How does a graph database perform graph traversals?

Graph databases perform graph traversals by navigating connections between nodes directly, using relationships as pathways. When a traversal starts, the database begins at one or more entry points (nodes) and follows specific relationship types and directions defined in the query. For example, a query might start at a “User” node, traverse “FRIENDS_WITH” relationships, and return all connected users within three hops. The traversal engine tracks visited nodes to avoid cycles and applies filters or conditions at each step, such as checking node properties or limiting traversal depth. This process is optimized for speed because graph databases store relationships natively, avoiding costly join operations typical in relational databases.

A key mechanism in graph traversal is index-free adjacency, where nodes directly reference their connected relationships and neighboring nodes. In databases like Neo4j, each node maintains pointers to its incoming and outgoing relationships, allowing the engine to “walk” the graph without searching global indexes. For instance, finding all products purchased by customers who bought a specific item involves starting at the item node, traversing “PURCHASED_BY” relationships to customer nodes, then traversing “PURCHASED” relationships from those customers to other products. This direct navigation skips the need for index lookups, making multi-hop queries efficient. Traversal strategies like breadth-first or depth-first search are chosen based on query patterns, and some databases allow hints to optimize pathfinding.

Graph databases optimize traversals further using techniques like caching adjacency lists, parallelizing operations across relationships, and pruning paths early when they fail conditions. For example, Apache Age uses PostgreSQL’s storage with graph-specific optimizations to batch-process relationships. Queries in languages like Cypher (Neo4j) or Gremlin (Amazon Neptune) express traversals declaratively, letting the engine handle execution details. A query like MATCH (u:User)-[:FRIENDS_WITH*1..3]->(f:User) RETURN f explicitly defines traversal depth and relationship type, enabling the database to apply optimizations like limiting memory usage for intermediate results. These features make graph databases particularly effective for social networks, fraud detection, or recommendation systems where relationship-based queries are frequent and complex.