LOCKING IN ORACLE

Two types of Locking


In Optimistic locking the row will not be locked until the actual update is performed. In order to prevent the lost update issue a version number column will be used.

In Pessimistic locking the row will be locked at the time when the data is selected from the database. This will prevent other users who want to update the same record. This should not prevent the readers but this behaviour varies from database to database.


Optimistic : One who usually expects a favorable outcome
Pessimistic : Expecting the worst possible outcome (negative)


The Oracle database uses Optimistic locking by default. Any command that begins with UPDATE SET that is not preceded by a SELECT FOR UPDATE is an example of optimistic locking.

SELECT ..... FOR UPDATE ,
SET TRANSACTION ISOLATION LEVEL,
LOCK TABLE PROJECT_MASTER IN ROW EXCLUSIVE MODE
are examples of Pessimistic locking

Pessimistic has two major problems

The Lockout - An application user selects a record for update, and then leaves for lunch without finishing or aborting the transaction. All other users that need to update that record are forced to wait until the user returns and completes the transaction, or until the DBA kills the offending transaction

The Deadlock - Users A and B are both updating the database at the same time. User A locks a record and then attempt to acquire a lock held by user B - who is waiting to obtain a lock held by user A. Both transactions go into an infinite wait state - the so-called deadly embrace or deadlock.

Some Good Queries

To get size of a table

select segment_name table_name,sum(bytes)/(1024*1024) table_size_meg
from user_extents where segment_type='TABLE'
and segment_name = 'EMP_MAST' group by segment_name

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To Select only unlocked rows

select * from emp_MAST for update skip locked;

CHECK THIS USING TWO SQL PLUS AND SESSION BROWSER IN TOAD

UPDATE EMP_MAST SET ENAME='p' WHERE EMPNO=7369 ; (IN FIRST SQLPLUS DONT EXECUTE COMMIT )

select * from emp_MAST for update skip locked; (IN SECOND SQLPLUS)
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To get numbers of records in all tables in a Schema

select table_name,to_number(extractvalue(xmltype(
dbms_xmlgen.getxml('select count(*) c from '||table_name)
),'/ROWSET/ROW/C')) count from user_tables order by 1

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To generate a CSV output of a Table

select regexp_replace(column_value,' *<[^>]*>[^>]*>',';')
from table(xmlsequence(cursor(select * from EMP_MAST)));

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To get Version and login naem

select OLAPSYS.version,sys.LOGIN_USER from dual
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select APEX_UTIL.get_since(sysdate-10) /* get how many days ago */,APEX_UTIL.url_encode('http://www.oracle4u.com')   from dual;

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select round(12.55555E78,2) from dual ---- For numeric overflow  round function will not work

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How to do pattern search in a subquery using LIKE


with dt as( select distinct  BOTTLETYPE  from  DRINKS WHERE BOTTLETYPE is not null )
select * from  tabled,dt where name like ''||dt.BOTTLETYPE||'%' order by 2

OR

select * from  tabled df,(select distinct BOTTLETYPE from DRINKS WHERE BOTTLETYPE is not null) gh   where name like ''||gh.BOTTLETYPE||'%' order by 2


OR

SELECT  * FROM  tabled df WHERE EXISTS (SELECT distinct BOTTLETYPE  FROM DRINKS gh WHERE BOTTLETYPE is not null and df.name LIKE ''||gh.BOTTLETYPE||'%') ;
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Check for hidden database user
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Run OS Commands via PLSQL
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Run OS Commands via DBMS_SCHEDULER
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Run OS Commands via Create Table
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