Overview of Extents and when extents are allocated

Extents is made up of a number of contiguous data blocks. One or more extents make a segment. When in a segment there is no space i.e segment is full, then to allow more space oracle allocates a new extent in that segment.

When Extents Are Allocated
When you create a table or index then oracle automatically creates an initial extent with a number of specified data blocks (with how many data blocks will be discussed later on) under the table's/ index's data segment.

In this case though no rows are inserted into the table or index but yet extents are allocated and reserve for the table's or index's rows.

Now, if the data blocks of a segment's initial extent become full and more space is required to hold new data, then oracle automatically allocates an incremental extent for that segment.

In this way, subsequent increment extents are allocated whenever data blocks of a segment's extent full.

When Extents Are Deallocated
With the Segment Advisor oracle automatically advice on which schema objects or on which tablespaces has space available for reclamation based on the level of space fragmentation within the object.

To access Segment Advisor from enterprise manager home go to Related Links> Select Advisor Central>Select Segment Advisor.

You can reclaim space manually of LB table by

alter table "LB" enable row movement;
alter table "LB" shrink space;

In general the extents of a segment do not return to the tablespace until you drop the schema objects using DROP TABLE or DROP CLUSTER statement.

The exception is following,
•TRUNCATE...DROP STORAGE statement.
•ALTER TABLE table_name DEALLOCATE UNUSED;
•Periodically, Oracle deallocates one or more extents of a rollback segment if it has the OPTIMAL size specified.

Comments

Post a Comment

Popular posts from this blog

ORA-04062: timestamp of procedure has been changed

Problem Description In the database I have created one procedure named a as below. create or replace procedure a(a number) as begin insert into t1 values(1); commit; end; / Now after creating database link using remote database machine whenever I access this procedure "A" it executes successfully and I get value "1" in table t1. Like below in example where orastdby_m is the database link, shaik is the schema name and value 1 is the argument value though argument value is not used in the procedure. SQL> exec shaik.a@orastdby_m(1); PL/SQL procedure successfully completed. Now in the source database machine I changed the procedure as below. Though you can change anything like any literal; adding space or remove space. I changed value to be inserted from 1 to 2. create or replace procedure a(a number) as begin insert into t1 values(2); commit; end; / Now in the other database whenever I execute the procedure using database link it throws error ORA-04062. But subseq...
Read more

ORA-00939: too many arguments -when case arguments exceed 255

Problem Description In all Oracle 10.1g and oracle 10.2.0.1, 10.2.0.2, 10.2.0.3 whenever there is more than 255 arguments inside CASE statements then following error is returned. ORA-00939: too many arguments for function But surprisingly on Oracle 9.2 database the same code does not return any error. Cause of the Problem As we see the same code return error in 10g but it is well on 9.2. In fact this is problem of oracle 9.2 that it does not return any error. It should return error if there is more than 255 arguments inside CASE statements. In oracle 9.2 this is bug number 2909927 and this bug is fixed in oracle 10.1 and 10.2. That's why in 10.2 and 10.1 error returns if arguments exceed 255. Up to Oracle 10.2.0.3 and earlier version, the arguments of CASE statement is limit to 255 and it does not support exceed over 255 and hence error return if it exceed. However in Oracle 10.2.0.4 and later the arguments limit of CASE statement has increased to 32767. Solution of the Problem Opt...
Read more

ORA-31655: no data or metadata objects selected for job

Problem Description You are going to do data pump export operation in order to export objects based on filtering via EXCLUDE or INCLUDE parameter of expdp. In this example you wanted tables 'ACCOUNT_GROUP','ADDRESS','AREA_GROUP' and all the tables starting with word CV(like 'CV', 'CV_EXPERIENCE', 'CV_EDUCATION' etc) Your parameter file is like below. userid=smilebd/a directory=d dumpfile=b.dmp include =TABLE:"IN('ACCOUNT_GROUP','ADDRESS','AREA_GROUP')" include =TABLE:"LIKE 'CV%' " And you invoke expdp as expdp parfile=d:\parfile.txt from command line. But it fails with below message on my windows PC. C:\>expdp parfile=d:\parfile.txt Export: Release 10.1.0.4.2 - Production on Thursday, 29 January, 2009 14:53 Copyright (c) 2003, Oracle. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Produc tion With the Partitioning, OLAP and Data Mining ...
Read more