ORA-30926: unable to get a stable set of rows in the source tables

Quite often, you may see this warning message when you use MERGE statement.

If you get this, then your Merge statement is trying to update multiple rows in "WHEN MATCHED" Clause.

Multiple updates to the same row in destination table is NOT allowed.

Multiple insert is allowed though.

To know for sure, If you comment out your "WHEN MATCHED" section, you will no longer get this warning.

Oracle: How to size UNDO Tablespace for Automatic Management

Here are some queries to figure out how to manage your UNDO Tablespace from 10g onwards. Oracle strongly recommends using UNDO_MANAGEMENT=AUTO.

If your UNDO tablespace is fixed size, some of the queries may help you. All of them are from Oracle Metalink.Replace UNDOTBS1 with name of your UNDO tablespace.

--UNDO Tablespace size
Select SUM(BYTES)/1024/1024 as "MB"
from dba_data_files
where tablespace_name = 'UNDOTBS1';

--Free space available
Select SUM(BYTES)/1024/1024 as "MB"
from dba_FREE_SPACE
where tablespace_name = 'UNDOTBS1';

--Active/Expired segments
SELECT DISTINCT STATUS, SUM(BYTES)/1024/1024 as "MB", COUNT(*)
FROM DBA_UNDO_EXTENTS GROUP BY STATUS;

--To calculate space required for your UNDO to grow
SELECT ((UR * (UPS * DBS)) + (DBS * 24))/1024/1024 AS "MB"
FROM (SELECT value AS UR FROM v$parameter WHERE name = 'undo_retention'),
(SELECT (SUM(undoblks)/SUM(((end_time - begin_time)*86400))) AS UPS FROM v$undostat),
(select block_size as DBS from dba_tablespaces where tablespace_name=
(select value from v$parameter where name = 'undo_tablespace'));

UR=UNDO_RETENTION (in seconds)
UPS=Number of UNDO data blocks generated per second
DBS=DB_BLOCK_SIZE (in bytes)

--SQL to find the active transaction that is consuming the UNDO
-- Replace UNDOTBS02 with your UNDO tablespace name

SELECT TO_CHAR (s.SID) || ',' || TO_CHAR (s.serial#) sid_serial,
NVL (s.username, 'None') orauser, s.machine,
--s.osuser,s.terminal,s.module,s.schemaname,
s.state,s.program, r.NAME undoseg,
t.used_ublk * TO_NUMBER (x.VALUE) / 1024 Undo_Size_KB,
t.status Transaction_Status,t.start_time,
t1.tablespace_name,sq.sql_text
FROM SYS.v_$rollname r, SYS.v_$session s, SYS.v_$transaction t, SYS.v_$parameter x, dba_rollback_segs t1, sys.v$sql sq
WHERE s.taddr = t.addr
and s.sql_id = sq.sql_id(+)
AND r.usn = t.xidusn(+)
AND x.NAME = 'db_block_size'
AND t1.segment_id = r.usn
AND t1.tablespace_name = 'UNDOTBS02'

Windows Explorer: Display security tab in Windows XP Professional

Click Start and launch Control Panel
Locate Folder Options
Click on the tab View, Scroll all the way down
Uncheck the option "Use simple file sharing (Recommended)"
Click OK. Now you should see Security tab on each folder.