|Oracle® Database Administrator's Guide
11g Release 2 (11.2)
Part Number E17120-05
This section describes guidelines to follow when managing tables. Following these guidelines can make the management of your tables easier and can improve performance when creating the table, as well as when loading, updating, and querying the table data.
The following topics are discussed:
Usually, the application developer is responsible for designing the elements of an application, including the tables. Database administrators are responsible for establishing the attributes of the underlying tablespace that will hold the application tables. Either the DBA or the applications developer, or both working jointly, can be responsible for the actual creation of the tables, depending upon the practices for a site.
Working with the application developer, consider the following guidelines when designing tables:
Use descriptive names for tables, columns, indexes, and clusters.
Be consistent in abbreviations and in the use of singular and plural forms of table names and columns.
Document the meaning of each table and its columns with the
Normalize each table.
Select the appropriate datatype for each column.
Consider whether your applications would benefit from adding one or more virtual columns to some tables.
Define columns that allow nulls last, to conserve storage space.
Cluster tables whenever appropriate, to conserve storage space and optimize performance of SQL statements.
Before creating a table, you should also determine whether to use integrity constraints. Integrity constraints can be defined on the columns of a table to enforce the business rules of your database automatically.
Here are the types of tables that you can create:
|Type of Table||Description|
|Ordinary (heap-organized) table||This is the basic, general purpose type of table which is the primary subject of this chapter. Its data is stored as an unordered collection (heap).|
|Clustered table||A clustered table is a table that is part of a cluster. A cluster is a group of tables that share the same data blocks because they share common columns and are often used together.
Clusters and clustered tables are discussed in Chapter 22, "Managing Clusters".
|Index-organized table||Unlike an ordinary (heap-organized) table, data for an index-organized table is stored in a B-tree index structure in a primary key sorted manner. Besides storing the primary key column values of an index-organized table row, each index entry in the B-tree stores the nonkey column values as well.
Index-organized tables are discussed in "Managing Index-Organized Tables".
|Partitioned table||Partitioned tables enable your data to be broken down into smaller, more manageable pieces called partitions, or even subpartitions. Each partition can have separate physical attributes, such as compression enabled or disabled, type of compression, physical storage settings, and tablespace, thus providing a structure that can be better tuned for availability and performance. In addition, each partition can be managed individually, which can simplify and reduce the time required for backup and administration.
Partitioned tables are discussed in Oracle Database VLDB and Partitioning Guide.
It is advisable to specify the
TABLESPACE clause in a
CREATE TABLE statement to identify the tablespace that is to store the new table. For partitioned tables, you can optionally identify the tablespace that is to store each partition. Ensure that you have the appropriate privileges and quota on any tablespaces that you use. If you do not specify a tablespace in a
CREATE TABLE statement, the table is created in your default tablespace.
When specifying the tablespace to contain a new table, ensure that you understand implications of your selection. By properly specifying a tablespace during the creation of each table, you can increase the performance of the database system and decrease the time needed for database administration.
The following situations illustrate how not specifying a tablespace, or specifying an inappropriate one, can affect performance:
If users' objects are created in the
SYSTEM tablespace, the performance of the database can suffer, since both data dictionary objects and user objects must contend for the same datafiles. Users' objects should not be stored in the
SYSTEM tablespace. To avoid this, ensure that all users are assigned default tablespaces when they are created in the database.
If application-associated tables are arbitrarily stored in various tablespaces, the time necessary to complete administrative operations (such as backup and recovery) for the data of that application can be increased.
You can utilize parallel execution when creating tables using a subquery (
AS SELECT) in the
CREATE TABLE statement. Because multiple processes work together to create the table, performance of the table creation operation is improved.
Parallelizing table creation is discussed in the section "Parallelizing Table Creation".
To create a table most efficiently use the
NOLOGGING clause in the
CREATE TABLE...AS SELECT statement. The
NOLOGGING clause causes minimal redo information to be generated during the table creation. This has the following benefits:
Space is saved in the redo log files.
The time it takes to create the table is decreased.
Performance improves for parallel creation of large tables.
NOLOGGING clause also specifies that subsequent direct loads using SQL*Loader and direct load
INSERT operations are not logged. Subsequent DML statements (
DELETE, and conventional path insert) are unaffected by the
NOLOGGING attribute of the table and generate redo.
If you cannot afford to lose the table after you have created it (for example, you will no longer have access to the data used to create the table) you should take a backup immediately after the table is created. In some situations, such as for tables that are created for temporary use, this precaution may not be necessary.
In general, the relative performance improvement of specifying
NOLOGGING is greater for larger tables than for smaller tables. For small tables,
NOLOGGING has little effect on the time it takes to create a table. However, for larger tables the performance improvement can be significant, especially when also parallelizing the table creation.
As your database grows in size, consider using table compression. Compression saves disk space, reduces memory use in the database buffer cache, and can significantly speed query execution during reads. Compression has a cost in CPU overhead for data loading and DML. However, this cost may be offset by reduced I/O requirements.
Table compression is completely transparent to applications. It is useful in both decision support systems (DSS) and online transaction processing (OLTP) systems.
You can specify compression for a tablespace, a table, or a partition. If specified at the tablespace level, then all tables created in that tablespace are compressed by default.
Compression can occur while data is being inserted, updated, or bulk loaded into a table. Operations that permit compression include:
Single-row or array inserts and updates
The following direct-path insert methods:
Direct path SQL*Loader
INSERT statements with an
Oracle Database support two methods of table compression. They are summarized in Table 20-1.
Table 20-1 Table Compression Methods
|Table Compression Method||Applications||CREATE/ALTER TABLE Syntax||Direct-Path Insert||DML|
Footnote 1 COMPRESS and COMPRESS BASIC are equivalent
Footnote 2 Inserted and updated rows are uncompressed
You specify table compression with the
COMPRESS clause of the
TABLE statement. You can enable compression for an existing table by using these clauses in an
TABLE statement. In this case, only data that is inserted or updated after compression is enabled is compressed. Similarly, you can disable table compression for an existing compressed table with the
NOCOMPRESS statement. In this case, all data that was already compressed remains compressed, and new data is inserted uncompressed.
To enable OLTP table compression, you must set the
COMPATIBLE initialization parameter to 11.1.0 or higher.
The following example enables OLTP table compression on the table
CREATE TABLE orders ... COMPRESS FOR OLTP;
Data for the
orders table is compressed during both direct-path insert and conventional DML.
The next two examples, which are equivalent, enable basic table compression on the
sales_history table, which is a fact table in a data warehouse. Frequent queries are run against this table, but no DML is expected.
CREATE TABLE sales_history ... COMPRESS BASIC; CREATE TABLE sales_history ... COMPRESS;
INSERT /*+ APPEND */ INTO sales_history SELECT * FROM sales WHERE year=2008; COMMIT;
Compression and Partitioned Tables
A table can have both compressed and uncompressed partitions, and different partitions can use different compression methods. If the compression settings for a table and one of its partitions disagree, the partition setting has precedence for the partition.
To change the compression method for a partition, do one of the following:
To change the compression method for new data only, use
To change the compression method for both new and existing data, use either
COMPRESS ... or online table redefinition.
Determining If a Table Is Compressed
*_TABLES data dictionary views, compressed tables have
ENABLED in the
COMPRESSION column. For partitioned tables, this column is null, and the
COMPRESSION column of the
*_TAB_PARTITIONS views indicates the partitions that are compressed. In addition, the
COMPRESS_FOR column indicates the compression method in use for the table or partition.
SELECT table_name, compression, compress_for FROM user_tables; TABLE_NAME COMPRESSION COMPRESS_FOR ---------------- ----------- ------------------ T1 DISABLED T2 ENABLED BASIC T3 ENABLED OLTP
SELECT table_name, partition_name, compression, compress_for FROM user_tab_partitions; TABLE_NAME PARTITION_NAME COMPRESSION COMPRESS_FOR ----------- ---------------- ----------- ------------------------------ SALES Q4_2008 ENABLED OLTP SALES Q1_2009 ENABLED OLTP SALES Q2_2009 ENABLED OLTP
Adding and Dropping Columns in Compressed Tables
The following restrictions apply when adding columns to compressed tables:
Basic compression—You cannot specify a default value for an added column.
OLTP compression—If a default value is specified for an added column, the column must be
NULL. Added nullable columns with default values are not supported.
The following restrictions apply when dropping columns in compressed tables:
Basic compression—Dropping a column is not supported.
COLUMN is supported, but internally the database sets the column
UNUSED to avoid long-running decompression and recompression operations.
Notes and Other Restrictions for Compressed Tables
Online segment shrink is not supported for compressed tables.
The table compression methods described in this section do not apply to SecureFile large objects (LOBs). SecureFile LOBs have their own compression methods. See Oracle Database SecureFiles and Large Objects Developer's Guide for more information.
Compression technology uses CPU. You should ensure that you have enough available CPU to handle the additional load.
Tables created with basic compression have the
PCT_FREE parameter automatically set to 0 unless you specify otherwise.
Packing Compressed Tables
If you use conventional DML on a table compressed with basic compression, then all inserted and updated rows are stored uncompressed. To "pack" the compressed table such that these rows are compressed, you can use an
MOVE statement. This operation takes an exclusive lock on the table, and therefore prevents any updates and loads until it completes. If this is not acceptable, you can use online table redefinition.
Oracle Database SQL Language Reference for more details on the
MOVE statements, including restrictions
Oracle Database VLDB and Partitioning Guide for more information on table partitioning
You can encrypt individual table columns that contain sensitive data. Examples of sensitive data include social security numbers, credit card numbers, and medical records. Column encryption is transparent to your applications, with some restrictions.
Although encryption is not meant to solve all security problems, it does protect your data from users who try to circumvent the security features of the database and access database files directly through the operating system file system.
Column encryption uses the transparent data encryption feature of Oracle Database, which requires that you create an Oracle wallet to store the master encryption key for the database. The wallet must be open before you can create a table with encrypted columns and before you can store or retrieve encrypted data. When you open the wallet, it is available to all sessions, and it remains open until you explicitly close it or until the database is shut down.
Transparent data encryption supports industry-standard encryption algorithms, including the following Advanced Encryption Standard (AES) and Triple Data Encryption Standard (3DES) algorithms:
You choose the algorithm to use when you create the table. All encrypted columns in the table use the same algorithm. The default is AES192. The encryption key length is implied by the algorithm name. For example, the AES128 algorithm uses 128-bit keys.
If you plan on encrypting many columns in one or more tables, you may want to consider encrypting an entire tablespace instead and storing these tables in that tablespace. Tablespace encryption, which also uses the transparent data encryption feature but encrypts at the physical block level, can perform better than encrypting many columns. Another reason to encrypt at the tablespace level is to address the following limitations of column encryption:
COMPATIBLE initialization parameter set to 10.2.0, which is the minimum setting to enable transparent data encryption, data from encrypted columns that is involved in a sort or hash-join and that must be written to a temporary tablespace is written in clear text, and thus exposed to attacks. You must set
COMPATIBLE to 11.1.0 or higher to ensure that encrypted data written to a temporary tablespace remains encrypted. Note that as long as
COMPATIBLE is set to 10.2.0 or higher, data from encrypted columns remains encrypted when written to the undo tablespace or the redo log.
Certain data types, such as object data types, are not supported for column encryption.
You cannot use the transportable tablespace feature for a tablespace that includes tables with encrypted columns.
Other restrictions, which are detailed in Oracle Database Advanced Security Administrator's Guide.
Oracle Database Advanced Security Administrator's Guide for more information about transparent data encryption and for instructions for creating and opening wallets
Oracle Database SQL Language Reference for information about the
Oracle Real Application Clusters Administration and Deployment Guide for information on using an Oracle wallet in an Oracle Real Application Clusters environment
Beginning with Oracle Database 11g Release 2, when you create heap-organized tables in a locally managed tablespace, the database defers table segment creation until the first row is inserted.
In addition, segment creation is deferred for any LOB columns of the table, any indexes created implicitly as part of table creation, and any indexes subsequently explicitly created on the table.
Note:In Release 188.8.131.52, deferred segment creation is not supported for partitioned tables. This restriction is removed in release 184.108.40.206 and later.
The advantages of this space allocation method are the following:
It saves a significant amount of disk space in applications that create hundreds or thousands of tables upon installation, many of which might never be populated.
It reduces application installation time.
There is a small performance penalty when the first row is inserted, because the new segment must be created at that time.
To enable deferred segment creation, compatibility must be set to '11.2.0' or higher.
The new clauses for the
TABLE statement are:
These clauses override the default setting of the
DEFERRED_SEGMENT_CREATION initialization parameter,
TRUE, which defers segment creation. To disable deferred segment creation, set this parameter to
Note that when you create a table with deferred segment creation, the new table appears in the
*_TABLES views, but no entry for it appears in the
*_SEGMENTS views until you insert the first row.
You can verify deferred segment creation by viewing the
SEGMENT_CREATED column in
*_LOBS views for non-partitioned tables, and in
*_LOB_PARTITIONS views for partitioned tables.
Note:With this new allocation method, it is essential that you do proper capacity planning so that the database has enough disk space to handle segment creation when tables are populated. See "Capacity Planning for Database Objects".
The following example creates two tables to demonstrate deferred segment creation. The first table uses the
DEFERRED clause. No segments are created for it initially. The second table uses the
IMMEDIATE clause and, therefore, segments are created for it immediately.
CREATE TABLE part_time_employees ( empno NUMBER(8), name VARCHAR2(30), hourly_rate NUMBER (7,2) ) SEGMENT CREATION DEFERRED; CREATE TABLE hourly_employees ( empno NUMBER(8), name VARCHAR2(30), hourly_rate NUMBER (7,2) ) SEGMENT CREATION IMMEDIATE PARTITION BY RANGE(empno) (PARTITION empno_to_100 VALUES LESS THAN (100), PARTITION empno_to_200 VALUES LESS THAN (200));
The following query against
USER_SEGMENTS returns two rows for
HOURLY_EMPLOYEES, one for each partition, but returns no rows for
PART_TIME_EMPLOYEES because segment creation for that table was deferred.
SELECT segment_name, partition_name FROM user_segments; SEGMENT_NAME PARTITION_NAME -------------------- ------------------------------ HOURLY_EMPLOYEES EMPNO_TO_100 HOURLY_EMPLOYEES EMPNO_TO_200
USER_TABLES view shows that
PART_TIME_EMPLOYEES has no segments:
SELECT table_name, segment_created FROM user_tables;
TABLE_NAME SEGMENT_CREATED ------------------------------ ---------------------------------------- PART_TIME_EMPLOYEES NO HOURLY_EMPLOYEES N/A
HOURLY_EMPLOYEES table, which is partitioned, the
segment_created column is
N/A because the
USER_TABLES view does not provide that information for partitioned tables. It is available from the
USER_TAB_PARTITIONS view, shown below.
SELECT table_name, segment_created, partition_name FROM user_tab_partitions; TABLE_NAME SEGMENT_CREATED PARTITION_NAME -------------------- -------------------- ------------------------------ HOURLY_EMPLOYEES YES EMPNO_TO_100 HOURLY_EMPLOYEES YES EMPNO_TO_200
The following statements add employees to these tables.
INSERT INTO hourly_employees VALUES (99, 'FRose', 20.00); INSERT INTO hourly_employees VALUES (150, 'LRose', 25.00); INSERT INTO part_time_employees VALUES (50, 'KReilly', 10.00);
Repeating the same
SELECT statements as before shows that
PART_TIME_EMPLOYEES now has a segment, due to the insertion of row data.
HOURLY_EMPLOYEES remains as before.
SELECT segment_name, partition_name FROM user_segments; SEGMENT_NAME PARTITION_NAME -------------------- ------------------------------ PART_TIME_EMPLOYEES HOURLY_EMPLOYEES EMPNO_TO_100 HOURLY_EMPLOYEES EMPNO_TO_200
SELECT table_name, segment_created FROM user_tables; TABLE_NAME SEGMENT_CREATED -------------------- -------------------- PART_TIME_EMPLOYEES YES HOURLY_EMPLOYEES N/A
USER_TAB_PARTITIONS view does not change.
See Also:Oracle Database SQL Language Reference for notes and restrictions on deferred segment creation
Beginning with Oracle Database 11g release 2 (220.127.116.11), the
DBMS_SPACE_ADMIN package includes the
MATERIALIZE_DEFERRED_SEGMENTS() procedure, which enables you to materialize segments for tables, table partitions, and dependent objects created with deferred segment creation enabled.
This enables you to add segments as needed, rather than starting with more than you need and using database resources unnecessarily.
The following example materializes segments for the
EMPLOYEES table in the
BEGIN DBMS_SPACE_ADMIN.MATERIALIZE_DEFERRED_SEGMENTS( schema_name => 'HR', table_name => 'EMPLOYEES'); END;
See Also:See Oracle Database PL/SQL Packages and Types Reference for details about this procedure
You can use the combined estimated size of tables, along with estimates for indexes, undo space, and redo log files, to determine the amount of disk space that is required to hold an intended database. From these estimates, you can make correct hardware purchases.
You can use the estimated size and growth rate of an individual table to better determine the attributes of a tablespace and its underlying datafiles that are best suited for the table. This can enable you to more easily manage the table disk space and improve I/O performance of applications that use the table.
See Also:"Capacity Planning for Database Objects"
Here are some restrictions that may affect your table planning and usage:
Tables containing object types cannot be imported into a pre-Oracle8 database.
You cannot merge an exported table into a preexisting table having the same name in a different schema.
You cannot move types and extent tables to a different schema when the original data still exists in the database.
Oracle Database has a limit on the total number of columns that a table (or attributes that an object type) can have. See Oracle Database Reference for this limit.
Further, when you create a table that contains user-defined type data, the database maps columns of user-defined type to relational columns for storing the user-defined type data. This causes additional relational columns to be created. This results in "hidden" relational columns that are not visible in a
DESCRIBE table statement and are not returned by a
SELECT * statement. Therefore, when you create an object table, or a relational table with columns of
REF, varray, nested table, or object type, be aware that the total number of columns that the database actually creates for the table can be more than those you specify.
See Also:Oracle Database Object-Relational Developer's Guide for more information about user-defined types