Question: I understand that there are
Oracle storage features that cause the standard table and index
storage options to be ignored. Under what conditions are the
storage parameters ignored?
Answer (by Chris Aiken): Oracle uses
various parameters to define how data blocks are handled within the
database. These storage parameters are all explained below,
giving a brief overview of what they do and how they operate, as
well as when the parameter is handled differently by Oracle.
It is important to note that with
Segment Space Management enabled, and in some cases with
Locally Managed Tablespaces, certain parameters are disregarded.
These storage parameters are PCTFREE, PCTUSED, MINIMUM EXTENT,
MAXEXTENTS, INITIAL, NEXT, PCTINCREASE, FREELIST GROUPS and
When PCTFREE is ignored
PCTFREE is a
block storage parameter which specifies the amount of free space
Oracle should leave in a database block for future updates. Oracle
will continue adding new rows to the block until the remaining space
is equal to the PCTFREE parameter. The default value for the PCTFREE
parameter is PCTFREE=10. Any value between 0 and 99 may be used as
long as the sum of PCTFREE and PCTUSED is not greater than 100.
When talking about PCTFREE, it must be
noted that indexes are a special case. PCTFREE may be specified for
an index, but only when the index is initially created.
The PCTFREE parameter is only used
when an index is created. After index creation, new index
block entries are made and PCTFREE is ignored.
PCTUSED is ignored
PCTUSED is a storage parameter in Oracle which specifies when a
database block is empty enough for Oracle to add it to the free
list. When the percentage of used space in a block is greater than
the PCTUSED parameter, Oracle will not add new rows to the block.
The default settings for all Oracle tables are PCTUSED=40.
When MINEXTENTS is ignored
In a dictionary-managed tablespace,
MINEXTENTS parameter defines the total number of extents to be
allocated when a segment is created. MINIMUM EXTENT is used for
locally managed tablespaces, but only to compute the initial amount
of space to be allocated.
When MAXEXTENTS is ignored
MAXEXTENTS, for dictionary-managed
tablespaces, is the maximum number of extents that can ever be
allocated for the segment.
Locally Managed Tablespaces ignores MAXEXTENTS. Because Oracle manages the extents
under a locally managed tablespace, MAXEXTENTS will be ignored.
When INITIAL is ignored
The INITIAL parameter defines the size
of the first extent of an object. The default value for this is the
size of 5 data blocks. In a dictionary-managed tablespace, if the
MINIMUM EXTENT parameter was set at the time the tablespace was
created, then Oracle rounds the value of INITIAL up to the specified
MINIMUM EXTENT size, but only if necessary.
In a locally managed tablespace,
Oracle uses the value of INITIAL together with the size of extents
specified for the tablespace to calculate the size of the first
extent of the object. It must also be noted that INITIAL cannot be
set by using an ALTER TABLE statement.
When NEXT is ignored
NEXT represents, for
dictionary-managed tablespaces, the size of the next incremental
extent to be allocated for a segment. The default value for NEXT is
always the size of 5 data blocks. The default value for the second
extent is always equal to the original setting for NEXT. For each
new extent, the NEXT is set to the previous size of NEXT multiplied
by (1 + PCTINCREASE/100). If you manually set the value of the NEXT
parameter, the next allocated extent will have the specified size
regardless of the size of the most recently allocated extent and the
NEXT is ignored with LMT (locally managed
tablespaces) because the database automatically manages extents.
When PCTINCREASE is ignored
PCTINCREASE is the storage parameter,
for dictionary-managed tablespaces, which specifies the percentage
by which each incremental extent grows over the previous extent
allocated for a segment.
Just like the
NEXT parameter, PCTINCREASE is not valid for locally managed tablespaces
so it is ignored.
When FREELISTS and FREELIST_GROUPS are ignored
The FREELIST GROUPS parameter
specifies the number of
groups of free lists for the database object you are creating.
The default, and minimum, is 1 which causes all free lists of a
segment to be available to all instances.
The FREELISTS parameter specifies the
number of free lists available for each of the free list groups for
the object. This parameter is not used for tablespaces. The default
and minimum value for FREELISTS is 1, and the maximum value depends
on block size. Attempting to specify FREELISTS to a value greater
than allowed will generate an error message informing you of the
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