FetchMode

FetchMode is a concept often encountered in object-relational mapping (ORM) frameworks, particularly in Java and other programming languages. It dictates how associated entities are loaded from a database when the parent entity is retrieved. Understanding fetchMode is crucial for optimizing database performance and managing application memory.

There are typically two primary fetch modes: Eager Fetch and Lazy Fetch. Eager Fetch, also known as immediate loading, means that when a parent entity is fetched, all its associated entities are immediately loaded from the database as well. This can be convenient as it ensures all necessary data is available without further database calls, but it can lead to over-fetching, where more data than immediately needed is retrieved, potentially impacting performance.

Lazy Fetch, conversely, involves delayed loading. Associated entities are only loaded from the database when they are explicitly accessed. This means that initially, only the parent entity is fetched. Subsequent access to a related entity triggers a separate database query to retrieve it. Lazy loading is generally more efficient for performance as it avoids fetching unnecessary data. However, it can result in the "N+1 select problem" if not managed carefully, where fetching a collection of parent entities leads to N additional queries for their associated entities, plus the initial query for the parents.

The choice between Eager and Lazy Fetch depends on the specific use case and the expected access patterns of the data. ORM frameworks often provide mechanisms to configure fetch modes at the entity mapping level, allowing developers to fine-tune performance.