Tip#12 STATUS column of dba_undo_extents

I am sure you might have seen loads of information regarding how to resize/drop/recreate UNDO tablspace but point to note during dropping (ex-default/previous) UNDO tablespace is to check a view dba_undo_extents to see if any undo statments is still used by any session or needed for flashback. Following SQL should tell you if you can go ahead and drop the ex-default/previous UNDO tablespace or not (assuming undo tablespace I want to drop is UNDOTBS2),

SELECT tablespace_name, status, COUNT (*)
FROM SYS.dba_undo_extents
WHERE tablespace_name = 'UNDOTBS2'
GROUP BY tablespace_name, status

What is of our interest is status column of dba_undo_extents, 3 possible values - ACTIVE, EXPIRED, UNEXPIRED. What are the meanings ?

ACTIVE means that this undo segment contains active transactions

EXPIRED means that this segment is not rqeuired at all after considering Undo retention period

UNEXPIRED means that this segment does not contain any active transactions but it contains transactions which are still required for Flashback option (after considering Undo retention period).

So if you have all extents EXPIRED then go ahead and drop the tablespace, as usual make sure that you have done proper backup before and after.

Tip#11 SQL*Plus - Ignore Blank Lines in the Script

Recently I came across a situation where during our database deploy one of our View script was exiting in SQLplus due to a blank line inside the script. To fix it, we had to set sqlblanllines setting in sqlplus to ignore the blank lines and that did the trick for us.

SQL> create or replace view xx
2 as
3 select
4 sysdate as x
5
SQL> from
SP2-0042: unknown command "from" - rest of line ignored.
SQL> dual
SP2-0042: unknown command "dual" - rest of line ignored.
SQL> set sqlblanklines on
SQL> create or replace view xx
2 as
3 select
4 sysdate as x
5
6 from
7 dual
8 ;

View created.

SQL>

Tip#10 Dump to text file using UTL_FILE

Dump the data from table/sql to text file (CSV or PIPE delimated etc) on the server using UTL_FILE

Note :
A) Specify the accessible directories for the UTL_FILE functions in the initialization file using the UTL_FILE_DIR parameter. e.g. : UTL_FILE_DIR = /app/oracle/my_dir
FYI, UTL_FILE_DIR = * means turn off directory access checking, and it makes any directory accessible to the UTL_FILE (NOT RECOMMANDED).

EDIT: if you are using > 9i , you can use DIRECTORY instead of UTL_FILE_DIR.

B) Since we have data with special character, we open file with FOPEN_NCHAR to make sure we don’t loose special characters in the text dump.

C) I have just tested it on Redhat Linux - Oracle 9.2.0.6

This is the Function which I use (adapted to my requirements but thanks to Tom Kyte to get me started)

CREATE OR REPLACE FUNCTION dump_to_file (
p_query IN VARCHAR2,
p_separator IN VARCHAR2 DEFAULT '|',
p_dir IN VARCHAR2,
p_filename IN VARCHAR2
)
RETURN NUMBER
IS
l_filehandle UTL_FILE.file_type;
l_thecursor INTEGER DEFAULT DBMS_SQL.open_cursor;
l_columnvalue VARCHAR2 (32767);
l_col_hdr VARCHAR2 (32767);
l_status INTEGER;
l_colcnt NUMBER DEFAULT 0;
l_separator VARCHAR2 (10) DEFAULT '';
l_cnt NUMBER DEFAULT 0;
col_cnt PLS_INTEGER;
rec_tab DBMS_SQL.desc_tab;
col_num NUMBER;
BEGIN
-- open a writable file in the UTL_FILE_DIR directory
l_filehandle := UTL_FILE.fopen_nchar (p_dir, p_filename, 'w', 32767);

-- parse the sql we got
DBMS_SQL.parse (l_thecursor, p_query, DBMS_SQL.native);

-- execute the sql cursor
l_status := DBMS_SQL.EXECUTE (l_thecursor);

-- get the column information l_colcnt OUT param to get us number of columns
-- and rec_tab is plsql table with all column related information like name, length,schema, precision, scale etc
DBMS_SQL.describe_columns (l_thecursor, l_colcnt, rec_tab);

-- we need to print the column names as the first line
l_separator := '';
l_col_hdr := '';


FOR i IN 1 .. l_colcnt
LOOP
-- we define which columns we need in the fetch
DBMS_SQL.define_column (l_thecursor, i, l_columnvalue, 4000);
-- print the column name in the file
l_col_hdr := l_col_hdr || l_separator || rec_tab (i).col_name;
l_separator := p_separator;
END LOOP;

UTL_FILE.put_nchar (l_filehandle, l_col_hdr);
UTL_FILE.new_line (l_filehandle);

-- lets print the data now
LOOP
EXIT WHEN (DBMS_SQL.fetch_rows (l_thecursor) <= 0); l_separator := ''; FOR i IN 1 .. l_colcnt LOOP DBMS_SQL.COLUMN_VALUE (l_thecursor, i, l_columnvalue); UTL_FILE.put_nchar (l_filehandle, l_separator || l_columnvalue); l_separator := p_separator; END LOOP; UTL_FILE.new_line (l_filehandle); l_cnt := l_cnt + 1; END LOOP; DBMS_SQL.close_cursor (l_thecursor); UTL_FILE.fclose (l_filehandle); RETURN l_cnt; EXCEPTION WHEN UTL_FILE.invalid_operation THEN UTL_FILE.fclose (l_filehandle); raise_application_error (-20061, 'Dump To File Error: Invalid Operation'); WHEN UTL_FILE.invalid_filehandle THEN UTL_FILE.fclose (l_filehandle); raise_application_error (-20062, 'Dump To File Error: Invalid File Handle'); WHEN UTL_FILE.write_error THEN UTL_FILE.fclose (l_filehandle); raise_application_error (-20063, 'Dump To File Error: Write Error'); WHEN OTHERS THEN UTL_FILE.fclose (l_filehandle); RAISE; END dump_to_file;

How to Use above function ?

To generate pipe (‘ | ’) delimated file called emp.txt in /app/oracle/my_dir for my query, I execute something like following

DECLARE
l_rows NUMBER := 0;
l_dir VARCHAR2 (100) := '/app/oracle/my_dir';
BEGIN
l_rows := dump_to_file ('select * FROM scott.emp ', '|', l_dir, 'emp.txt');
DBMS_OUTPUT.put_line ('Number of rows dumped to emp.txt : ' || l_rows);
END;

EDIT:
if you want to use DIRECTORY instead of UTL_FILE_DIR, just create one like following,

create or replace directory my_dir as '/app/oracle/my_dir';

then use above code with 'my_dir' instead of l_dir.

Tip#9 Some OS utilities (Unix)

NOHUP :

nohup (no hangups) is a UNIX command that allows you to execute a command in the background while you are logged into UNIX. The background process will continue to run until completion, even if you log off. For long running processes, this is a nice way to execute them. In my case, it allows me to log into a client server remotely, execute a process interactively with nohup, and then disconnect. Another benefit of nohup is that by default, a log of all activity is recorded in the current directory in the file nohup.out . e.g.

nohup sqlplus user/pwd @sqlscript &

Note : & at the end is for running the process in the background

SKILL:

Sometime I have a situations where I find that a process is consuming a lot of CPU and memory, but I don't want to kill it and I just wished if I could just 'pause' the process and that's where SKILL really helps me. e.g Process ID 1234 is taking too much resources and I need to somehow get other 'important' things done I will execute following command,

skill -STOP 1234

Once I have finished other 'important' things done I can free the process with following command

skill -CONT 1234

The nice feature is you can pass not just PID but also User, Terminal ID OR Command which makes it more powerful then I initially thought. e.g. You can 'pause' all RMAN commands on DB server with

skill -STOP rman

SNICE:

The command snice is similar to skill but Instead of stopping a process it makes its priority a lower one. e.g. for the same heavy process 1234 I could decrease the priority by using something like this,

snice +4 -p 1234

Note: +4 means we are increasing the Nice Value and effectively the higher the number, the lower the priority.

This utility is quite useful in reducing priorities.