15.1. How are queries executed?

Each query is turned into an execution plan by something called the execution planner. The execution plan tells Neo4j which operations to perform when executing the query. Two different execution planning strategies are included in Neo4j:

Rule
This planner has rules that are used to produce execution plans. The planner considers available indexes, but does not use statistical information to guide the query compilation.
Cost
This planner uses the statistics service in Neo4j to assign cost to alternative plans and picks the cheapest one. While this should lead to superior execution plans in most cases, it is still under development.

By default, Neo4j 3.1.0-SNAPSHOT will use the cost planner for some queries, but not all. You can force it to use a specific planner by using the cypher.planner configuration setting (see cypher.planner), or by prepending your query with CYPHER planner=cost or CYPHER planner=rule. Neo4j might still not use the planner you selected — not all queries are solvable by the cost planner at this point.

You can see which planner was used by looking at the execution plan.

[Note]Note

When Cypher is building execution plans, it looks at the schema to see if it can find indexes it can use. These index decisions are only valid until the schema changes, so adding or removing indexes leads to the execution plan cache being flushed.