Managing Storage Parameters

Identifying the Storage Parameters

Storage parameters determine space allocation for objects when their segments are created in a tablespace. Not all storage parameters can be specified for every type of database object, and not all storage parameters can be specified in both the CREATE and ALTER statements. Storage parameters for objects in locally managed tablespaces are supported mainly for backward compatibility.

The Oracle Database server manages extents for locally managed tablespaces. If you specified the UNIFORM clause when the tablespace was created, then the database creates all extents of a uniform size that you specified (or a default size) for any objects created in the tablespace. If you specified the AUTOALLOCATE clause, then the database determines the extent sizing policy for the tablespace. So, for example, if you specific the INITIAL clause when you create an object in a locally managed tablespace you are telling the database to preallocate at least that much space. The database then determines the appropriate number of extents needed to allocate that much space.

Table 17-1 contains a brief description of each storage parameter. For a complete description of these parameters, including their default, minimum, and maximum settings, see the Oracle Database SQL Language Reference.

Specifying Storage Parameters at Object Creation

At object creation, you can specify storage parameters for each individual schema object. These parameter settings override any default storage settings. Use the STORAGE clause of the CREATE or ALTER statement for specifying storage parameters for the individual object.

Setting Storage Parameters for Clusters

Use the STORAGE clause of the CREATE TABLE or ALTER TABLE statement to set the storage parameters for non-clustered tables.

In contrast, set the storage parameters for the data segments of a cluster using the STORAGE clause of the CREATE CLUSTER or ALTER CLUSTER statement, rather than the individual CREATE or ALTER statements that put tables into the cluster. Storage parameters specified when creating or altering a clustered table are ignored. The storage parameters set for the cluster override the table storage parameters.

Setting Storage Parameters for Partitioned Tables

With partitioned tables, you can set default storage parameters at the table level. When creating a new partition of the table, the default storage parameters are inherited from the table level (unless you specify them for the individual partition). If no storage parameters are specified at the table level, then they are inherited from the tablespace.

Setting Storage Parameters for Index Segments

Storage parameters for an index segment created for a table index can be set using the STORAGE clause of the CREATE INDEX or ALTER INDEX statement.

Storage parameters of an index segment created for the index used to enforce a primary key or unique key constraint can be set in either of the following ways:

• In the ENABLE ... USING INDEX clause of the CREATE TABLE or ALTER TABLE statement

• In the STORAGE clause of the ALTER INDEX statement

Setting Storage Parameters for LOBs, Varrays, and Nested Tables

A table or materialized view can contain LOB, varray, or nested table column types. These entities can be stored in their own segments. LOBs and varrays are stored in LOB segments, while a nested table is stored in a storage table. You can specify a STORAGE clause for these segments that will override storage parameters specified at the table level.

Changing Values of Storage Parameters

You can alter default storage parameters for tablespaces and specific storage parameters for individual objects if you so choose. Default storage parameters can be reset for a tablespace; however, changes affect only new objects created in the tablespace or new extents allocated for a segment. As discussed previously, you cannot specify default storage parameters for locally managed tablespaces, so this discussion does not apply.

The INITIAL and MINEXTENTS storage parameters cannot be altered for an existing table, cluster, index. If only NEXT is altered for a segment, the next incremental extent is the size of the new NEXT, and subsequent extents can grow by PCTINCREASE as usual.

If both NEXT and PCTINCREASE are altered for a segment, the next extent is the new value of NEXT, and from that point forward, NEXT is calculated using PCTINCREASE as usual.

Understanding Precedence in Storage Parameters

Starting with default values, the storage parameters in effect for a database object at a given time are determined by the following, listed in order of precedence (where higher numbers take precedence over lower numbers):

1. Oracle Database default values

2. DEFAULT STORAGE clause of CREATE TABLESPACE statement

3. DEFAULT STORAGE clause of ALTER TABLESPACE statement

4. STORAGE clause of CREATE [TABLE | CLUSTER | MATERIALIZED VIEW | MATERIALIZED VIEW LOG | INDEX] statement

5. STORAGE clause of ALTER [TABLE | CLUSTER | MATERIALIZED VIEW | MATERIALIZED VIEW LOG | INDEX] statement

Any storage parameter specified at the object level overrides the corresponding option set at the tablespace level. When storage parameters are not explicitly set at the object level, they default to those at the tablespace level. When storage parameters are not set at the tablespace level, Oracle Database system defaults apply. If storage parameters are altered, the new options apply only to the extents not yet allocated.