[20170206]为什么少1个段.txt

--前几天的测试,链接:http://blog.itpub.net/267265/viewspace-2133066/

1.环境:
SCOTT@book> @ &r/ver1

PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

CREATE TABLESPACE TEA DATAFILE
  '/mnt/ramdisk/book/tea01.dbf' SIZE 100M AUTOEXTEND ON NEXT 1280K MAXSIZE UNLIMITED
LOGGING
ONLINE
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 10M
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;

drop table mytest purge;
create table mytest(x number) tablespace tea;
create index i_mytest_x on mytest(x) tablespace tea;
insert into  mytest select level from dual connect by level <= 200126;
commit;

2.测试:
SCOTT@book> select * from DBA_FREE_SPACE where file_id=6;
TABLESPACE_NAME                   FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------------------------ ---------- ---------- ---------- ---------- ------------
TEA                                     6       2688   73400320       8960            6

--73400320/1024/1024=70M.

SCOTT@book> column PARTITION_NAME noprint
SCOTT@book> select * from dba_extents where file_id=6;
OWNER  SEGMENT_NAME         SEGMENT_TYPE       TABLESPACE_NAME  EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------ -------------------- ------------------ --------------- ---------- ---------- ---------- ---------- ---------- ------------
SCOTT  MYTEST               TABLE              TEA                      0          6        128   10485760       1280            6
SCOTT  I_MYTEST_X           INDEX              TEA                      0          6       1408   10485760       1280            6

--10485760/1024/1024=10M.
--别人问的问题:100-10-10=80,为什么自由空间是70M呢?

3.分析:

$ ls -l /mnt/ramdisk/book/tea01.dbf
-rw-r----- 1 oracle oinstall 104865792 2017-02-06 15:34:33 /mnt/ramdisk/book/tea01.dbf

--//实际上建立的文件大小是 100M+8192 = 100*1024*1024+8192=104865792.
--//文件的第0块是OS块,1-127是位图区,这样前面占用1M.

--//如果我增大数据文件1M,这样剩余的9M就可以凑成一个段.

SCOTT@book> ALTER DATABASE DATAFILE '/mnt/ramdisk/book/tea01.dbf' RESIZE 101M;
Database altered.

SCOTT@book> select * from DBA_FREE_SPACE where file_id=6;
TABLESPACE_NAME                   FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------------------------ ---------- ---------- ---------- ---------- ------------
TEA                                     6       2688   83886080      10240            6

--83886080/1024/1024=80M.