Relational DataBases
Relational DataBases are the most common type of database and are sometimes referred to as a SQL DBMS. A relational database presents data as rows in tables with a fixed schema (structure that defines its columns). A database most often contains one or more tables. Each table is identified by a name. Each column is identified by a name. The schema defines the type of a column (numbers, strings of characters, possibly empty etc.)
RDBMS products can be quite expensive, but there are high-quality, open source options, such as PostgreSQL, MariaDB and MySQL.
Database Indexes
An index is an additional data structure that helps improve the performance of a query.
Each index must be associated with a specific table. An index consists of one or more columns, but all columns of an index must be in the same table.
Imagine an index in the database like an index of a book. By looking at the index, you can quickly identify page numbers based on the keywords.
A table may have multiple indexes.
Conversely, if an index is created with multiple columns, the additional columns are used as subsequent sort keys.