Primary and Secondary Indexes

1. Primary Index:

   • The primary index is a type of index that is automatically created on the primary key of a table by the database management system (DBMS) itself.

   • The primary key is a unique identifier for each row in the table, and the primary index is built based on this key to ensure that each row in the table can be quickly and uniquely located.

   • Since the primary key must be unique for each row, the primary index provides an efficient way to enforce uniqueness and ensure data integrity.

   • The primary index is typically a clustered index, meaning that it determines the physical order of data storage on disk. In a clustered index, the data rows are physically organized based on the primary key, which can improve the retrieval speed for queries involving the primary key.

2. Secondary Index:

   • A secondary index is an index created on a column or set of columns that are not the primary key of the table.

   • Unlike the primary index, the secondary index is not automatically created and must be explicitly defined by the database administrator or developer.

   • The secondary index allows for fast retrieval of rows based on the indexed column(s) values. It enables efficient querying even if the search criteria do not involve the primary key.

   • Secondary indexes are non-clustered, meaning they do not determine the physical order of data storage on disk. Instead, they contain pointers to the actual data rows in the table.

   • Since a table can have multiple secondary indexes, it can improve the performance of a wide range of query conditions that involve different columns.

The primary index is automatically created on the primary key of a table and enforces uniqueness and data integrity. It is typically a clustered index. On the other hand, secondary indexes are explicitly defined on non-primary key columns and allow for fast retrieval of rows based on different search criteria. They are non-clustered and can enhance the performance of a variety of query conditions. Both primary and secondary indexes are essential for optimizing data retrieval operations in databases and improving overall query performance.