Oracle 数据块(data block)的结构和解析

?First of all let’s create a new tablespace and a table:

SQL> create tablespace my_tbs datafile '/u01/oradata/chenlin/my_tbs.dbf' size 200m ;

?Tablespace created.

?then:

SQL> create table my_tab tablespace my_tbs as select * from dba_objects ?;

?Table created.

?Now let’s get more information about this table from DBA_SEGMENTS view:

SQL> select header_file, header_block, bytes, blocks, extents from dba_segments where segment_name='MY_TAB';

      Total Blocks............................1024      Total Bytes.............................4194304      Total MBytes............................4      Unused Blocks...........................192      Unused Bytes............................786432      Last Used Ext FileId....................12      Last Used Ext BlockId...................785      Last Used Block.........................64

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PL/SQL procedure successfully completed.

As it has 831 blocks below and the number of block is 1023 (dump file didn’t count the header block), then there’re 1023-831=192 data blocks free (and above the hightwater mark)

Moreover, as the highwater mark is at block 64 ?and the extent has ?256 ?data blocks, then 256-64 = 192 data blocks are free,

So this means that there’s 192*4096 = 786432 bytes (768Kb) free space left .

As you see comparison of both values led us to the same results .

So in this post you’ve learned how to dump a header block of the segment and how to read it.?

ps: 如果你想真正知道oracle的block块里面真正有些什么，请耐心读完这篇文章～

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