storage_clause

storage_clause

Purpose

The storage_clause lets you specify how Oracle should store a database object. Storage parameters affect both how long it takes to access data stored in the database and how efficiently space in the database is used.

See Also: Oracle9i Database Performance Tuning Guide and Reference for a discussion of the effects of the storage parameters

When you create a cluster, index, rollback segment, materialized view, materialized view 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, then Oracle uses the value of that parameter specified for the tablespace in which the object resides.

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

The storage_clause is part of the physical_attributes_clause, so you can specify this clause in any of the statements where you can specify the physical attributes clause (see physical_attributes_clause on page 7-52).

In addition, you can specify the storage_clause in the following statements:

■CREATE CLUSTER and ALTER CLUSTER: to set or change the storage characteristics of the cluster and all tables in the cluster (see CREATE CLUSTER on page 13-2 and ALTER CLUSTER on page 9-6).

■CREATE INDEX and ALTER INDEX: to set or change the storage characteristics of an index or index partition (see CREATE INDEX on page 13-65 and ALTER INDEX on page 9-62).

■CREATE MATERIALIZED VIEW and ALTER MATERIALIZED VIEW: to set or change the storage characteristics of a materialized view, one of its partitions, or the index Oracle generates to maintain the materialized view (see CREATE MATERIALIZED VIEW on page 14-5 and ALTER MATERIALIZED VIEW on page 9-90).

■CREATE MATERIALIZED VIEW LOG and ALTER MATERIALIZED VIEW LOG: to set or change the storage characteristics of the materialized view log (see

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CREATE MATERIALIZED VIEW LOG on page 14-34 and ALTER

MATERIALIZED VIEW LOG on page 9-110).

■CREATE ROLLBACK SEGMENT and ALTER ROLLBACK SEGMENT: to set or change the storage attributes of a rollback segment (see CREATE ROLLBACK SEGMENT on page 14-82 and ALTER ROLLBACK SEGMENT on page 9-136).

■CREATE TABLE and ALTER TABLE: to set the storage characteristics of a LOB data segment of the table or one of its partitions or subpartitions (see CREATE TABLE on page 15-7 and ALTER TABLE on page 11-2).

■CREATE TABLESPACE and ALTER TABLESPACE: to set or change the default storage characteristics for objects created in the tablespace (see CREATE TABLESPACE on page 15-80 and ALTER TABLESPACE on page 11-102).

■constraints: to specify storage for the index (and its partitions, if it is a partitioned index) used to enforce the constraint (see constraints on page 7-5).

Prerequisites

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

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Semantics

This section describes the parameters of the storage_clause. For additional information, refer to the SQL statement in which you set or reset these parameters for a particular database object.

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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 on page 15-80.

See Also: "Specifying Table Storage Attributes: Example" on page 7-64 and "Specifying Rollback Segment Storage Attributes: Example" on page 7-65

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. In tablespaces with manual segmentspace management, the minimum value is the size of 2 data blocks plus one data block for each free list group you specify. In tablespaces with automatic segmentspace management, the minimum value is 5 data blocks. The maximum value depends on your operating system.

In dictionary-managed tablespaces, if MINIMUM EXTENT was specified for the tablespace when it was created, then Oracle rounds the value of INITIAL up to the specified MINIMUM EXTENT size if necessary. If MINIMUM EXTENT was not specified, then Oracle rounds the INITIAL extent size for segments created in that tablespace up to the minimum value (see preceding paragraph), or to multiples of 5 blocks if the requested size is greater than 5 blocks.

In locally managed tablespaces, Oracle uses the value of INITIAL in conjunction with the size of extents specified for the tablespace to determine the object’s first extent. For example, in a uniform locally managed tablespace with 5M extents, if you specify an INITIAL value of 1M, then Oracle creates five 1M extents.

Restriction on INITIAL You cannot specify INITIAL in an ALTER statement.

See Also: FREELIST GROUPS on page 7-62 for information on freelist groups

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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

Oracle9i Database Administrator’s Guide.

If you change the value of the NEXT parameter (that is, if you specify it in an ALTER statement), then 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: Oracle9i Database Administrator’s Guide 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), then 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 on PCTINCREASE You cannot specify PCTINCREASE for rollback segments. Rollback segments always have a PCTINCREASE value of 0.

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MINEXTENTS

Specify the total number of extents to allocate when the object is created. This parameter lets you 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 storage parameters.

When changing the value of MINEXTENTS (that is, in an ALTER statement), you can reduce the value from its current value, but you cannot increase it. Resetting MINEXTENTS to a smaller value might be useful, for example, before a TRUNCATE ...

DROP STORAGE statement, if you want to ensure that the segment will maintain a minimum number of extents after the TRUNCATE operation.

Restriction on MINEXTENTS You cannot change the value of MINEXTENTS for an object that resides in a locally managed tablespace.

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.

Restriction on MAXEXTENTS You cannot change the value of MAXEXTENTS for an object that resides in a locally managed tablespace.

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.

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Caution: A rollback segment that you create without specifying the storage_clause has the same storage parameters as the tablespace in which the rollback segment is created. 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 Real Application Clusters 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, then Oracle increases the value of INITIAL the necessary amount.

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

Note: Oracle ignores a setting of FREELIST GROUPS if the tablespace in which the object resides is in automatic segmentspace management mode. If you are running your database in this mode, please refer to the FREEPOOLS parameter of the LOB_ storage_clause on page 15-36.

Restriction on FREELIST GROUPS 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: Oracle9i Real Application Clusters Administration

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

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specify a FREELISTS value that is too large, then Oracle returns an error indicating the maximum value.

Note: Oracle ignores a setting of FREELISTS if the tablespace in which the object resides is in automatic segment-space management mode. If you are running your database in this mode, please refer to the FREEPOOLS parameter of the LOB_storage_ clause on page 15-36.

Restriction on FREELISTS 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.

The value of OPTIMAL cannot be less than the space initially allocated 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.

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.

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 you define a buffer pool 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.

■For an index-organized table, you can specify a buffer pool separately for the index segment and the overflow segment.

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Restrictions on BUFFER_POOL

■You cannot specify this clause for a cluster table. However, you can specify it for a cluster.

■You cannot specify this clause for a tablespace or for a rollback segment.

KEEP Specify KEEP to put blocks from the segment into the KEEP buffer pool. Maintaining an appropriately sized KEEP buffer pool lets Oracle 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 put blocks from the segment into the RECYCLE pool. An appropriately sized RECYCLE pool reduces the number of objects whose default pool is the RECYCLE pool 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.

See Also: Oracle9i Database Performance Tuning Guide and Reference for more information about using multiple buffer pools

Examples

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, then Oracle allocates a second extent. The NEXT value is 50K, so the second extent’s size would be 50 kilobytes.

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■If the table data subsequently grows to exceed the first two extents, then 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, then Oracle rounds this value to 52 kilobytes.

If the table data continues to grow, then 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.

Specifying Rollback Segment Storage Attributes: Example The following statement creates a rollback segment and provides storage parameter values:

CREATE ROLLBACK SEGMENT rs_store

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, then 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.

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