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结合truncate系列一的原理《TRUNCATE TABLE原理解析》（详戳），再加上truncate table还有一个特点，就是所释放的空间会变成free space，这就给没有备份的情况下恢复表数据提供了另外一种思路。 恢复思路如下：
抽取的范围也有两种思路：
环境构造： SQL> create table rescureora.rescureora_table as select * from dba_objects;Table created. SQL> select count(*) from rescureora.rescureora_table; COUNT(*)---------- 86877 SQL> truncate table rescureora.rescureora_table;Table truncated.1.通过LogMiner或者redodump找到dataobj#的变化。 因为表可能会truncate过多次，原dataobj#不一定就等于obj#，所以需要通过redo来确认，如果最小补充日志没有打开，LogMiner可能会有遗漏。如果遗漏则使用redodump来寻找。 SQL> select obj#,dataobj# from obj$ where name='RESCUREORA_TABLE'; OBJ# DATAOBJ#---------- ---------- 87903 87904 SQL> select SQL_REDO from V$LOGMNR_CONTENTS where table_name='OBJ$' and SQL_REDO LIKE '%87903%'; SQL_REDO--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------update "SYS"."OBJ$" set "OBJ#" = '87903', "DATAOBJ#" = '87904', "TYPE#" = '2', "CTIME" = TO_DATE('01-DEC-19', 'DD-MON-RR'), "MTIME" = TO_DATE('01-DEC-19', 'DD-MON-RR'), "STIME" = TO_DATE('01-DEC-19','DD-MON-RR'), "STATUS" = '1', "FLAGS" = '0', "OID$" = NULL, "SPARE1" = '6', "SPARE2" = '1' where "OBJ#" = '87903' and "DATAOBJ#" = '87903' and "TYPE#" = '2' and "CTIME" = TO_DATE('01-DEC-19', 'DD-MON-RR') and "MTIME" = TO_DATE('01-DEC-19', 'DD-MON-RR') and "STIME" = TO_DATE('01-DEC-19', 'DD-MON-RR') and "STATUS" = '1' and "FLAGS" = '0' and "OID$" IS NULL and "SPARE1" = '6' and "SPARE2" = '1' and ROWID = 'AAAAASAABAAAVKtAAE';dataobj#从87903变成了87904。 2.遍历表所在tablespace的free block和该表的第一个extent，以及该表空间所有segment的最后一个extent，使用dbms_rowid.rowid_create抽取数据。 dbms_rowid.rowid_create参数解释如下： 由于需要根据dataobj#去匹配，所以需要修改obj$，修改完成之后需要flush shared_pool注意：如果有lob，truncate不会更改lob index的dataobj#,只需要修改lob的dataobj#即可 SQL> update obj$ set dataobj#=87903 where obj#=87903;1 row updated. SQL> commit; Commit complete. SQL> alter system flush shared_pool; System altered. 设置表空间为read only，避免数据被覆盖SQL> alter tablespace users read only;Tablespace altered. 具体抽取脚本如下：注意：恢复的表需要在重新指定一个表空间，避免覆盖数据。本脚本示例将表恢复到了system表空间。 declare v_fno number; v_s_bno number; v_e_bno number; v_rowid rowid; nrows number; v_owner varchar2(100):='RESCUREORA'; v_table varchar2(100):='RESCUREORA_TABLE'; v_o_owner varchar2(100):='SYS'; v_o_table varchar2(100):='RESCUREORA_TABLE'; v_dataobj number; v_sql varchar2(4000); v_tablespace varchar2(100);begin select data_object_id into v_dataobj from dba_objects where owner=v_owner and object_name=v_table; select tablespace_name into v_tablespace from dba_tables where owner=v_owner and table_name=v_table; for i in (select relative_fno,block_id,blocks from dba_extents where owner=v_owner and segment_name=v_table and extent_id=0 union all select relative_fno,block_id,blocks from dba_free_space where tablespace_name=v_tablespace union all select relative_fno,block_id,blocks from ( select relative_fno,block_id,blocks,row_number()over(partition by owner,segment_name,partition_name order by extent_id desc) rn from dba_extents where tablespace_name=v_tablespace and extent_id>0) where rn=1) loop v_fno:=i.relative_fno; v_s_bno:=i.block_id; v_e_bno:=i.block_id+i.blocks-1; for j in v_s_bno .. v_e_bno loop begin for x in 0 .. 999 loop v_rowid:=dbms_rowid.rowid_create(1,v_dataobj,v_fno,j,x); v_sql:='insert into '||v_o_owner||'.'||v_o_table||' select * from '||v_owner||'.'||v_table||' where rowid=:1'; execute immediate v_sql using v_rowid; end loop; exception when others then null; end; commit; end loop; end loop;end;/ SQL> select count(*) from sys.rescureora_table; COUNT(*)---------- 86877 SQL> update obj$ set dataobj#=87904 where obj#=87903; 1 row updated. SQL> commit;Commit complete. SQL> alter system flush shared_pool;SQL> alter system flush buffer_cache;至此，该方法在无数据覆盖的情况下实现了完全恢复。
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