Which Data Structure Is Used in Oracle Database?


Larry Thompson

Oracle Database is one of the most popular and widely used relational database management systems in the world. It provides a robust and scalable platform for storing, managing, and retrieving data.

When it comes to the internal organization of data within Oracle Database, a specific data structure is employed to ensure efficient storage and retrieval operations. Let’s explore the data structure used in Oracle Database.

Tablespace: The Foundation

Tablespace is the fundamental building block of Oracle Database’s storage architecture. It is a logical container that holds various database objects such as tables, indexes, and other schema objects. A tablespace is represented by a collection of physical files on disk.

Segments: Organizing Data

Within a tablespace, data is organized into segments. Each segment corresponds to a specific type of database object. For example, there are segments for tables, indexes, clusters, partitions, etc.

Segments are further divided into extents. An extent represents a contiguous set of data blocks that are allocated to store the actual data. The size of an extent varies depending on the configuration settings and can range from a few kilobytes to several megabytes.

Data Blocks: Storing Data

At the lowest level of the data structure hierarchy are data blocks. A data block is the smallest unit of storage in Oracle Database. It represents a fixed-size chunk of disk space (typically 8 KB) that stores actual data rows or index entries.

Data blocks are used to hold both user-defined tables and system-generated objects such as indexes and rollback segments. They form the basis for efficient data retrieval by minimizing disk I/O operations.

Indexes: Enhancing Performance

In addition to the above-mentioned structures, Oracle Database utilizes indexes to accelerate data access. An index is a data structure that provides a quick way to locate specific rows in a table based on the values in one or more columns.

Indexes are created on tables to improve query performance by reducing the amount of disk I/O required to retrieve data. They are organized in a B-tree structure, which allows for efficient searching and sorting operations.

Caches: Speeding Up Operations

To further enhance performance, Oracle Database utilizes various caches that store frequently accessed data and metadata in memory. These caches eliminate the need for disk I/O by providing quicker access to commonly used information.

One such cache is the buffer cache, which stores copies of frequently accessed data blocks. When a user requests data, Oracle Database checks if the required block is already present in the buffer cache. If so, it is quickly retrieved from memory, avoiding the need to read from disk.

Another important cache is the library cache, which stores parsed SQL statements and execution plans. This cache allows Oracle Database to reuse previously parsed statements, saving valuable time during query execution.


In summary, Oracle Database employs a sophisticated data structure consisting of tablespaces, segments, extents, and data blocks to efficiently store and retrieve data. Indexes and caches further enhance performance by speeding up query execution and reducing disk I/O operations.

Understanding this underlying data structure is crucial for database administrators and developers working with Oracle Database as it enables them to optimize schema design, improve query performance, and ensure efficient utilization of system resources.

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