Graph Traversal

Anchor of a query

When the execution plan is created, it determines the set of nodes that will be the starting points for the query. The anchor for a query is often based upon a MATCH clause. The anchor is typically determined by meta-data that is stored in the graph or a filter that is provided inline or in a WHERE clause. The anchor for a query will be based upon the fewest number of nodes that need to be retrieved into memory.

Next, we will look at some examples of how queries are anchored based upon the hueristics used by the graph engine.

PROFILE MATCH (p:Person)-[:ACTED_IN]->(m)
RETURN p.name, m.title LIMIT 100

In the above query, the Person nodes are the anchor for the query. This is because there are a total of 28,863 nodes in the graph which is what m represents. There are only 19,047 Person nodes so the execution will retrieve fewer nodes if it anchors with the Person nodes.

PROFILE MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
RETURN p.name, m.title LIMIT 100

In the above query the Movie nodes will be the anchor for the query because there are fewer Movie nodes (9,125) than Person nodes (19,047).

PROFILE MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
WHERE p.name = 'Eminem'
RETURN p.name, m.title

In the above query, a filter is specified which reduces the number of nodes that will be retrieved for the Person node. Satisfying the filter is the anchor for the query. If the Person nodes has an index on name, it only retrieves one record. If there is no index, it needs to scan/filter all Person nodes for the name property.

PROFILE
MATCH (p1:Person)-[:ACTED_IN]->(m1)
MATCH (m2)<-[:ACTED_IN]-(p2:Person)
WHERE p1.name = 'Tom Hanks'
AND p2.name = 'Meg Ryan'
AND m1 = m2
RETURN m1.title

Expand to follow paths

After the anchor nodes have been retrieved, the next step if the query specifies a path is to follow the path. The loaded, initial nodes that are part of the anchor set have pointers to relationships that point to nodes on the other end of the relationships.

The goal here is to eliminate paths from the nodes in memory to nodes that will need to be retrieved. This is where specificity in the relationship types is important in your data model.

PROFILE MATCH (m:Movie)<--(p:Person)
WHERE p.name = 'Clint Eastwood'
RETURN  m.title

This query expands to 70 rows because Clint Eastwood has 70 relationships to movies.

PROFILE MATCH (m:Movie)<-[:DIRECTED]-(p:Person)
WHERE p.name = 'Clint Eastwood'
RETURN  m.title

This query expands to fewer rows because Clint Eastwood has fewer DIRECTED relationships to movies.

Cypher queries with multiple MATCH statements may execute differently than what you may expect. You should always profile your queries to understand how the graph is traversed.

Basic query traversal

Now let’s learn a little more about how traversal works in Neo4j.

With this query:

MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
WHERE p.name = 'Eminem'
RETURN  m.title AS movies

Here is what happens when this query executes:

Query traversal using multiple patterns

Here is another query that contains two patterns:

MATCH (p:Person)-[:ACTED_IN]->(m:Movie),
(coActors:Person)-[:ACTED_IN]->(m)
WHERE p.name = 'Eminem'
RETURN m.title AS movie ,collect(coActors.name) AS coActors

Here is what happens when this query executes:

Notice that for this query, a depth-first traversal occurs.

Query traversal using multiple MATCH clauses

Here is another query that contains two MATCH clauses:

MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
WHERE p.name = 'Eminem'
MATCH (allActors:Person)-[:ACTED_IN]->(m)
RETURN m.title AS movie, collect(allActors.name) AS allActors

Here is what happens when this query executes:

Notice that for this query, a depth-first traversal occurs just as it did for the previous query. The one difference in the outcome, however is that the Eminem node is added as a result of the second MATCH.

Avoiding labels for better performance

Another graph optimization that you can take advantage of is to reduce labels used in your query patterns. Having a label for the anchor nodes in a pattern is good:

PROFILE MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
WHERE p.name = 'Tom Hanks'
RETURN m.title AS movie

The Person label for the anchor node retrieval is good here, but the label for the other side of the pattern is unnecessary. Having the label on the non-anchor node forces a label check, which is really not necessary.

Here is a more performant way to do this query:

PROFILE MATCH (p:Person)-[:ACTED_IN]->(m)
WHERE p.name = 'Tom Hanks'
RETURN m.title AS movie

With this second query, you see that there are fewer db hits.

Returning paths

In Neo4j Browser, when you return nodes, by default the relationships are visualized. For example, this query is visualized with its associated paths that were traversed for the query:

MATCH (person:Person)-[]->(movie)
WHERE person.name = 'Walt Disney'
RETURN person, movie

The visualization includes one Person node that is connected to four Movie nodes using five relationships. The objects that are returned (table view), are five rows where the Person node values are in each row and the one Movie node is repeated. There is no relationship information returned.

You can return paths in your query as follows:

MATCH p = ((person:Person)-[]->(movie))
WHERE person.name = 'Walt Disney'
RETURN p

This query returns 5 paths. If you view the objects (table view in Neo4j Browser), you will see that each row returned represents the Person node, the Movie node, and the relationship.

In some applications, it may be useful to work with path objects. Cypher has some useful functions that can be used to analyze paths: