DBMS_REPAIR example

PURPOSE

 This document provides an example of DBMS_REPAIR as introduced in Oracle 8i.
 Oracle provides different methods for detecting and correcting data block
 corruption - DBMS_REPAIR is one option.

 WARNING: Any corruption that involves the loss of data requires analysis to
 understand how that data fits into the overall database system. Depending on
 the nature of the repair, you may lose data and logical inconsistencies can
 be introduced; therefore you need to carefully weigh the gains and losses
 associated with using DBMS_REPAIR.

SCOPE & APPLICATION

 This article is intended to assist an experienced DBA working with an Oracle
 Worldwide Support analyst only.  This article does not contain general
 information regarding the DBMS_REPAIR package, rather it is designed to provide
 sample code that can be customized by the user (with the assistance of
 an Oracle support analyst) to address database corruption.  The
 "Detecting and Repairing Data Block Corruption" Chapter of the Oracle8i
 Administrator's  Guide should be read and risk assessment analyzed prior to
 proceeding.

RELATED DOCUMENTS

  Oracle 8i Administrator's Guide,  DBMS_REPAIR Chapter

Introduction
=============

Note: The DBMS_REPAIR package is used to work with corruption in the
transaction layer and the data layer only (software corrupt blocks).
Blocks with physical corruption (ex. fractured block) are marked as
the block is read into the buffer cache and DBMS_REPAIR ignores all
blocks marked corrupt.

The only block repair in the initial release of DBMS_REPAIR is to
*** mark the block software corrupt ***.

DB_BLOCK_CHECKING and DB_BLOCK_CHECKSUM must both be set to FALSE.

A backup of the file(s) with corruption should be made before using package.

Database Summary
===============

A corrupt block exists in table T1.

SQL> desc t1
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 COL1                                      NOT NULL NUMBER(38)
 COL2                                               CHAR(512)

SQL> analyze table t1 validate structure;
analyze table t1 validate structure
*
ERROR at line 1:
ORA-01498: block check failure - see trace file

---> Note: In the trace file produced from the ANALYZE, it can be determined
---        that the corrupt block contains 3 rows of data (nrows = 3).
---        The leading lines of the trace file follows:

Dump file /export/home/oracle/product/8.1.5/admin/V815/udump/v815_ora_2835.trc
Oracle8 Enterprise Edition Release 8.1.5.0.0 - Beta
With the Partitioning option

*** 1998.12.16.15.53.02.000
*** SESSION ID:(7.6) 1998.12.16.15.53.02.000
kdbchk: row locked by non-existent transaction
        table=0   slot=0
        lockid=32   ktbbhitc=1
Block header dump:  0x01800003
 Object id on Block? Y
 seg/obj: 0xb6d  csc: 0x00.1cf5f  itc: 1  flg: -  typ: 1 - DATA
     fsl: 0  fnx: 0x0 ver: 0x01

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   xid:  0x0002.011.00000121    uba: 0x008018fb.0345.0d  --U-    3  fsc
0x0000.0001cf60

data_block_dump
===============
tsiz: 0x7b8
hsiz: 0x18
pbl: 0x28088044
bdba: 0x01800003
flag=-----------
ntab=1
nrow=3
frre=-1
fsbo=0x18
fseo=0x19d
avsp=0x185
tosp=0x185
0xe:pti[0]      nrow=3  offs=0
0x12:pri[0]     offs=0x5ff
0x14:pri[1]     offs=0x3a6
0x16:pri[2]     offs=0x19d
block_row_dump:

[... remainder of file not included]

end_of_block_dump

DBMS_REPAIR.ADMIN_TABLES (repair and orphan key
================================================

ADMIN_TABLES provides administrative functions for repair and orphan key tables.

SQL> @adminCreate
SQL> connect sys/change_on_install
Connected.
SQL>
SQL> -- Repair Table
SQL>
SQL> declare
  2  begin
  3  -- Create repair table
  4  dbms_repair.admin_tables (
  5  --    table_name => 'REPAIR_TABLE',
  6      table_type => dbms_repair.repair_table,
  7      action => dbms_repair.create_action,
  8      tablespace => 'USERS');          -- default TS of SYS if not specified
  9  end;
 10  /

PL/SQL procedure successfully completed.

SQL> select owner, object_name, object_type
  2  from dba_objects
  3  where object_name like '%REPAIR_TABLE';

OWNER                 OBJECT_NAME                      OBJECT_TYPE
------------------------------------------------------------------
SYS                   DBA_REPAIR_TABLE                 VIEW
SYS                   REPAIR_TABLE                     TABLE

SQL>
SQL> -- Orphan Key Table
SQL>
SQL> declare
  2  begin
  3  -- Create orphan key table
  4  dbms_repair.admin_tables (
  5      table_type => dbms_repair.orphan_table,
  6      action => dbms_repair.create_action,
  7      tablespace => 'USERS');          -- default TS of SYS if not specified
  8  end;
  9  /

PL/SQL procedure successfully completed.

SQL> select owner, object_name, object_type
  2  from dba_objects
  3  where object_name like '%ORPHAN_KEY_TABLE';

OWNER                 OBJECT_NAME                      OBJECT_TYPE
------------------------------------------------------------------
SYS                   DBA_ORPHAN_KEY_TABLE             VIEW
SYS                   ORPHAN_KEY_TABLE                 TABLE

DBMS_REPAIR.CHECK_OBJECT
=========================

CHECK_OBJECT procedure checks the specified object and populates the repair
table with information about corruption and repair directive(s).  Validation
consists of block checking all blocks in the object.  All blocks previously
marked corrupt will be skipped.

Note: In the initial release of DBMS_REPAIR the only repair is to mark the
      block as software corrupt.

SQL> @checkObject
SQL> set serveroutput on
SQL>
SQL> declare
  2     rpr_count int;
  3  begin
  4     rpr_count := 0;
  5  dbms_repair.check_object (
  6     schema_name => 'SYSTEM',
  7     object_name => 'T1',
  8     repair_table_name => 'REPAIR_TABLE',
  9     corrupt_count => rpr_count);
 10     dbms_output.put_line('repair count: ' || to_char(rpr_count));
 11  end;
 12  /
repair count: 1

PL/SQL procedure successfully completed.

SQL> desc repair_table
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OBJECT_ID                                 NOT NULL NUMBER
 TABLESPACE_ID                             NOT NULL NUMBER
 RELATIVE_FILE_ID                          NOT NULL NUMBER
 BLOCK_ID                                  NOT NULL NUMBER
 CORRUPT_TYPE                              NOT NULL NUMBER
 SCHEMA_NAME                               NOT NULL VARCHAR2(30)
 OBJECT_NAME                               NOT NULL VARCHAR2(30)
 BASEOBJECT_NAME                                    VARCHAR2(30)
 PARTITION_NAME                                     VARCHAR2(30)
 CORRUPT_DESCRIPTION                                VARCHAR2(2000)
 REPAIR_DESCRIPTION                                 VARCHAR2(200)
 MARKED_CORRUPT                            NOT NULL VARCHAR2(10)
 CHECK_TIMESTAMP                           NOT NULL DATE
 FIX_TIMESTAMP                                      DATE
 REFORMAT_TIMESTAMP                                 DATE

SQL> select object_name, block_id, corrupt_type, marked_corrupt,
  2  corrupt_description, repair_description
  3  from repair_table;

OBJECT_NAME                      BLOCK_ID CORRUPT_TYPE MARKED_COR
------------------------------ ---------- ------------ ----------
CORRUPT_DESCRIPTION
--------------------------------------------------------------------------------
REPAIR_DESCRIPTION
--------------------------------------------------------------------------------
T1                                      3            1 FALSE
kdbchk: row locked by non-existent transaction
        table=0   slot=0
        lockid=32   ktbbhitc=1
mark block software corrupt

Data Extraction
===============

The repair table indicates that block 3 of file 6 is corrupt - but remember
that this block has not yet been marked as corrupt, therefore now is the
time to extract any meaningful data.  After the block is marked corrupt,
the entire block must be skipped.

1. Determine the number of rows in the block from ALTER SYSTEM DUMP (nrows = 3).
2. Query the corrupt object and extract as much information as possible.

SQL> -- The following query can be used to salvage data from a corrupt block.
SQL> -- Creating a temporary table facilitates data insertion.

SQL> create table temp_t1 as
  2  select * from system.t1
  3  where dbms_rowid.rowid_block_number(rowid) = 3
  4  and dbms_rowid.rowid_to_absolute_fno (rowid, 'SYSTEM','T1') = 6;

Table created.

SQL> select col1 from temp_t1;

      COL1
----------
         2
         3

DBMS_REPAIR.FIX_CORRUPT_BLOCKS  (ORA-1578)
============================================

FIX_CORRUPT_BLOCKS procedure fixes the corrupt blocks in the specified objects
based on information in the repair table.  After the block has been marked as
corrupt,  an ORA-1578 results when a full table scan is performed.

SQL> declare
  2     fix_count int;
  3  begin
  4     fix_count := 0;
  5  dbms_repair.fix_corrupt_blocks (
  6     schema_name => 'SYSTEM',
  7     object_name => 'T1',
  8     object_type => dbms_repair.table_object,
  9     repair_table_name => 'REPAIR_TABLE',
 10     fix_count => fix_count);
 11     dbms_output.put_line('fix count: ' || to_char(fix_count));
 12  end;
 13  /
fix count: 1

PL/SQL procedure successfully completed.

SQL> select object_name, block_id, marked_corrupt
  2  from repair_table;

OBJECT_NAME                      BLOCK_ID MARKED_COR
------------------------------ ---------- ----------
T1                                      3 TRUE

SQL> select * from system.t1;
select * from system.t1
                     *
ERROR at line 1:
ORA-01578: ORACLE data block corrupted (file # 6, block # 3)
ORA-01110: data file 6: '/tmp/ts_corrupt.dbf'

DBMS_REPAIR.DUMP_ORPHAN_KEYS
==============================

DUMP_ORPHAN_KEYS reports on index entries that point to rows in corrupt data
blocks.

SQL> select index_name from dba_indexes
  2  where table_name in (select distinct object_name from repair_table);

INDEX_NAME
------------------------------
T1_PK

SQL> @dumpOrphanKeys
SQL> set serveroutput on
SQL>
SQL> declare
  2     key_count int;
  3  begin
  4     key_count := 0;
  5  dbms_repair.dump_orphan_keys (
  6     schema_name => 'SYSTEM',
  7     object_name => 'T1_PK',
  8     object_type => dbms_repair.index_object,
  9     repair_table_name => 'REPAIR_TABLE',
 10     orphan_table_name => 'ORPHAN_KEY_TABLE',
 11     key_count => key_count);
 12     dbms_output.put_line('orphan key count: ' || to_char(key_count));
 13  end;
 14  /
orphan key count: 3
PL/SQL procedure successfully completed.

SQL> desc orphan_key_table
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 SCHEMA_NAME                               NOT NULL VARCHAR2(30)
 INDEX_NAME                                NOT NULL VARCHAR2(30)
 IPART_NAME                                         VARCHAR2(30)
 INDEX_ID                                  NOT NULL NUMBER
 TABLE_NAME                                NOT NULL VARCHAR2(30)
 PART_NAME                                          VARCHAR2(30)
 TABLE_ID                                  NOT NULL NUMBER
 KEYROWID                                  NOT NULL ROWID
 KEY                                       NOT NULL ROWID
 DUMP_TIMESTAMP                            NOT NULL DATE

SQL> select index_name, count(*) from orphan_key_table
  2  group by index_name;

INDEX_NAME                       COUNT(*)
------------------------------ ----------
T1_PK                                   3

Note: Index entry in the orphan key table implies that the index should be
rebuilt to guarantee the a table probe and an index probe return the same
result set.

DBMS_REPAIR.SKIP_CORRUPT_BLOCKS
===============================

SKIP_CORRUPT_BLOCKS enables/disables the skipping of corrupt blocks during
index and table scans of a specified object.

Note: If an index and table are out of sync, then a SET TRANSACTION READ ONLY
transaction may be inconsistent in situations where one query probes only
the index and then a subsequent query probes both the index and the table.
If the table block is marked corrupt, then the two queries will return
different results.

Suggestion: If SKIP_CORRUPT_BLOCKS is enabled, then rebuild any indexes
identified in the orphan key table (or all index associated with object
if DUMP_ORPHAN_KEYS was omitted).

SQL> @skipCorruptBlocks
SQL> declare
  2  begin
  3  dbms_repair.skip_corrupt_blocks (
  4     schema_name => 'SYSTEM',
  5     object_name => 'T1',
  6     object_type => dbms_repair.table_object,
  7     flags => dbms_repair.skip_flag);
  8  end;
  9  /

PL/SQL procedure successfully completed.

SQL> select table_name, skip_corrupt from dba_tables
  2  where table_name = 'T1';

TABLE_NAME                     SKIP_COR
------------------------------ --------
T1                             ENABLED

SQL> -- rows in corrupt block skipped, no errors on full table scan
SQL> select * from system.t1;

COL1              COL2
--------------------------------------------------------------------------------
4                 dddd
5                 eeee

--> Notice the pk index has not yet been corrected.

SQL> insert into system.t1 values (1,'aaaa');
insert into system.t1 values (1,'aaaa')
                   *
SQL> select * from system.t1 where col1 = 1;

no rows selected

DBMS_REPAIR.REBUILD_FREELISTS
===============================

REBUILD_FREELISTS rebuilds freelists for the specified object.

SQL> declare
  2  begin
  3  dbms_repair.rebuild_freelists (
  4     schema_name => 'SYSTEM',
  5     object_name => 'T1',
  6     object_type => dbms_repair.table_object);
  7  end;
  8  /

PL/SQL procedure successfully completed.

Rebuild Index
=============

Note:  Every index identified in the orphan key table should be rebuilt to
ensure consistent results.

SQL> alter index system.t1_pk rebuild online;

Index altered.

SQL> insert into system.t1 values (1, 'aaaa');

1 row created.

SQL> select * from system.t1;

COL1              COL2
--------------------------------------------------------------------------------
4                 dddd
5                 eeee
1                 aaaa

Note - The above insert statement was used to provide a simple example.
This is the perfect world - we know the data that was lost.  The temporary
table (temp_t1) should also be used to include all rows extracted from
the corrupt block.

Conclusion
==========

At this point the table T1 is available but data loss was incurred.  In general,
data loss must be seriously considered before using the DBMS_REPAIR package for
mining the index segment and/or table block dumps is very complicated and
logical inconsistencies may be introduced.  In the initial release, the only
repair affected by DBMS_REPAIR is to mark the block as software corrupt.

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