|Oracle® Database Concepts
10g Release 1 (10.1)
Part Number B10743-01
Oracle allocates logical database space for all data in a database. The units of database space allocation are data blocks, extents, and segments. Figure 2-1 shows the relationships among these data structures:
Figure 2-1 The Relationships Among Segments, Extents, and Data Blocks
At the finest level of granularity, Oracle stores data in data blocks (also called logical blocks, Oracle blocks, or pages). One data block corresponds to a specific number of bytes of physical database space on disk.
The level of logical database storage greater than an extent is called a segment. A segment is a set of extents, each of which has been allocated for a specific data structure and all of which are stored in the same tablespace. For example, each table's data is stored in its own data segment, while each index's data is stored in its own index segment. If the table or index is partitioned, each partition is stored in its own segment.
Oracle allocates space for segments in units of one extent. When the existing extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.
A segment and all its extents are stored in one tablespace. Within a tablespace, a segment can include extents from more than one file; that is, the segment can span datafiles. However, each extent can contain data from only one datafile.
Although you can allocate additional extents, the blocks themselves are allocated separately. If you allocate an extent to a specific instance, the blocks are immediately allocated to the free list. However, if the extent is not allocated to a specific instance, then the blocks themselves are allocated only when the high water mark moves. The high water mark is the boundary between used and unused space in a segment.
Note:Oracle recommends that you manage free space automatically. See "Free Space Management".
Oracle manages the storage space in the datafiles of a database in units called data blocks. A data block is the smallest unit of data used by a database. In contrast, at the physical, operating system level, all data is stored in bytes. Each operating system has a block size. Oracle requests data in multiples of Oracle data blocks, not operating system blocks.
The standard block size is specified by the
DB_BLOCK_SIZE initialization parameter . In addition, you can specify of up to five nonstandard block sizes. The data block sizes should be a multiple of the operating system's block size within the maximum limit to avoid unnecessary I/O. Oracle data blocks are the smallest units of storage that Oracle can use or allocate.
The Oracle data block format is similar regardless of whether the data block contains table, index, or clustered data. Figure 2-2 illustrates the format of a data block.
Figure 2-2 Data Block Format
After the space has been allocated in the row directory of a data block's overhead, this space is not reclaimed when the row is deleted. Therefore, a block that is currently empty but had up to 50 rows at one time continues to have 100 bytes allocated in the header for the row directory. Oracle reuses this space only when new rows are inserted in the block.
The data block header, table directory, and row directory are referred to collectively as overhead. Some block overhead is fixed in size; the total block overhead size is variable. On average, the fixed and variable portions of data block overhead total 84 to 107 bytes.
See Also:"Row Chaining and Migrating "
In data blocks allocated for the data segment of a table or cluster, or for the index segment of an index, free space can also hold transaction entries. A transaction entry is required in a block for each
UPDATE statement accessing one or more rows in the block. The space required for transaction entries is operating system dependent; however, transaction entries in most operating systems require approximately 23 bytes.
Free space can be managed automatically or manually.
Free space can be managed automatically inside database segments. The in-segment free/used space is tracked using bitmaps, as opposed to free lists. Automatic segment-space management offers the following benefits:
Ease of use
Better space utilization, especially for the objects with highly varying row sizes
Better run-time adjustment to variations in concurrent access
Better multi-instance behavior in terms of performance/space utilization
You specify automatic segment-space management when you create a locally managed tablespace. The specification then applies to all segments subsequently created in this tablespace.
Two types of statements can increase the free space of one or more data blocks:
DELETE statements, and
UPDATE statements that update existing values to smaller values. The released space from these types of statements is available for subsequent
INSERT statements under the following conditions:
INSERT statement is in the same transaction and subsequent to the statement that frees space, then the
INSERT statement can use the space made available.
INSERT statement is in a separate transaction from the statement that frees space (perhaps being run by another user), then the
INSERT statement can use the space made available only after the other transaction commits and only if the space is needed.
Released space may or may not be contiguous with the main area of free space in a data block. Oracle coalesces the free space of a data block only when (1) an
UPDATE statement attempts to use a block that contains enough free space to contain a new row piece, and (2) the free space is fragmented so the row piece cannot be inserted in a contiguous section of the block. Oracle does this compression only in such situations, because otherwise the performance of a database system decreases due to the continuous compression of the free space in data blocks.
In two circumstances, the data for a row in a table may be too large to fit into a single data block. In the first case, the row is too large to fit into one data block when it is first inserted. In this case, Oracle stores the data for the row in a chain of data blocks (one or more) reserved for that segment. Row chaining most often occurs with large rows, such as rows that contain a column of datatype
RAW. Row chaining in these cases is unavoidable.
However, in the second case, a row that originally fit into one data block is updated so that the overall row length increases, and the block's free space is already completely filled. In this case, Oracle migrates the data for the entire row to a new data block, assuming the entire row can fit in a new block. Oracle preserves the original row piece of a migrated row to point to the new block containing the migrated row. The rowid of a migrated row does not change.
When a row is chained or migrated, I/O performance associated with this row decreases because Oracle must scan more than one data block to retrieve the information for the row.
For manually managed tablespaces, two space management parameters,
PCTUSED, enable you to control the use of free space for inserts and updates to the rows in all the data blocks of a particular segment. Specify these parameters when you create or alter a table or cluster (which has its own data segment). You can also specify the storage parameter
PCTFREE when creating or altering an index (which has its own index segment).
Note:This discussion does not apply to LOB datatypes (
See "Overview of LOB Datatypes" for information.
PCTFREE parameter sets the minimum percentage of a data block to be reserved as free space for possible updates to rows that already exist in that block. For example, assume that you specify the following parameter within a
This states that 20% of each data block in this table's data segment be kept free and available for possible updates to the existing rows already within each block. New rows can be added to the row data area, and corresponding information can be added to the variable portions of the overhead area, until the row data and overhead total 80% of the total block size. Figure 2-3 illustrates
Figure 2-3 PCTFREE
PCTUSED parameter sets the minimum percentage of a block that can be used for row data plus overhead before new rows are added to the block. After a data block is filled to the limit determined by
PCTFREE, Oracle considers the block unavailable for the insertion of new rows until the percentage of that block falls beneath the parameter
PCTUSED. Until this value is achieved, Oracle uses the free space of the data block only for updates to rows already contained in the data block. For example, assume that you specify the following parameter in a
In this case, a data block used for this table's data segment is considered unavailable for the insertion of any new rows until the amount of used space in the block falls to 39% or less (assuming that the block's used space has previously reached
PCTFREE). Figure 2-4 illustrates this.
Figure 2-4 PCTUSED
PCTUSED work together to optimize the use of space in the data blocks of the extents within a data segment. Figure 2-5 illustrates the interaction of these two parameters.
Figure 2-5 Maintaining the Free Space of Data Blocks with PCTFREE and PCTUSED
In a newly allocated data block, the space available for inserts is the block size minus the sum of the block overhead and free space (
PCTFREE). Updates to existing data can use any available space in the block. Therefore, updates can reduce the available space of a block to less than
PCTFREE, the space reserved for updates but not accessible to inserts.
For each data and index segment, Oracle maintains one or more free lists—lists of data blocks that have been allocated for that segment's extents and have free space greater than
PCTFREE. These blocks are available for inserts. When you issue an
INSERT statement, Oracle checks a free list of the table for the first available data block and uses it if possible. If the free space in that block is not large enough to accommodate the
INSERT statement, and the block is at least
PCTUSED, then Oracle takes the block off the free list. Multiple free lists for each segment can reduce contention for free lists when concurrent inserts take place.
After you issue a
UPDATE statement, Oracle processes the statement and checks to see if the space being used in the block is now less than
PCTUSED. If it is, then the block goes to the beginning of the transaction free list, and it is the first of the available blocks to be used in that transaction. When the transaction commits, free space in the block becomes available for other transactions.
An extent is a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a segment. When the existing space in a segment is completely used, Oracle allocates a new extent for the segment.
When you create a table, Oracle allocates to the table's data segment an initial extent of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.
If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment.
Note:This chapter applies to serial operations, in which one server process parses and runs a SQL statement. Extents are allocated somewhat differently in parallel SQL statements, which entail multiple server processes.
Storage parameters expressed in terms of extents define every segment. Storage parameters apply to all types of segments. They control how Oracle allocates free database space for a given segment. For example, you can determine how much space is initially reserved for a table's data segment or you can limit the number of extents the table can allocate by specifying the storage parameters of a table in the
STORAGE clause of the
TABLE statement. If you do not specify a table's storage parameters, then it uses the default storage parameters of the tablespace.
You can have dictionary managed tablespaces, which rely on data dictionary tables to track space utilization, or locally managed tablespaces, which use bitmaps (instead of data dictionary tables) to track used and free space. Because of the better performance and easier manageability of locally managed tablespaces, the default for non-
SYSTEM permanent tablespaces is locally managed whenever the type of extent management is not explicitly specified.
A tablespace that manages its extents locally can have either uniform extent sizes or variable extent sizes that are determined automatically by the system. When you create the tablespace, the
AUTOALLOCATE (system-managed) clause specifies the type of allocation.
For uniform extents, you can specify an extent size or use the default size, which is 1 MB. Ensure that each extent contains at least five database blocks, given the database block size. Temporary tablespaces that manage their extents locally can only use this type of allocation.
For system-managed extents, Oracle determines the optimal size of additional extents, with a minimum extent size of 64 KB. If the tablespaces are created with 'segment space management auto', and if the database block size is 16K or higher, then Oracle manages segment spze by creating extents with a minimum size of 1M. This is the default for permanent tablespaces.
The storage parameters
MINEXTENTS cannot be specified at the tablespace level for locally managed tablespaces. They can, however, be specified at the segment level. In this case,
MINEXTENTS are used together to compute the initial size of the segment. After the segment size is computed, internal algorithms determine the size of each extent.
With locally managed tablespaces, Oracle looks for free space to allocate to a new extent by first determining a candidate datafile in the tablespace and then searching the datafile's bitmap for the required number of adjacent free blocks. If that datafile does not have enough adjacent free space, then Oracle looks in another datafile.
Note:Oracle strongly recommends that you use locally managed tablespaces.
In general, the extents of a segment do not return to the tablespace until you drop the schema object whose data is stored in the segment (using a
CLUSTER statement). Exceptions to this include the following:
The owner of a table or cluster, or a user with the
ANY privilege, can truncate the table or cluster with a
A database administrator (DBA) can deallocate unused extents using the following SQL syntax:
Periodically, Oracle deallocates one or more extents of a rollback segment if it has the
OPTIMAL size specified.
When extents are freed, Oracle modifies the bitmap in the datafile (for locally managed tablespaces) or updates the data dictionary (for dictionary managed tablespaces) to reflect the regained extents as available space. Any data in the blocks of freed extents becomes inaccessible.
As long as a nonclustered table exists or until you truncate the table, any data block allocated to its data segment remains allocated for the table. Oracle inserts new rows into a block if there is enough room. Even if you delete all rows of a table, Oracle does not reclaim the data blocks for use by other objects in the tablespace.
After you drop a nonclustered table, this space can be reclaimed when other extents require free space. Oracle reclaims all the extents of the table's data and index segments for the tablespaces that they were in and makes the extents available for other schema objects in the same tablespace.
In dictionary managed tablespaces, when a segment requires an extent larger than the available extents, Oracle identifies and combines contiguous reclaimed extents to form a larger one. This is called coalescing extents. Coalescing extents is not necessary in locally managed tablespaces, because all contiguous free space is available for allocation to a new extent regardless of whether it was reclaimed from one or more extents.
Clustered tables store information in the data segment created for the cluster. Therefore, if you drop one table in a cluster, the data segment remains for the other tables in the cluster, and no extents are deallocated. You can also truncate clusters (except for hash clusters) to free extents.
See Also:"Overview of Materialized Views"
All extents allocated to an index segment remain allocated as long as the index exists. When you drop the index or associated table or cluster, Oracle reclaims the extents for other uses within the tablespace.
When Oracle completes the execution of a statement requiring a temporary segment, Oracle automatically drops the temporary segment and returns the extents allocated for that segment to the associated tablespace. A single sort allocates its own temporary segment in a temporary tablespace of the user issuing the statement and then returns the extents to the tablespaces.
Multiple sorts, however, can use sort segments in temporary tablespaces designated exclusively for sorts. These sort segments are allocated only once for the instance, and they are not returned after the sort, but remain available for other multiple sorts.
A temporary segment in a temporary table contains data for multiple statements of a single transaction or session. Oracle drops the temporary segment at the end of the transaction or session, returning the extents allocated for that segment to the associated tablespace.
Oracle periodically checks the rollback segments of the database to see if they have grown larger than their optimal size. If a rollback segment is larger than is optimal (that is, it has too many extents), then Oracle automatically deallocates one or more extents from the rollback segment.
A segment is a set of extents that contains all the data for a specific logical storage structure within a tablespace. For example, for each table, Oracle allocates one or more extents to form that table's data segment, and for each index, Oracle allocates one or more extents to form its index segment.
This section contains the following topics:
The storage parameters for a table or cluster determine how its data segment's extents are allocated. You can set these storage parameters directly with the appropriate
ALTER statement. These storage parameters affect the efficiency of data retrieval and storage for the data segment associated with the object.
Note:Oracle creates segments for materialized views and materialized view logs in the same manner as for tables and clusters.
Oracle creates the index segment for an index or an index partition when you issue the
INDEX statement. In this statement, you can specify storage parameters for the extents of the index segment and a tablespace in which to create the index segment. (The segments of a table and an index associated with it do not have to occupy the same tablespace.) Setting the storage parameters directly affects the efficiency of data retrieval and storage.
When processing queries, Oracle often requires temporary workspace for intermediate stages of SQL statement parsing and execution. Oracle automatically allocates this disk space called a temporary segment. Typically, Oracle requires a temporary segment as a database area for sorting. Oracle does not create a segment if the sorting operation can be done in memory or if Oracle finds some other way to perform the operation using indexes.
SELECT ... ORDER BY
SELECT DISTINCT ...
SELECT ... GROUP BY
SELECT ... INTERSECT
SELECT ... MINUS
Some unindexed joins and correlated subqueries can require use of a temporary segment. For example, if a query contains a
DISTINCT clause, a
BY, and an
BY, Oracle can require as many as two temporary segments.
Oracle allocates temporary segments differently for queries and temporary tables.
Oracle allocates temporary segments as needed during a user session in one of the temporary tablespaces of the user issuing the statement. Specify these tablespaces with a
USER or an
USER statement using the
Note:You cannot assign a permanent tablespace as a user's temporary tablespace.
If no temporary tablespace is defined for the user, then the default temporary tablespace is the
SYSTEM tablespace. The default storage characteristics of the containing tablespace determine those of the extents of the temporary segment. Oracle drops temporary segments when the statement completes.
Because allocation and deallocation of temporary segments occur frequently, create at least one special tablespace for temporary segments. By doing so, you can distribute I/O across disk devices, and you can avoid fragmentation of the
SYSTEM and other tablespaces that otherwise hold temporary segments.
Oracle allocates segments for a temporary table when the first
INSERT into that table is issued. (This can be an internal insert operation issued by
SELECT.) The first
INSERT into a temporary table allocates the segments for the table and its indexes, creates the root page for the indexes, and allocates any
Segments for a temporary table are allocated in a temporary tablespace of the user who created the temporary table.
Oracle drops segments for a transaction-specific temporary table at the end of the transaction and drops segments for a session-specific temporary table at the end of the session. If other transactions or sessions share the use of that temporary table, the segments containing their data remain in the table.
Oracle maintains information to nullify changes made to the database. Such information consists of records of the actions of transactions, collectively known as 'undo.' Oracle uses the undo to do the following:
Rollback an active transaction
Recover a terminated transaction
Provide read consistency
Recovery from logical corruptions
Automatic undo management is undo-tablespace based. You allocate space in the form of an undo tablespace, instead of allocating many rollback segments in different sizes.
Automatic undo management eliminates the complexities of managing rollback segment space and lets you exert control over how long undo is retained before being overwritten. Oracle strongly recommends that you use undo tablespaces to manage undo rather than rollback segments. The system automatically tunes the period for which undo is retained in the undo tablespace to satisfy queries that require undo information. You can set the
UNDO_RETENTION parameter to a low threshold value so that the system retains the undo for at least the time specified in the parameter, provided that the current undo tablespace has enough space.
See Also:Oracle Database Administrator's Guide for more information
V$UNDOSTAT view to monitor and configure your database system to achieve efficient use of undo space.
V$UNDOSTAT shows various undo and transaction statistics, such as the amount of undo space consumed in the instance.
Note:Earlier releases of Oracle used rollback segments to store undo, also known as manual undo management mode. Space management for these rollback segments was complex, and Oracle has now deprecated that method of storing undo.
The Oracle Database contains an Undo Advisor that provides advice on and helps automate the establishment of your undo environment.
See Also:Oracle Database 2 Day DBA for information on the Undo Advisor and on how to use advisors
Undo mode provides a more flexible way to migrate from manual undo management to automatic undo management. A database system can run in either manual undo management mode or automatic undo management mode. In manual undo management mode, undo space is managed through rollback segments. In automatic undo management mode, undo space is managed in undo tablespaces. To use automatic undo management mode, the database administrator needs only to create an undo tablespace for each instance and set the
UNDO_MANAGEMENT initialization parameter to
AUTO. You are strongly encouraged to run in automatic undo management mode.
In automatic undo management mode, the system controls exclusively the assignment of transactions to undo segments, and controls space allocation for undo segments. An ill-behaved transaction can potentially consume much of the undo space, thus paralyzing the entire system. The Resource Manager directive
UNDO_POOL is a more explicit way to control large transactions. This lets database administrators group users into consumer groups, with each group assigned a maximum undo space limit. When the total undo space consumed by a group exceeds the limit, its users cannot make further updates until undo space is freed up by other member transactions ending.
The default value of
UNLIMITED, where users are allowed to consume as much undo space as the undo tablespace has. Database administrators can limit a particular user by using the
Long-running queries sometimes fail because undo information required for consistent read operations is no longer available. This happens when committed undo blocks are overwritten by active transactions. The success of several flashback features can also depend upon older undo information.
Oracle automatically tunes undo retention by collecting database usage statistics and estimating undo capacity needs for the successful completion of the queries. You can set a low threshold value for the
UNDO_RETENTION parameter so that the system retains the undo for at least the time specified in the parameter, provided that the current undo tablespace has enough space. The setting of the
UNDO_RETENTION parameter should take into account any flashback requirements of the system.
Monitor transaction and undo information with
V$ROLLSTAT. For automatic undo management, the information in
V$ROLLSTAT reflects the behaviors of the automatic undo management undo segments.
V$UNDOSTAT view displays a histogram of statistical data to show how well the system is working. You can see statistics such as undo consumption rate, transaction concurrency, and lengths of queries run in the instance. Using this view, you can better estimate the amount of undo space required for the current workload.
See Also:Oracle Database Administrator's Guide for more details about using