|Lesson 6||Setting the extent size for an object |
|Objective||Use the storage clause to set the extent size for a table in Oracle |
Setting Oracle extent size
Use the storage clause to set the extent size for a table in Oracle.
In the previous lesson, you learned about the different attributes that determine how Oracle uses extents within a segment.
There are a set of SQL keywords you can use in the DDL statements that create and alter database objects and thus control how extents are used.
How do I use the storage clause to set the extent size for a table in Oracle?
In Oracle Database Management System, you can use the STORAGE clause to define the storage characteristics of a table, including the extent size. The STORAGE clause allows you to set parameters such as INITIAL, NEXT, PCTINCREASE, MINEXTENTS, and MAXEXTENTS, which control how the table's extents are allocated and resized as the table grows.
To set the extent size for a table using the STORAGE clause, follow these steps:
1. Define the INITIAL and NEXT parameters:
The INITIAL parameter specifies the size of the first extent allocated to the table, while the NEXT parameter determines the size of subsequent extents. Both parameters can be expressed in bytes or using K, M, or G for kilobytes, megabytes, or gigabytes, respectively. For example, to set the initial extent size to 1 MB and the next extent size to 2 MB, you would use the following syntax:
INITIAL 1M NEXT 2M
2. Set the PCTINCREASE parameter:
The PCTINCREASE parameter controls the growth rate of the extent size for the table. It is expressed as a percentage of the size of the previous extent. A PCTINCREASE value of 0 indicates that each subsequent extent will have a fixed size equal to the NEXT parameter value. To set the PCTINCREASE value to 50%, for example, you would use the following syntax:
3. Specify the MINEXTENTS and MAXEXTENTS parameters:
The MINEXTENTS parameter defines the minimum number of extents that the table should have initially.
The MAXEXTENTS parameter determines the maximum number of extents the table can have.
To set the MINEXTENTS value to 1 and the MAXEXTENTS value to 100, for example, you would use the following syntax:
MINEXTENTS 1 MAXEXTENTS 100
4. Incorporate the STORAGE clause into the CREATE TABLE statement:
By using the STORAGE clause with the appropriate parameters, you can set the extent size for a table in an Oracle database, as well as control the growth pattern of the extents as the table expands. This allows you to optimize the storage allocation and performance of your database objects.
To create a table with the specified storage parameters, include the STORAGE clause in the CREATE TABLE statement, as shown in the following example:
CREATE TABLE sample_table (
Syntax for extents
All of these storage parameters can be used with a
CREATE TABLE, CREATE INDEX, ALTER TABLE
| This keyword is used with an integer and either K or M, to specify either Kilobytes or Megabytes. The |
INITIAL size is the size of the initial extent allocated when the object is created.
| This keyword uses the same integer notation as |
INITIAL and describes the size of the first extent allocated after the initial extent is full.
| This keyword specifies the percentage each subsequent extent should grow by. If the |
NEXT size is 20 K and the
PCTINCREASE is 50 (its default value), the third extent allocated would be 30 K (20 K * 150%), the fourth extent allocated would be 45 K (30 K * 150%), and so on.
| This keyword is followed by an integer that indicates the minimum number of extents that are initially allocated. If the value for this parameter is greater than 1, Oracle will allocate the additional extents when the object is created, based on the |
INITIAL, NEXT, and
| This parameter limits the overall size of the database object. |
View the code below.
CREATE TABLE COINS_IN_LOT (auction_id NUMBER, lot_id NUMBER, coin_id NUMBER)
STORAGE (INITIAL 100K NEXT 50K MINEXTENTS 1 MAXEXTENTS 5 PCTINCREASE 50 );
If you use the storage parameters when creating a table, the syntax will look like this: This SQL statement causes the table to begin with a single extent of 100 K allocated. Subsequent extents would contain 50K, 75 K (50 K *150%), and so on. The table could not contain more than five extents, which would limit its overall size to 506.25 K.
You do not have to specify storage parameters for any object.
Implications of the storage Clause
The amount of space used by a segment is determined by its storage parameters
These parameters are determined by the database at segment-creation time; if no specific storage parameters are
given in the
- create table,
- create index, or
- create cluster
command, then the database will use the default storage parameters for the tablespace in which the segment is to be stored.
You can assign default tablespaces to users, and assign space quotas within those tablespaces, via the create user, alter user, and grant commands.
When you create a table, index, or other segment, you can use the default values for a locally managed tablespace (the recommended option) or specify a storage clause as part of the create command. You can also specify a tablespace clause, enabling you to direct Oracle to store the data in a particular tablespace. For example, a create table command in a dictionary-managed
tablespace may include the following clauses:
storage (initial 1M next 1M pctincrease 0
minextents 1 maxextents 200)
If no storage clause is specified, the default storage parameters of the tablespace will be used. For a locally managed USERS tablespace, the create table command would only need to include
The storage parameters
specify the initial extent size, the next extent size, the pctincrease (a factor by which each successive extent will geometrically grow),
- the maxextents (maximum number of extents), and
- the minextents (minimum number of extents).
After the segment has been created, the initial and minextents values cannot be altered unless you perform a reorganization of the object.
The default values for the storage parameters for each tablespace are available in the DBA_TABLESPACES and USER_TABLESPACES views.
The next lesson shows how to set default storage attributes for all the objects in a tablespace.