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Posted Jun 10, 2004

Creating NOT NULL Columns in Huge Oracle Tables - Page 2

By Amar Kumar Padhi

Parallel Processing

Parallel processing can be used for running huge updates. This is achieved by setting the table's degree to a higher value. This would result in an UPDATE statement being fired in multiple threads. The number of threads invoked would be equal to what is set as the table degree. The maximum threads used by Oracle does not exceed the value specified by the initialization parameter PARALLEL_MAX_SERVERS.

The tables V$PX_PROCESS, V$PX_SESSION, V$PX_SESSTAT and V$PX_PROCESS_SYSSTAT are used to monitor parallel processing activities. Before running the update process, one large rollback segment is made available for it, all the rest are made offline. If using Undo tablespace, the size should be verified and increased if necessary. In my case, I keep a rollback segment of 500MB online.

session 1:

SQL> select name, value from v$parameter 
  where name = 'parallel_max_servers';

NAME                                   VALUE
-------------------------------------- --------------
parallel_max_servers                   8

SQL>  alter rollback segment rbs01 offline;  
 -- This is done for all rbs (not system!) 
     except the big one.

Rollback segment altered.

SQL> alter table mtl_trx parallel 
  (degree 6);

SQL> alter table mtl_trx add lovind varchar2(1);

Table altered.

SQL> update mtl_trx set lovind = 'X';

Session 2 (While the above statement is running):

SQL> select * from v$px_process;

SERV STATUS           PID SPID             SID    SERIAL#
---- --------- ---------- --------- ---------- ----------
P000 IN USE            62 5478              26      19690
P001 IN USE            65 5480              81        946
P002 IN USE            68 5482              78       1465
P003 IN USE            70 5484               9      16174
P004 IN USE            71 5486              56       6246
P005 IN USE            72 5488              82        272

6 rows selected.

SQL> select sid, serial# from v$session where program like '%P00%';

       SID    SERIAL#
---------- ----------
         9      16174
        26      19690
        56       6246
        78       1465
        81        946
        82        272

6 rows selected.

SQL> select saddr, sid, serial#, server#, degree,  req_degree
  2  from   v$px_session
  3  where sid in (9, 26, 56, 78, 81, 82);

-------- ---------- ---------- ---------- ---------- ----------
8C0A7ED8         26      19690          1          6          6
8C0C4E0C         81        946          2          6          6
8C0C34C8         78       1465          3          6          6
8C09EFAC          9      16174          4          6          6
8C0B7B80         56       6246          5          6          6
8C0C5678         82        272          6          6          6

6 rows selected.

SQL> select * from v$px_process_sysstat;

STATISTIC                           VALUE
------------------------------ ----------
Servers In Use                          6
Servers Available                       0
Servers Started                        18
Servers Shutdown                       12
Servers Highwater                       6
Servers Cleaned Up                      0
Server Sessions                         6
Memory Chunks Allocated                 1
Memory Chunks Freed                     0
Memory Chunks Current                   1
Memory Chunks HWM                       1

STATISTIC                           VALUE
------------------------------ ----------
Buffers Allocated                  104146
Buffers Freed                      104128
Buffers Current                        18
Buffers HWM                            28

15 rows selected.

Session 1:

The session  completes update.

2205961 rows updated.

SQL> commit;

Commit complete.

SQL> alter table mtl_trx modify lovind default 'X' not null;

Table altered.

SQL> alter table mtl_trx noparallel; 

Table altered.

SQL> select degree from dba_tables where table_name = 'MTL_TRX';


The last step of setting the degree back to 1 should not be skipped, as it may affect online queries. The total process took around 16-18 minutes (excluding statistics generation). This can be further reduced by increasing the number of threads.

Advantage: Oracle provided threading option is used, process time reduced.

Disadvantage: Resource intensive, undo segment errors are common if sizing is not done before hand, no commits are possible in intervals, the complete process fails if a single thread returns an error (rolling back changes takes considerable time), temporary alteration of degree is required for the table (the initialization parameter PARALLEL_MAX_SERVERS may also need to be set).


Data can be exported and re-imported with a default value on the column. I personally don't recommend this, as the idea of taking data out of the table in a production database is risky and sometimes dependencies exists between objects (e.g.: materialized views, foreign key relations etc.). A copy could be made for safety sake but this would depend on space availability.

Running export at OS level:

$ exp userid=apps/apps file=mtl_trx.dmp log=1.log tables=mtl_trx rows=y triggers=y grants=y indexes=y

Export: Release - Production on Wed May 26 15:06:32 2004

(c) Copyright 2000 Oracle Corporation.  All rights reserved.

Connected to: Oracle8i Enterprise Edition Release - Production
With the Partitioning option
JServer Release - Production
Export done in US7ASCII character set and US7ASCII NCHAR character set

About to export specified tables via Conventional Path ...
. . exporting table                        MTL_TRX    2205961 rows exported
Export terminated successfully without warnings.

Instead of truncating, I rename the tables and the indexes and create a new structure. This will have the new column with the default value.

SQL> rename mtl_trx to bkp_mtl_trx;

Table renamed.

SQL> alter index mtl_trx_u1 rename to bkp_mtl_trx_u1;

Index altered.

SQL> create table mtl_trx as select * from bkp_mtl_trx where 1 = 2;

Table created.

SQL> alter table mtl_trx add xtcind varchar2(1) default 'N' not null;

Table altered.

data imported back:

$ imp userid=apps/apps file=mtl_trx.dmp log=2.log buffer=2097152 ignore=y commit
=y tables=mtl_trx

Import: Release - Production on Wed May 26 15:59:18 2004

(c) Copyright 2000 Oracle Corporation.  All rights reserved.

Connected to: Oracle8i Enterprise Edition Release - Production
With the Partitioning option
JServer Release - Production

Export file created by EXPORT:V08.01.07 via conventional path
import done in US7ASCII character set and US7ASCII NCHAR character set
. importing APPS's objects into APPS
. . importing table                      "MTL_TRX"    2205961 rows imported
Import terminated successfully without warnings.

The new column is populated with the default value when the import takes place. The total process takes around 20-24 minutes (excluding statistics generation).

Advantage: Process time reduced, commits are possible at intervals with BUFFER and COMMIT setting in import, no Undo-space issues.

Disadvantage: Complete downtime is required, structural changes are done, sufficient space is required at both OS and database level, a risk involved as data is being moved out of the database, the more indexes that are present on the table the more the process time increases, connected objects like triggers and materialized views have to be evaluated for their role.

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