storage_clause

Purpose

Use the storage_clause to specify storage characteristics for any of the following schema objects:

• clusters

• indexes

• rollback segments

• materialized views

• materialized view logs

• tables

• tablespaces

• partitions

Storage parameters affect both how long it takes to access data stored in the database and how efficiently space in the database is used. For a discussion of the effects of these parameters, see Oracle8i Performance Guide and Reference.

When you create a tablespace, you can specify values for the storage parameters. These values serve as default values for segments allocated in the tablespace.

When you alter a tablespace, you can change the values of storage parameters. The new values serve as default values only for subsequently allocated segments (or subsequently created objects).

Note: The storage_clause is interpreted differently for locally managed tablespaces. At creation, Oracle ignores MAXEXTENTS and uses the remaining parameter values to calculate the initial size of the segment. For more information, see CREATE TABLESPACE.

When you create a cluster, index, rollback segment, snapshot, snapshot log, table, or partition, you can specify values for the storage parameters for the segments allocated to these objects. If you omit any storage parameter, Oracle uses the value of that parameter specified for the tablespace.

When you alter a cluster, index, rollback segment, snapshot, snapshot log, table, or partition, you can change the values of storage parameters. The new values affect only future extent allocations.

Additional Topics

• Prerequisites

• Syntax

• Keywords and Parameters

• Examples

Prerequisites

To change the value of a STORAGE parameter, you must have the privileges necessary to use the appropriate CREATE or ALTER statement.

Syntax

Keywords and Parameters

INITIAL

Specify in bytes the size of the object's first extent. Oracle allocates space for this extent when you create the schema object. Use K or M to specify this size in kilobytes or megabytes.

The default value is the size of 5 data blocks. The minimum value is the size of 2 data blocks for nonbitmapped segments or 3 data blocks for bitmapped segments, plus one data block for each free list group you specify. The maximum value depends on your operating system. Oracle rounds values up to the next multiple of the data block size for values less than 5 data blocks, and rounds up to the next multiple of 5 data blocks for values greater than 5 data blocks.

Restriction: You cannot specify INITIAL in an ALTER statement.

See Also: FREELIST GROUPS for information on freelist groups

NEXT

Specify in bytes the size of the next extent to be allocated to the object. Use K or M to specify the size in kilobytes or megabytes. The default value is the size of 5 data blocks. The minimum value is the size of 1 data block. The maximum value depends on your operating system. Oracle rounds values up to the next multiple of the data block size for values less than 5 data blocks. For values greater than 5 data blocks, Oracle rounds up to a value that minimizes fragmentation, as described in .

If you change the value of the NEXT parameter (that is, if you specify it in an ALTER statement), the next allocated extent will have the specified size, regardless of the size of the most recently allocated extent and the value of the PCTINCREASE parameter.

See Also: Oracle8i Concepts for information on how Oracle minimizes fragmentation

PCTINCREASE

Specify the percent by which the third and subsequent extents grow over the preceding extent. The default value is 50, meaning that each subsequent extent is 50% larger than the preceding extent. The minimum value is 0, meaning all extents after the first are the same size. The maximum value depends on your operating system.

Oracle rounds the calculated size of each new extent to the nearest multiple of the data block size.

If you change the value of the PCTINCREASE parameter (that is, if you specify it in an ALTER statement), Oracle calculates the size of the next extent using this new value and the size of the most recently allocated extent.

Suggestion: If you wish to keep all extents the same size, you can prevent SMON from coalescing extents by setting the value of PCTINCREASE to 0. In general, Oracle Corporation recommends a setting of 0 as a way to minimize fragmentation and avoid the possibility of very large temporary segments during processing.

Restriction: You cannot specify PCTINCREASE for rollback segments. Rollback segments always have a PCTINCREASE value of 0.

MINEXTENTS

Specify the total number of extents to allocate when the object is created. This parameter enables you to allocate a large amount of space when you create an object, even if the space available is not contiguous. The default and minimum value is 1, meaning that Oracle allocates only the initial extent, except for rollback segments, for which the default and minimum value is 2. The maximum value depends on your operating system.

If the MINEXTENTS value is greater than 1, then Oracle calculates the size of subsequent extents based on the values of the INITIAL, NEXT, and PCTINCREASE parameters.

Restriction: You cannot specify MINEXTENTS in an ALTER statement.

MAXEXTENTS

Specify the total number of extents, including the first, that Oracle can allocate for the object. The minimum value is 1 (except for rollback segments, which always have a minimum value of 2). The default value depends on your data block size.

UNLIMITED	Specify UNLIMITED if you want extents to be allocated automatically as needed. Oracle Corporation recommends this setting as a way to minimize fragmentation. However, do not use this clause for rollback segments. Rogue transactions containing inserts, updates, or deletes that continue for a long time will continue to create new extents until a disk is full.
	Caution: A rollback segment that you create without specifying the storage_clause has the same storage parameters as the tablespace that the rollback segment is created in. Thus, if you create the tablespace with MAXEXTENTS UNLIMITED, then the rollback segment will also have the same default.

FREELIST GROUPS

Specify the number of groups of free lists for the database object you are creating. The default and minimum value for this parameter is 1. Oracle uses the instance number of Oracle Parallel Server instances to map each instance to one free list group.

Each free list group uses one database block. Therefore:

• If you do not specify a large enough value for INITIAL to cover the minimum value plus one data block for each free list group, Oracle increases the value of INITIAL the necessary amount.

• If you are creating an object in a uniform locally managed tabledspace, and the extent size is not large enough to acommodate the number of freelist groups, the create operation will fail.

Restriction: You can specify the FREELIST GROUPS parameter only in CREATE TABLE, CREATE CLUSTER, CREATE MATERIALIZED VIEW, CREATE MATERIALIZED VIEW LOG, and CREATE INDEX statements.

See Also: Oracle8i Parallel Server Concepts

FREELISTS

For objects other than tablespaces, specify the number of free lists for each of the free list groups for the table, partition, cluster, or index. The default and minimum value for this parameter is 1, meaning that each free list group contains one free list. The maximum value of this parameter depends on the data block size. If you specify a FREELISTS value that is too large, Oracle returns an error indicating the maximum value.

Restriction: You can specify FREELISTS in the storage_clause of any statement except when creating or altering a tablespace or rollback segment.

OPTIMAL

The OPTIMAL keyword is relevant only to rollback segments. It specifies an optimal size in bytes for a rollback segment. Use K or M to specify this size in kilobytes or megabytes. Oracle tries to maintain this size for the rollback segment by dynamically deallocating extents when their data is no longer needed for active transactions. Oracle deallocates as many extents as possible without reducing the total size of the rollback segment below the OPTIMAL value.

NULL	Specify NULL for no optimal size for the rollback segment, meaning that Oracle never deallocates the rollback segment's extents. This is the default behavior.

The value of OPTIMAL cannot be less than the space initially allocated for the rollback segment specified by the MINEXTENTS, INITIAL, NEXT, and PCTINCREASE parameters. The maximum value depends on your operating system. Oracle rounds values up to the next multiple of the data block size.

BUFFER_POOL

The BUFFER_POOL clause lets you specify a default buffer pool (cache) for a schema object. All blocks for the object are stored in the specified cache. If a buffer pool is defined for a partitioned table or index, then the partitions inherit the buffer pool from the table or index definition, unless overridden by a partition-level definition.

Note: BUFFER_POOL is not a valid clause for creating or altering tablespaces or rollback segments.

KEEP	Specify KEEP to retain the schema object in memory to avoid I/O operations. KEEP takes precedence over any NOCACHE clause you specify for a table, cluster, materialized view, or materialized view log.
RECYCLE	Specify RECYCLE to eliminate blocks from memory as soon as they are no longer needed, thus preventing an object from taking up unnecessary cache space.
DEFAULT	Specify DEFAULT to indicate the default buffer pool. This is the default for objects not assigned to KEEP or RECYCLE.

Examples

Creating a table with storage attributes

The following statement creates a table and provides storage parameter values:

CREATE TABLE dept 
    (deptno     NUMBER(2), 
     dname      VARCHAR2(14), 
     loc        VARCHAR2(13) ) 
     STORAGE  ( INITIAL 100K  NEXT     50K 
                MINEXTENTS 1  MAXEXTENTS 50  PCTINCREASE 5); 

Oracle allocates space for the table based on the STORAGE parameter values as follows:

• The MINEXTENTS value is 1, so Oracle allocates 1 extent for the table upon creation.

• The INITIAL value is 100K, so the first extent's size is 100 kilobytes.

• If the table data grows to exceed the first extent, Oracle allocates a second extent. The NEXT value is 50K, so the second extent's size would be 50 kilobytes.

• If the table data subsequently grows to exceed the first two extents, Oracle allocates a third extent. The PCTINCREASE value is 5, so the calculated size of the third extent is 5% larger than the second extent, or 52.5 kilobytes. If the data block size is 2 kilobytes, Oracle rounds this value to 52 kilobytes.

  If the table data continues to grow, Oracle allocates more extents, each 5% larger than the previous one.

• The MAXEXTENTS value is 50, so Oracle can allocate as many as 50 extents for the table.

Creating a rollback segment with storage attributes

The following statement creates a rollback segment and provides storage parameter values:

CREATE ROLLBACK SEGMENT rsone 
    STORAGE ( INITIAL  10K  NEXT 10K 
              MINEXTENTS 2  MAXEXTENTS 25 
               OPTIMAL 50K ); 

Oracle allocates space for the rollback segment based on the STORAGE parameter values as follows:

• The MINEXTENTS value is 2, so Oracle allocates 2 extents for the rollback segment upon creation.

• The INITIAL value is 10K, so the first extent's size is 10 kilobytes.

• The NEXT value is 10K, so the second extent's size is 10 kilobytes.

• If the rollback data exceeds the first two extents, Oracle allocates a third extent. The PCTINCREASE value for rollback segments is always 0, so the third and subsequent extents are the same size as the second extent, 10 kilobytes.

• The MAXEXTENTS value is 25, so Oracle can allocate as many as 25 extents for the rollback segment.

• The OPTIMAL value is 50K, so Oracle deallocates extents if the rollback segment exceeds 50 kilobytes. Oracle deallocates only extents that contain data for transactions that are no longer active.