Empty String IS NOT NULL?

Today friend of mine show me interesting case:

declare
  var varchar2(3);
  chr char(3);
begin
  var := ''; -- Empty string here
  chr := var;
  if var is null then
    dbms_output.put_line('VAR IS NULL');
  end if;
  if chr is null then
    dbms_output.put_line('CHR IS NULL');
  else
    dbms_output.put_line('CHR IS NOT NULL');
  end if;
end;
/
VAR IS NULL
CHR IS NOT NULL

I must say that I was expecting that both variables will be null. It might be my lack of experience with CHAR datatype as I usually use VARCHAR2 even when it is expected that string will have constant length.

declare
  var varchar2(3);
  chr char(3);
begin
  var := null;
  chr := var;
  if var is null then
    dbms_output.put_line('VAR IS NULL');
  end if;
  if chr is null then
    dbms_output.put_line('CHR IS NULL');
  else
    dbms_output.put_line('CHR IS NOT NULL');
  end if;
end;
/
VAR IS NULL
CHR IS NULL

Just as expected. So it seems that in some cases NULL and empty string is not equivalent. But this difference can be only seen in PL/SQL. When I tried to reproduce this in pure SQL it was always NULL:

SQL> create table a(v1 varchar2(3), v2 varchar2(3), c1 char(3), c2 char(3));
Table created.
SQL> insert into a values (null, '', null, '');
1 row created.
SQL> select dump(v1) v1,dump(v2) v2,dump(c1)c1,dump(c2) c2 from a;
V1    V2    C1    C2
----- ----- ----- -----
NULL  NULL  NULL  NULL
SQL> update a set c1 = v1, c2 = v2;
1 row updated.
SQL> select dump(v1) v1,dump(v2) v2,dump(c1)c1,dump(c2) c2 from a;
V1    V2    C1    C2
----- ----- ----- -----
NULL  NULL  NULL  NULL

Cheers, Paweł

Interesting posts 2007-02-06

Here are some interesting posts I've read recently:

• User Interface Design: Complexity vs. Flexibility - Rob Walling is traing to find aswer to question: can a design be simple and flexible at the same time?
• How to scale... Tom Kyte post about scalability of middleware and database. Do not forget to read Tim Hall response Middle-tier diet….
• Difference between Jeff Bezos and Bill Gates? by Kathy Sierra - very interesting story about top CEO's attitude to users.

Cheers, Paweł

Restore CONTROLFILE and/or SPFILE from autobackup

As I mentioned earlier in post about setting up autobackup, its time to describe how to restore CONTROLFILE and SPFILE from autobackups. If your database is still running and what You want is just get historical version of SPFILE or CONTROLFILE then it is easy task. Just start RMAN:

RMAN TARGET=/

And enter command to restore CONTROLFILE:

RMAN> restore until time 'sysdate-3'
2> CONTROLFILE to 'c:\temp\cfile' from autobackup;
Starting restore at 01-FEB-07
using channel ORA_DISK_1
recovery area destination: C:\oracle\product\10.2.0/flash_recovery_area
database name (or database unique name) used for search: BAR
channel ORA_DISK_1: no autobackups found in the recovery area
channel ORA_DISK_1: looking for autobackup on day: 20070129
channel ORA_DISK_1: autobackup found: C:\Backup\c-669001291-20070129-03
channel ORA_DISK_1: control file restore from autobackup complete
Finished restore at 01-FEB-07

The until time clause sets time of validity for CONTROLFILE. By default RAM looks for autobackups for seven days into past starting from that time. It can happen that for several days there were no changes, and autobackup was not invoked. In that case add MAXDAYS clause:

RMAN> restore until time 'sysdate-3'
2> CONTROLFILE to 'c:\temp\cfile' from autobackup maxdays 100;

It will force RMAN to look for copies for more days into past. Similarly You can restore SPFILE:

RMAN> restore until time 'sysdate-3'
2> SPFILE to 'c:\temp\SPFILE' from autobackup maxdays 100;
Starting restore at 01-FEB-07
using channel ORA_DISK_1
recovery area destination: C:\oracle\product\10.2.0/flash_recovery_area
database name (or database unique name) used for search: BAR
channel ORA_DISK_1: no autobackups found in the recovery area
channel ORA_DISK_1: looking for autobackup on day: 20070129
channel ORA_DISK_1: autobackup found: C:\Backup\c-669001291-20070129-03
channel ORA_DISK_1: SPFILE restore from autobackup complete
Finished restore at 01-FEB-07

After restoring SPFILE You can convert it to PFILE, so You can easily read settings:

RMAN> sql "create PFILE = ''c:\temp\PFILE'' from SPFILE = ''c:\temp\SPFILE''";
sql statement: create PFILE = ''c:\temp\PFILE'' from SPFILE = ''c:\temp\SPFILE''

Now more complex scenario: You have lost all database files including CONTROLFILE and SPFILE. You only have backups created by RMAN. In that case You have to eventually install Database Software. Then setup environment variables and start recovery. On Windows platform You have to create Service to be able to startup oracle. You probably remember instance name and SID. If You don't it can be usually found in some TNSNAMES files on client machines. But You probably do not know DBID. You can easily find DBID – it'is part of the name of autobackup file. Details in my previous post. So if autobackup file name is 'c-669001291-20070129-03' then DBID is 669001291. Now we can start recovery:

RMAN target=/

At the beginning set Database ID:

RMAN> SET DBID 669001291;

Then startup instance:

RMAN> startup force nomount;

and run recovery of SPFILE

RMAN> RUN {
2> SET CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO 'C:\Backup\%F'; 
3> RESTORE SPFILE FROM AUTOBACKUP; }

I suggest also creating PFILE from SPFILE. Then You should check if all file locations are correct, all directories exists and Oracle have rights to write in them.

RMAN> shutdown;
RMAN> startup force nomount;
RMAN> RUN { 
2> SET CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO 'C:\Backup\%F'; 
3> RESTORE CONTROLFILE FROM AUTOBACKUP; }

Now You can continue recovery of datafiles.

Paweł

Autobackup of SPFILE and CONTROLFILE

Even if you do not use RMAN to take backups of your database, you might find it useful to take snapshots of your SPFILE and CONTROLFILE automatically. You can configure Recovery Manager (RMAN) to automatically create copies of CONTROLFILE and SPFILE when you make changes to database such as:

• adding/dropping a new tablespace or datafile
• altering the state of a tablespace or datafile (online/offline/read only)
• adding a new online redo log,
• renaming a file
• adding a new redo thread
• and many more...

To setup autobackup just start RMAN and connect to database

RMAN target=/

or

RMAN target=sys/password@db

set directory and format of backup files:

RMAN> CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO 'C:\Backup\%F';

In name of autbackup file you have to use %F. It will be expanded to C-XXXXXXXXX-YYYYMMDD-NN, where:

• XXXXXXXXX – database id
• YYYYMMDD – day, when backuped
• NN – change number during day, starts with 00, and represented in hexadecimal

Activate autobackups:

RMAN> CONFIGURE CONTROLFILE AUTOBACKUP ON;

Now autobackups are active, and every time you change database structure controlfile will be automatically backed up.
If you use RMAN for backups it will create copy of SPFILE or CONTROLFILE also after each backup operation.
I've tested it on Oracle 10g (10.2) but it should work in Oracle since version 8i. Next time i'll write how to restore SPFILE or CONTROLFILE from these backup.

Cheers, Paweł

Update: How to restore CONTROLFILE and SPFILE from RMAN autobackups

Oracle XMLType: exctractvalue vs. extract

I always thought that

exctarctvalue(xml, '/Node')

is equivalent to

xml.extract('/Node/text()').getstringval()

Usually it is true, but in some cases it is not. When node contains special characters like &, >, < etc... then results are different. Let me give an example:

SQL> set define off
SQL> select aa.a.extract('Node/text()').getStringVal() extr
  2       , extractvalue(aa.a, 'Node') extractval
  3    from (select XMLRoot(
  4              XMLElement("Node", 'test & < and >'),
  5             VERSION '1.0') a from dual) aa;

EXTR
----------------------------------------
EXTRACTVAL
----------------------------------------
test &amp; &lt; and &gt;
test & < and >

First look on documentation even confirms my expectations:
"extractValue – This takes an XPath expression and returns the corresponding leaf node. The XPath expression passed to extractValue should identify a single attribute or an element that has precisely one text node child. The result is returned in the appropriate SQL data type. Function extractValue is essentially a shortcut for extract plus either getStringVal() or getNumberVal()."
XML DB Developer's Guide - b14259.pdf page 1-17.

But there is small note on that:
"Note: Function extractValue and XMLType method getStringVal() differ in their treatment of entity encoding. Function extractValue unescapes any encoded entities; method getStringVal() returns the data with entity encoding intact."
XML DB Developer's Guide - b14259.pdf page 4-10.

Cheers, Paweł

RMAN connection to Catalog - updated

I must say that I was completely wrong in my post RMAN connection to Catalog. In fact when connection to Catalog fails, RMAN quits and rest of script is ignored. So I've changed this. RMAN connect only to Target Database during backup process, and after that RMAN is run second time. This time it connects to Target Database and Catalog Database, then run command RESYNC CATALOG to force synchronization of control file with Catalog. It should work now.

Cheers, Paweł

ORA-03297 on empty datafile

Today I've noticed strange behavior of Oracle 10gR2 (10.2.0.3) while shrinking datafile. I had empty datafile (no objects) and i couldn't resize it. I was getting ORA-03297: file contains used data beyond requested RESIZE value while DBA_EXTENTS show no rows for that file. I've discovered that there are objects in Recycle Bin that were located on that file. After purging recycle bin file was shirked successfully.
Let me make an example:

SQL> create tablespace tb_test
  2  datafile 'c:\temp\tb_test.dbf' size 100 K autoextend on;
Tablespace created.
SQL> select count(*) from dba_extents where tablespace_name='TB_TEST';
  COUNT(*)                                                                      
----------                                                                      
         0                                                                      
SQL> select bytes from v$datafile where name = 'C:\TEMP\TB_TEST.DBF';
     BYTES                                                                      
----------                                                                      
    106496                                                                      
SQL> create table t_big tablespace tb_test as
  2  select a.* from all_objects a, dba_users for_cartesian;
Table created.
SQL> select count(*) from dba_extents where tablespace_name='TB_TEST';
  COUNT(*)                                                                      
----------                                                                      
        34                                                                      
SQL> select bytes from v$datafile where name = 'C:\TEMP\TB_TEST.DBF';
     BYTES                                                                      
----------                                                                      
  19963904

Now we have datafile quite big. Let's drop table and shrink file:

SQL> drop table t_big;
Table dropped.
SQL> select count(*) from dba_extents where tablespace_name='TB_TEST';
  COUNT(*)                                                                      
----------                                                                      
         0                                                                      
SQL> alter database datafile 'c:\temp\tb_test.dbf' resize 100 K;
alter database datafile 'c:\temp\tb_test.dbf' resize 100 K
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value 

So. even there is nothing reported in DBA_EXTENT, file cannot be shinked. Let's purge recyclebin and try again:

SQL> purge recyclebin;
Recyclebin purged.
SQL> alter database datafile 'c:\temp\tb_test.dbf' resize 100 K;
Database altered.

Now works fine! Don't forget to clear after tests :)

SQL> drop tablespace tb_test including contents and datafiles;
Tablespace dropped.

Cheers, Paweł