The absence or presence of an index does not require a change in the wording of any SQL statement.

An index is a fast access path to a single row of data. It affects only the speed of execution. Given a data value that has been indexed, the index points directly to the location of the rows containing that value.

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When an index exists on one or more columns of a table, the database can in some cases retrieve a small set of randomly distributed rows from the table. Indexes are one of many means of reducing disk I/O. If a heap-organized table has no indexes, then the database must perform a full table scan to find a value. For example, a query of location 2700 in the unindexed hr.departments table requires the database to search every row in every block. This approach does not scale well as data volumes increase.

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In general, consider creating an index on a column in any of the following situations:

• The indexed columns are queried frequently and return a small percentage of the total number of rows in the table.
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• A referential integrity constraint exists on the indexed column or columns. The index is a means to avoid a full table lock that would otherwise be required if you update the parent table primary key, merge into the parent table, or delete from the parent table.
• A unique key constraint will be placed on the table and you want to manually specify the index and all index options.

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Indexes are schema objects that can speed access to table rows. Index-organized tables are tables stored in an index structure.

This chapter contains the following sections:

• Introduction to Indexes
• Overview of B-Tree Indexes
• Overview of Bitmap Indexes
• Overview of Function-Based Indexes
• Overview of Application Domain Indexes
• Overview of Index-Organized Tables

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Introduction to Indexes

An index is an optional structure, associated with a table or table cluster, that can sometimes speed data access.

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Indexes are schema objects that are logically and physically independent of the data in the objects with which they are associated. Thus, you can drop or create an index without physically affecting the indexed table.

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Note

If you drop an index, then applications still work. However, access of previously indexed data can be slower.

For an analogy, suppose an HR manager has a shelf of cardboard boxes. Folders containing employee information are inserted randomly in the boxes. The folder for employee Whalen (ID 200) is 10 folders up from the bottom of box 1, whereas the folder for King (ID 100) is at the bottom of box 3. To locate a folder, the manager looks at every folder in box 1 from bottom to top, and then moves from box to box until the folder is found. To speed access, the manager could create an index that sequentially lists every employee ID with its folder location:

CopyID 100: Box 3, position 1 (bottom)
ID 101: Box 7, position 8 
ID 200: Box 1, position 10
.
.
.

Similarly, the manager could create separate indexes for employee last names, department IDs, and so on.

This section contains the following topics:

• Benefits of Indexes
• Index Usability and Visibility
• Keys and Columns
• Composite Indexes
• Unique and Nonunique Indexes
• Types of Indexes
• How the Database Maintains Indexes
• Index Storage

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An Oracle object type is a user-defined type with a name, attributes, and methods. An object table is a special kind of table in which each row represents an object. Object types make it possible to model real-world entities such as customers and purchase orders as objects in the database.

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An object type defines a logical structure, but does not create storage. The following example creates an object type named department_typ:

CopyCREATE TYPE department_typ AS OBJECT
   ( d_name     VARCHAR2(100),
     d_address  VARCHAR2(200) );
/

The following example creates an object table named departments_obj_t of the object type department_typ, and then inserts a row into the table. The attributes (columns) of the departments_obj_t table are derived from the definition of the object type.

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CopyCREATE TABLE departments_obj_t OF department_typ;
INSERT INTO departments_obj_t 
  VALUES ('hr', '10 Main St, Sometown, CA');

Like a relational column, an object table can contain rows of just one kind of thing, namely, object instances of the same declared type as the table. By default, every row object in an object table has an associated logical object identifier (OID) that uniquely identifies it in an object table. The OID column of an object table is a hidden column.

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See also

• Oracle Database Object-Relational Developer’s Guide to learn about object-relational features in Oracle Database
• Oracle Database SQL Language Reference for CREATE TYPE syntax and semantics

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Internally, creating an external table means creating metadata in the data dictionary. Unlike an ordinary table, an external table does not describe data stored in the database, nor does it describe how data is stored externally. Rather, external table metadata describes how the external table layer must present data to the database.

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A CREATE TABLE ... ORGANIZATION EXTERNAL statement has two parts. The external table definition describes the column types. This definition is like a view that enables SQL to query external data without loading it into the database. The second part of the statement maps the external data to the columns.

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External tables are read-only unless created with CREATE TABLE AS SELECT with the ORACLE_DATAPUMP access driver. Restrictions for external tables include no support for indexed columns and column objects.

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See also

• Oracle Database Utilities to learn about external tables
• Oracle Database Administrator’s Guide to learn about managing external tables, external connections, and directory objects
• Oracle Database SQL Language Reference for information about creating and querying external tables

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An access driver is an API that interprets the external data for the database. The access driver runs inside the database, which uses the driver to read the data in the external table. The access driver and the external table layer are responsible for performing the transformations required on the data in the data file so that it matches the external table definition.

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The following figure represents SQL access of external data.

Figure 2-8 External Tables
Description of “Figure 2-8 External Tables”

Oracle provides the following access drivers for external tables:

• ORACLE_LOADER (default)Enables read-only access to external files using SQL*Loader. You cannot create, update, or append to an external file using the ORACLE_LOADER driver.
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• ORACLE_DATAPUMPEnables you to unload or load external data. An unload operation reads data from the database and inserts the data into an external table, represented by one or more external files. After external files are created, the database cannot update or append data to them. A load operation reads an external table and loads its data into a database.
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• ORACLE_HDFSEnables the extraction of data stored in a Hadoop Distributed File System (HDFS).
• ORACLE_HIVEEnables access to data stored in an Apache Hive database. The source data can be stored in HDFS, HBase, Cassandra, or other systems. Unlike the other access drivers, you cannot specify a location because ORACLE_HIVE obtains location information from an external metadata store.

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An external table accesses data in external sources as if this data were in a table in the database.

The data can be in any format for which an access driver is provided. You can use SQL (serial or parallel), PL/SQL, and Java to query external tables.

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Purpose of External Tables

External tables are useful when an Oracle database application must access non-relational data.

For example, a SQL-based application may need to access a text file whose records are in the following form:

Copy100,Steven,King,SKING,515.123.4567,17-JUN-03,AD_PRES,31944,150,90
101,Neena,Kochhar,NKOCHHAR,515.123.4568,21-SEP-05,AD_VP,17000,100,90 
102,Lex,De Haan,LDEHAAN,515.123.4569,13-JAN-01,AD_VP,17000,100,90

You could create an external table, copy the text file to the location specified in the external table definition, and then use SQL to query the records in the text file. Similarly, you could use external tables to give read-only access to JSON documents or LOBs.

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In data warehouse environments, external tables are valuable for performing extraction, transformation, and loading (ETL) tasks. For example, external tables enable you to pipeline the data loading phase with the transformation phase. This technique eliminates the need to stage data inside the database in preparation for further processing inside the database.

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Starting in Oracle Database 12c Release 2 (12.2), you can partition external tables on virtual or non-virtual columns. Thus, you can take advantage of performance improvements provided by partition pruning and partition-wise joins. For example, you could use partitioned external tables to analyze large volumes of non-relational data stored on Hadoop Distributed File System (HDFS) or a NoSQL database.

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When an Oracle instance starts, it reads initialization parameters from an initialization parameter file. This file must at a minimum specify the DB_NAME parameter. All other parameters have default values.

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The initialization parameter file can be either a read-only text file, a PFILE, or a read/write binary file.

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The binary file is called a server parameter file. A server parameter file enables you to change initialization parameters with ALTER SYSTEM commands and to persist the changes across a shutdown and startup. It also provides a basis for self-tuning by Oracle Database. For these reasons, it is recommended that you use a server parameter file. You can create one manually from your edited text initialization file, or automatically by using Database Configuration Assistant (DBCA) to create your database.

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Before you manually create a server parameter file, you can start an instance with a text initialization parameter file. Upon startup, the Oracle instance first searches for a server parameter file in a default location, and if it does not find one, searches for a text initialization parameter file. You can also override an existing server parameter file by naming a text initialization parameter file as an argument of the STARTUP command.

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You can add or edit basic initialization parameters before you create your new database.

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• About Initialization Parameters and Initialization Parameter Files
  When an Oracle instance starts, it reads initialization parameters from an initialization parameter file. This file must at a minimum specify the DB_NAME parameter. All other parameters have default values.
• Determining the Global Database Name
  The global database name consists of the user-specified local database name and the location of the database within a network structure.
• Specifying a Fast Recovery Area
  The Fast Recovery Area is a location in which Oracle Database can store and manage files related to backup and recovery. It is distinct from the database area, which is a location for the current database files (data files, control files, and online redo logs).

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• Specifying Control Files
  Every database has a control file, which contains entries that describe the structure of the database (such as its name, the timestamp of its creation, and the names and locations of its data files and redo files). The CONTROL_FILES initialization parameter specifies one or more names of control files, separated by commas.
• Specifying Database Block Sizes
  The DB_BLOCK_SIZE initialization parameter specifies the standard block size for the database.
• Specifying the Maximum Number of Processes
  The PROCESSES initialization parameter determines the maximum number of operating system processes that can be connected to Oracle Database concurrently.
• Specifying the DDL Lock Timeout
  You can specify the amount of time that blocking DDL statements wait for locks.

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• Specifying the Method of Undo Space Management
  Every Oracle Database must have a method of maintaining information that is used to undo changes to the database. Such information consists of records of the actions of transactions, primarily before they are committed. Collectively these records are called undo data.
• Specifying the Database Compatibility Level
  The COMPATIBLE initialization parameter controls the database compatibility level.
• Setting the License Parameter
  If you use named user licensing, Oracle Database can help you enforce this form of licensing. You can set a limit on the number of users created in the database. Once this limit is reached, you cannot create more users.

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The CREATE ... TEMPORARY TABLE statement creates a temporary table.

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Specify either GLOBAL TEMPORARY TABLE or PRIVATE TEMPORARY TABLE. In both cases, the ON COMMIT clause specifies whether the table data is transaction-specific (default) or session-specific. You create a temporary table for the database itself, not for every PL/SQL stored procedure.

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You can create indexes for global (not private) temporary tables with the CREATE INDEX statement. These indexes are also temporary. The data in the index has the same session or transaction scope as the data in the temporary table. You can also create a view or trigger on a global temporary table.

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See also

• “Overview of Views”
• “Overview of Triggers”
• Oracle Database Administrator’s Guide to learn how to create and manage temporary tables
• Oracle Database SQL Language Reference for CREATE ... TEMPORARY TABLE syntax and semantics

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FORCE LOGGING mode results in some performance degradation.

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If the primary reason for specifying FORCE LOGGING is to ensure complete media recovery, and there is no standby database active, then consider the following:

• How many media failures are likely to happen?
• How serious is the damage if unlogged direct writes cannot be recovered?
• Is the performance degradation caused by forced logging tolerable?

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If the database is running in NOARCHIVELOG mode, then generally there is no benefit to placing the database in FORCE LOGGING mode. Media recovery is not possible in NOARCHIVELOG mode, so if you combine it with FORCE LOGGING, the result may be performance degradation with little benefit.

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Starting with Oracle Database 18c, the following two new nologging clauses are introduced, which enable non-logged operations to be carried out and have Active Data Guard standby databases receive all the data, thus preventing performance degradation caused by large redo log generation by the FORCE LOGGING mode:

• STANDBY NOLOGGING FOR DATA AVAILABILITY
• STANDBY NOLOGGING FOR LOAD PERFORMANCE

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