12 Logical Storage Structures

Locally Managed Tablespaces

A locally managed tablespace maintains a bitmap in each data file to keep track of the free or used blocks in the data file. Each bit in the bitmap corresponds to a block or a group of blocks. When an extent is allocated or freed, Oracle Database changes the bitmap values to show the new status of the blocks.

A locally managed tablespace has the following advantages:

• Avoids using the data dictionary to manage extents

  Recursive operations can occur in dictionary-managed tablespaces if consuming or releasing space in an extent results in another operation that consumes or releases space in a data dictionary table or undo segment.

• Tracks adjacent free space automatically

  In this way, the database eliminates the need to coalesce free extents.

• Determines the size of locally managed extents automatically

  Alternatively, all extents can have the same size in a locally managed tablespace and override object storage options.

Segment space management is an attribute inherited from the tablespace that contains the segment. Within a locally managed tablespace, the database can manage segments automatically or manually. For example, segments in tablespace users can be managed automatically while segments in tablespace tools are managed manually.

Dictionary-Managed Tablespaces

A dictionary-managed tablespace uses the data dictionary to manage its extents. Oracle Database updates tables in the data dictionary whenever an extent is allocated or freed for reuse. For example, when a table needs an extent, the database queries the data dictionary tables, and searches for free extents. If the database finds space, then it modifies one data dictionary table and inserts a row into another. In this way, the database manages space by modifying and moving data.

The SQL that the database executes in the background to obtain space for database objects is recursive SQL. Frequent use of recursive SQL can have a negative impact on performance because updates to the data dictionary must be serialized. Locally managed tablespaces, which are the default, avoid this performance problem.

Database Block Size

Every database has a database block size. The DB_BLOCK_SIZE initialization parameter sets the data block size for a database when it is created. The size is set for the SYSTEM and SYSAUX tablespaces and is the default for all other tablespaces. The database block size cannot be changed except by re-creating the database.

If DB_BLOCK_SIZE is not set, then the default data block size is operating system-specific. The standard data block size for a database is 4 KB or 8 KB. If the size differs for data blocks and operating system blocks, then the data block size must be a multiple of the operating system block size.

Tablespace Block Size

You can create individual tablespaces whose block size differs from the DB_BLOCK_SIZE setting. A nonstandard block size can be useful when moving a transportable tablespace to a different platform.

Data Block Overhead

Oracle Database uses the block overhead to manage the block itself. The block overhead is not available to store user data. As shown in Figure 12-5, the block overhead includes the following parts:

• Block header

  This part contains general information about the block, including disk address and segment type. For blocks that are transaction-managed, the block header contains active and historical transaction information.

  A transaction entry is required for every transaction that updates the block. Oracle Database initially reserves space in the block header for transaction entries. In data blocks allocated to segments that support transactional changes, free space can also hold transaction entries when the header space is depleted. The space required for transaction entries is operating system dependent. However, transaction entries in most operating systems require approximately 23 bytes.

• Table directory

  For a heap-organized table, this directory contains metadata about tables whose rows are stored in this block. Multiple tables can store rows in the same block.

• Row directory

  For a heap-organized table, this directory describes the location of rows in the data portion of the block.

  After space has been allocated in the row directory, the database does not reclaim this space after row deletion. Thus, a block that is currently empty but formerly had up to 50 rows continues to have 100 bytes allocated for the row directory. The database reuses this space only when new rows are inserted in the block.

Some parts of the block overhead are fixed in size, but the total size is variable. On average, the block overhead totals 84 to 107 bytes.

Row Format

The row data part of the block contains the actual data, such as table rows or index key entries. Just as every data block has an internal format, every row has a row format that enables the database to track the data in the row.

Oracle Database stores rows as variable-length records. A row is contained in one or more row pieces. Each row piece has a row header and column data.

Figure 12-6 shows the format of a row.

Figure 12-6 The Format of a Row Piece

Description of "Figure 12-6 The Format of a Row Piece"

Rowid Format

Oracle Database uses a rowid to uniquely identify a row. Internally, the rowid is a structure that holds information that the database needs to access a row. A rowid is not physically stored in the database, but is inferred from the file and block on which the data is stored.

An extended rowid includes the data in the restricted rowid plus a data object number. This rowid type uses a base 64 encoding of the physical address for each row. The encoding characters are A-Z, a-z, 0-9, +, and /.

Example 12-1 queries the ROWID pseudocolumn to show the extended rowid of the row in the employees table for employee 100.

Example 12-1 ROWID Pseudocolumn

SQL> SELECT ROWID FROM employees WHERE employee_id = 100;
 
ROWID
------------------
AAAPecAAFAAAABSAAA

Figure 12-7 illustrates the format of an extended rowid.

Figure 12-7 ROWID Format

Description of "Figure 12-7 ROWID Format"

An extended rowid is displayed in a four-piece format, OOOOOOFFFBBBBBBRRR, with the format divided into the following components:

• OOOOOO

  The data object number identifies the segment (data object AAAPec in Example 12-1). A data object number is assigned to every database segment. Schema objects in the same segment, such as a cluster of tables, have the same data object number.

• FFF

  The tablespace-relative data file number identifies the data file that contains the row (file AAF in Example 12-1).

• BBBBBB

  The data block number identifies the block that contains the row (block AAAABS in Example 12-1). Block numbers are relative to their data file, not their tablespace. Thus, two rows with identical block numbers could reside in different data files of the same tablespace.

• RRR

  The row number identifies the row in the block (row AAA in Example 12-1).

After a rowid is assigned to a row piece, the rowid can change in special circumstances. For example, if row movement is enabled, then the rowid can change because of partition key updates, Flashback Table operations, shrink table operations, and so on. If row movement is disabled, then a rowid can change if the row is exported and imported using Oracle Database utilities.

Note:

Internally, the database performs row movement as if the row were physically deleted and reinserted. However, row movement is considered an update, which has implications for triggers.

See Also:

• "Rowid Data Types"

• Oracle Database SQL Language Reference to learn about rowids

Percentage of Free Space in Data Blocks

The PCTFREE storage parameter is essential to how the database manages free space. This SQL parameter sets the minimum percentage of a data reserved as free space for updates to existing rows. Thus, PCTFREE is important for preventing row migration and avoiding wasted space.

For example, assume that you create a table that will require only occasional updates, most of which will not increase the size of the existing data. You specify the PCTFREE parameter within a CREATE TABLE statement as follows:

CREATE TABLE test_table (n NUMBER) PCTFREE 20;

Figure 12-8 shows how a PCTFREE setting of 20 affects space management. The database adds rows to the block over time, causing the row data to grow upwards toward the block header, which is itself expanding downward toward the row data. The PCTFREE setting ensures that at least 20% of the data block is free. For example, the database prevents an insert from filling the block so that the row data and header occupy a combined 90% of the total block space, leaving only 10% free.

Figure 12-8 PCTFREE

Description of "Figure 12-8 PCTFREE"

Optimization of Free Space in Data Blocks

While the percentage of free space cannot be less than PCTFREE, the amount of free space can be greater. For example, a PCTFREE setting of 20% prevents the total amount of free space from dropping to 5% of the block, but permits 50% of the block to be free space. The following SQL statements can increase free space:

• DELETE statements

• UPDATE statements that either update existing values to smaller values or increase existing values and force a row to migrate

• INSERT statements on a table that uses OLTP compression

  If inserts fill a block with data, then the database invokes block compression, which may result in the block having more free space.

The space released is available for INSERT statements under the following conditions:

• If the INSERT statement is in the same transaction and after the statement that frees space, then the statement can use the space.

• If the INSERT statement is in a separate transaction from the statement that frees space (perhaps run by another user), then the statement can use the space made available only after the other transaction commits and only if the space is needed.

Coalescing Fragmented Space

Released space may or may not be contiguous with the main area of free space in a data block, as shown in Figure 12-9. Noncontiguous free space is called fragmented space.

Figure 12-9 Data Block with Fragmented Space

Description of "Figure 12-9 Data Block with Fragmented Space"

Oracle Database automatically and transparently coalesces the free space of a data block only when the following conditions are true:

• An INSERT or UPDATE statement attempts to use a block that contains sufficient free space to contain a new row piece.

• The free space is fragmented so that the row piece cannot be inserted in a contiguous section of the block.

After coalescing, the amount of free space is identical to the amount before the operation, but the space is now contiguous. Figure 12-10 shows a data block after space has been coalesced.

Figure 12-10 Data Block After Coalescing Free Space

Description of "Figure 12-10 Data Block After Coalescing Free Space"

Oracle Database performs coalescing only in the preceding situations because otherwise performance would decrease because of the continuous coalescing of the free space in data blocks.

Reuse of Index Space

The database can reuse space within an index block. For example, if you insert a value into a column and delete it, and if an index exists on this column, then the database can reuse the index slot when a row requires it.

The database can reuse an index block itself. Unlike a table block, an index block only becomes free when it is empty. The database places the empty block on the free list of the index structure and makes it eligible for reuse. However, Oracle Database does not automatically compact the index: an ALTER INDEX REBUILD or COALESCE statement is required.

Figure 12-11 represents an index of the employees.department_id column before the index is coalesced. The first three leaf blocks are only partially full, as indicated by the gray fill lines.

Figure 12-11 Index Before Coalescing

Description of "Figure 12-11 Index Before Coalescing"

Figure 12-12 shows the index in Figure 12-11 after the index has been coalesced. The first two leaf blocks are now full, as indicated by the gray fill lines, and the third leaf block has been freed.

Figure 12-12 Index After Coalescing

Description of "Figure 12-12 Index After Coalescing"

See Also:

• Oracle Database Administrator's Guide to learn how to coalesce and rebuild indexes

• Oracle Database SQL Language Reference to learn about the COALESCE statement

Chained and Migrated Rows

Oracle Database must manage rows that are too large to fit into a single block. The following situations are possible:

• The row is too large to fit into one data block when it is first inserted.

  In row chaining, Oracle Database stores the data for the row in a chain of one or more data blocks reserved for the segment. Row chaining most often occurs with large rows. Examples include rows that contain a column of data type LONG or LONG RAW, a VARCHAR2(4000) column in a 2 KB block, or a row with a huge number of columns. Row chaining in these cases is unavoidable.

• A row that originally fit into one data block is updated so that the overall row length increases, but insufficient free space exists to hold the updated row.

  In row migration, Oracle Database moves the entire row to a new data block, assuming the row can fit in a new block. The original row piece of a migrated row contains a pointer or "forwarding address" to the new block containing the migrated row. The rowid of a migrated row does not change.

• A row has more than 255 columns.

  Oracle Database can only store 255 columns in a row piece. Thus, if you insert a row into a table that has 1000 columns, then the database creates 4 row pieces, typically chained over multiple blocks.

Figure 12-13 depicts the difference between chaining and migration.

Figure 12-13 Row Chaining and Migration

Description of "Figure 12-13 Row Chaining and Migration"

When a row is chained or migrated, the amount of I/O necessary to retrieve the data increases. This situation results because Oracle Database must scan multiple blocks to retrieve the information for the row. For example, if the database performs one I/O to read an index and one I/O to read a nonmigrated table row, then an additional I/O is required to obtain the data for a migrated row.

The Segment Advisor, which can be run both manually and automatically, is an Oracle Database component that identifies segments that have space available for reclamation. The advisor can offer advice about objects that have significant free space or too many chained rows.

User Segment Creation

By default, Oracle Database creates a user segment when you create a user object with a CREATE statement. For example, Oracle Database creates a segment for a table, table cluster, index, or materialized view.

The database allocates one or more extents when the segment is created. Storage parameters for the object determine how the extents for each segment are allocated (see "Storage Parameters for Extents"). The parameters affect the efficiency of data retrieval and storage for the data segment associated with the object.

For example, assume that you create a table as follows:

CREATE TABLE test_table (my_column NUMBER);

As shown in Figure 12-16, the data for the preceding table is stored in one segment.

Figure 12-16 Creation of a User Segment

Description of "Figure 12-16 Creation of a User Segment"

When you create a table with a primary key or unique key, Oracle Database automatically creates an index for this key. Thus, a single CREATE TABLE statement can create multiple segments. For example, suppose you create a table as follows:

CREATE TABLE lob_table (my_column NUMBER PRIMARY KEY, clob_column CLOB);

Figure 12-17 shows that the data for lob_table is stored in one segment, while the implicitly created index is in a different segment. Also, the CLOB data is stored in its own segment, as is its associated CLOB index (see "Internal Lobs"). Thus, the statement results in the creation of four different segments.

Figure 12-17 Multiple Segments

Description of "Figure 12-17 Multiple Segments"

In some cases, installation of an application can create thousands of objects, consuming significant disk space. Many of these objects may never be used. By default, the database uses delayed segment creation to update only database metadata when creating nonpartitioned tables and indexes, avoiding the initial creation of user segments. When a user inserts the first row into a table, the database creates segments for the table, its LOB columns, and its indexes.

Allocation of Temporary Segments for Queries

Oracle Database allocates temporary segments for queries as needed during a user session and drops them when the query completes. Changes to temporary segments are not recorded in the online redo log, except for space management operations on the temporary segment (see "Overview of the Online Redo Log").

The database creates temporary segments in the temporary tablespace assigned to the user. The default storage characteristics of the tablespace determine the characteristics of the extents in the temporary segment. Because allocation and deallocation of temporary segments occurs frequently, the best practice is to create at least one special tablespace for temporary segments. The database distributes I/O across disks and avoids fragmenting SYSTEM and other tablespaces with temporary segments.

Allocation of Temporary Segments for Temporary Tables and Indexes

Oracle Database can also allocate temporary segments for temporary tables and their indexes. Temporary tables hold data that exists only for the duration of a transaction or session. Each session accesses only the extents allocated for the session and cannot access extents allocated for other sessions.

Oracle Database allocates segments for a temporary table when the first INSERT into that table occurs. The insert can occur explicitly or because of CREATE TABLE AS 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 LOB segments.

Segments for a temporary table are allocated in a temporary tablespace of the current user. Assume that the temporary tablespace assigned to user1 is temp1 and the temporary tablespace assigned to user2 is temp2. In this case, user1 stores temporary data in the temp1 segments, while user2 stores temporary data in the temp2 segments.

Smallfile and Bigfile Tablespaces

A tablespace is either a bigfile or a smallfile tablespace. The tablespaces are indistinguishable in terms of execution of SQL statements that do not explicitly refer to data files. The difference is as follows:

• A smallfile tablespace can contain multiple data files or temp files, but the files cannot be as large as in bigfile tablespaces. This is the default tablespace type.

• A bigfile tablespace contains one very large data file or temp file. This type of tablespaces can do the following:

  • Increase the storage capacity of a database

    The maximum number of data files in a database is limited (usually to 64 KB files), so increasing the size of each data file increases the overall storage.

  • Reduce the burden of managing many data files

    Bigfile tablespaces simplify data file management with Oracle-managed files and Automatic Storage Management by eliminating the need for adding new data files and dealing with multiple files.

  • Perform operations on tablespaces rather than data files

    Bigfile tablespaces make the tablespace the main unit of the disk space administration, backup and recovery, and so on.

  Bigfile tablespaces are supported only for locally managed tablespaces with ASSM. However, locally managed undo and temporary tablespaces can be bigfile tablespaces even though segments are manually managed.

Read/Write and Read-Only Tablespaces

Every tablespace is in a write mode that specifies whether it can be written to. The mutually exclusive modes are as follows:

• Read/write mode

  Users can read and write to the tablespace. All tablespaces are initially created as read/write.

• Read-only mode

  Write operations to the data files in the tablespace are prevented. A read-only tablespace can reside on read-only media such as DVDs or WORM drives.

  Read-only tablespaces eliminate the need to perform backup and recovery of large, static portions of a database. Read-only tablespaces do not change and thus do not require repeated backup. If you recover a database after a media failure, then you do not need to recover read-only tablespaces.

Online and Offline Tablespaces

Any tablespace other than the SYSTEM tablespace can be online (accessible) or offline (not accessible) whenever the database is open. A tablespace is usually online so that its data is available to users.

A tablespace can go offline manually or automatically. For example, a database administrator can take a tablespace offline for maintenance or backup and recovery. The database automatically takes a tablespace offline when certain errors are encountered, as when the database writer (DBWn) process fails in several attempts to write to a data file of the tablespace. Users trying to access tables in the offline tablespace receive an error.

When a tablespace goes offline, the database does the following:

• The database does not permit subsequent DML statements to reference objects in the offline tablespace. An offline tablespace cannot be read or edited by any utility other than Oracle Database.

• Active transactions with completed statements that refer to data in that tablespace are not affected at the transaction level.

• The database saves undo data corresponding to those completed statements in a deferred undo segment in the SYSTEM tablespace. When the tablespace is brought online, the database applies the undo data to the tablespace, if needed.

The SYSTEM Tablespace

Oracle Database uses the SYSTEM tablespace to manage the database. This tablespace includes the following information, all owned by the SYS user:

• The data dictionary

• Tables and views that contain administrative information about the database

• Compiled stored objects such as triggers, procedures, and packages

The SYSTEM tablespace is managed as any other tablespace, but requires a higher level of privilege and is restricted in some ways. For example, you cannot rename or drop the SYSTEM tablespace or take it offline.

By default, Oracle Database sets all newly created user tablespaces to be locally managed. In a database with a locally managed SYSTEM tablespace, dictionary-managed tablespaces (which are deprecated) cannot be created. However, if you execute the CREATE DATABASE statement manually and accept the defaults, then the SYSTEM tablespace is dictionary managed. You can migrate an existing dictionary-managed SYSTEM tablespace to a locally managed format.

The SYSAUX Tablespace

The SYSAUX tablespace is an auxiliary tablespace to the SYSTEM tablespace. The SYSAUX tablespace provides a centralized location for database metadata that does not reside in the SYSTEM tablespace. It reduces the number of tablespaces created by default, both in the seed database and in user-defined databases.

Several database components, including Oracle Enterprise Manager and Oracle Streams, use the SYSAUX tablespace as their default storage location. Therefore, the SYSAUX tablespace is created during database creation or upgrade.

During normal database operation, the database does not allow the SYSAUX tablespace to be dropped or renamed. If the SYSAUX tablespace becomes unavailable, then core database functionality remains operational. The database features that use the SYSAUX tablespace could fail, or function with limited capability.

Automatic Undo Management Mode

Undo tablespaces require the database to be in the default automatic undo management mode. This mode eliminates the complexities of manually administering undo segments. The database automatically tunes itself to provide the best possible retention of undo data to satisfy long-running queries that may require this data.

An undo tablespace is automatically created with a new installation of Oracle Database. Earlier versions of Oracle Database may not include an undo tablespace and use legacy rollback segments instead, known as manual undo management mode. When upgrading to Oracle Database 11g, you can enable automatic undo management mode and create an undo tablespace. Oracle Database contains an Undo Advisor that provides advice on and helps automate your undo environment.

A database can contain multiple undo tablespaces, but only one can be in use at a time. When an instance attempts to open a database, Oracle Database automatically selects the first available undo tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo records in the SYSTEM tablespace. Storing undo in SYSTEM is not recommended.

Automatic Undo Retention

The undo retention period is the minimum amount of time that Oracle Database attempts to retain old undo data before overwriting it. Undo retention is important because long-running queries may require older block images to supply read consistency. Also, some Oracle Flashback features can depend on undo availability.

In general, it is desirable to retain old undo data as long as possible. After a transaction commits, undo data is no longer needed for rollback or transaction recovery. The database can retain old undo data if the undo tablespace has space for new transactions. When available space is low, the database begins to overwrite old undo data for committed transactions.

Oracle Database automatically provides the best possible undo retention for the current undo tablespace. The database collects usage statistics and tunes the retention period based on these statistics and the undo tablespace size. If the undo tablespace is configured with the AUTOEXTEND option, and if the maximum size not specified, then undo retention tuning is different. In this case, the database tunes the undo retention period to be slightly longer than the longest-running query, if space allows.

Default Temporary Tablespace

When the SYSTEM tablespace is locally managed, a default temporary tablespace is included in the database by default during database creation. A locally managed SYSTEM tablespace cannot be used for default temporary storage.

You can specify a user-named default temporary tablespace when you create a database by using the DEFAULT TEMPORARY TABLESPACE extension to the CREATE DATABASE statement. If SYSTEM is dictionary managed, and if a default temporary tablespace is not defined at database creation, then SYSTEM is the default temporary storage. However, the database writes a warning in the alert log saying that a default temporary tablespace is recommended.