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For traditional relational database management systems (RDBMS), such as Oracle and MySQL, data are stored and retrieved in the database. In Oracle, schema is used to represent the collection of logical data structures or schema objects, which are created by users and used to directly reference the data in the database. The most important objects are tables and indexes.

A table is the basic unit of data organization in a database. A table describes an entity, such as a students table. We use a table name (students) and some columns to define the entity, each column represents an attribute of the student, while the table represents a collection of rows, each row records a specific student's information.

Index is an optional data structure in the database. We can create an index on one or more columns in the table to improve the performance of data retrieval. The function of the index is to quickly locate the data we want to retrieve. In Oracle, indexes are logically and physically independent of the data, so we can delete and create indexes without any impact on other indexes or data in the table.

In Oracle, there are many kinds of tables, including related system tables and object tables, and tables are organized into heaps, indexes, external tables and so on according to their organizational characteristics.

The index is divided into primary key index, unique index, non-empty index, general index, bitmap index and so on.

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