How to create Index-organized table with desc order

To create an Index-Organized Table (IOT) sorted in descending order (DESC) in Oracle Database, follow these steps:

1. Define the Table Structure: First, define the table structure for the Index-Organized Table (IOT), specifying which columns are key columns as they will serve as the primary key and influence the physical storage order of the data.

2. Create the Primary Key Index: When creating an Index-Organized Table (IOT), specify a primary key and explicitly define the sorting order. In Oracle, to set the index in descending order, append the keyword DESC to the column.

Here is a specific SQL example demonstrating how to create an Index-Organized Table sorted in descending order:

sql
CREATE TABLE employees_iot (
  employee_id NUMBER(6),
  first_name VARCHAR2(20),
  last_name VARCHAR2(25),
  email VARCHAR2(25),
  hire_date DATE,
  job_id VARCHAR2(10),
  salary NUMBER(8,2),
  PRIMARY KEY (employee_id DESC)
)
ORGANIZATION INDEX;

In this example:

• The table employees_iot is an Index-Organized Table (IOT).
• PRIMARY KEY (employee_id DESC) defines employee_id as the primary key and specifies the descending order (DESC).

Notes:

• Data in an Index-Organized Table (IOT) is physically stored based on the primary key index, resulting in efficient insertion and query operations, particularly for primary key operations.
• Choosing a descending order may affect the query optimizer's choice and performance, so the sorting order should be determined based on specific query requirements during design.
• Index-Organized Tables (IOTs) are particularly suitable for applications that frequently require full key-value queries, such as Online Transaction Processing (OLTP) systems.

Using Index-Organized Tables (IOTs) can improve data retrieval speed, but they typically require more maintenance, such as additional index rebuild operations during bulk data updates. Therefore, before deciding to use Index-Organized Tables, carefully consider the application scenario and maintenance costs.