What are partial indexes in databases?

Partial (or, as they are also called, conditional) indexes are indexes that cover only a subset of rows in a table, defined by a given condition (predicate). They allow indexing only those rows that are important for a specific query, thereby reducing the size of the index and speeding up searches on these rows.

The use of partial indexes is justified in the following cases:

• When the main part of queries filters data by a specific value or range.
• When indexing all rows would be excessive or inefficient due to a large volume of data that does not satisfy frequently used predicates.
• To create a unique index for a subset of rows, which cannot be done with a regular unique index if uniqueness is required only for certain field values.

Example in PostgreSQL:

CREATE INDEX active_users_idx ON users (email) WHERE status = 'active';

In this example, the index active_users_idx is created only for rows where the value of the status field is 'active'. Queries like SELECT * FROM users WHERE status = 'active' AND email = 'test@example.com' will be significantly faster thanks to this index. Queries without the WHERE status = 'active' condition or with a different status value will not use this partial index.

Advantages:

• Reduction in index size.
• Lower write costs (less data to update in the index).
• Improved query performance using the partial index predicate.

Disadvantages:

• The index is only used for queries that exactly match its predicate.
• It may complicate understanding which index will be used by the query planner.