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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