert /*+ append */ into a select * from all_objects ;
9891 rows created.

Statistics
----------------------------------------------------------
302 recursive calls
137 db block gets
6040 consistent gets
0 physical reads
1055332 redo size
627 bytes sent via SQL*Net to client
558 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
9891 rows processed
SQL> commit ;
Commit complete.

第一次查询数据：
SQL> select count(*) from a ;
COUNT(*)
----------
19782

Statistics
----------------------------------------------------------
0 recursive calls
1 db block gets
255 consistent gets
248 physical reads
168 redo size　　　　--------------------------------->　？？？产生redo？？？ 395 bytes sent via SQL*Net to client
507 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed


第二次查询：

SQL> select count(*) from a ;
COUNT(*)
----------
19782

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
252 consistent gets
1 physical reads
0 redo size　　　　　　
395 bytes sent via SQL*Net to client
507 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed


=================================================
如上所示，为什么在查询的时候会产生 redo ？ 产生的redo 到底是做什么的？
=================================================
----

取消 hint append 插入数据，第一次查询不会产生redo

SQL> insert into a select * from a ;

19782 rows created.


Statistics
----------------------------------------------------------
112 recursive calls
21100 db block gets
699 consistent gets
0 physical reads
7149196 redo size
642 bytes sent via SQL*Net to client
534 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
19782 rows processed

SQL>
SQL>
SQL> select count(*) from a ;

COUNT(*)
----------
39564


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
502 consistent gets
0 physical reads
0 redo size 395 bytes sent via SQL*Net to client
507 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
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