Shortest path planning
This page contains an example of how to plan queries using the shortestPath() function.
Planning shortest paths in Cypher® can lead to different query plans depending on the predicates that need to be evaluated.
Internally, Neo4j will use a fast bidirectional breadth-first search algorithm if the predicates can be evaluated whilst searching for the path.
Therefore, this fast algorithm will always be certain to return the right answer when there are universal predicates on the path; for example, when searching for the shortest path where all nodes have the Person label, or where there are no nodes with a name property.
If the predicates need to inspect the whole path before deciding on whether it is valid or not, this fast algorithm cannot be relied on to find the shortest path, and Neo4j may have to resort to using a slower exhaustive depth-first search algorithm to find the path.
This means that query plans for shortest path queries with non-universal predicates will include a fallback to running the exhaustive search to find the path should the fast algorithm not succeed.
For example, depending on the data, an answer to a shortest path query with existential predicates — such as the requirement that at least one node contains the property name='Kevin Bacon' — may not be able to be found by the fast algorithm.
In this case, Neo4j will fall back to using the exhaustive search to enumerate all paths and potentially return an answer.
The running times of these two algorithms may differ by orders of magnitude, so it is important to ensure that the fast approach is used for time-critical queries.
When the exhaustive search is planned, it is still only executed when the fast algorithm fails to find any matching paths. The fast algorithm is always executed first, since it is possible that it can find a valid path even though that could not be guaranteed at planning time.
Please note that falling back to the exhaustive search may prove to be a very time consuming strategy in some cases; such as when there is no shortest path between two nodes.
Therefore, in these cases, it is recommended to set cypher.forbid_exhaustive_shortestpath to true, as explained in Operations Manual → Configuration settings.
Shortest path — fast algorithm
This query can be evaluated with the fast algorithm — there are no predicates that need to see the whole path before being evaluated.
PROFILE
MATCH
(KevinB:Person {name: 'Kevin Bacon'}),
(Al:Person {name: 'Al Pacino'}),
p = shortestPath((KevinB)-[:ACTED_IN*]-(Al))
WHERE all(r IN relationships(p) WHERE r.role IS NOT NULL)
RETURN pPlanner COST Runtime PIPELINED Runtime version 5.25 Batch size 128 +---------------------+------------------------------------------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+ | Operator | Details | Estimated Rows | Rows | DB Hits | Memory (Bytes) | Page Cache Hits/Misses | Time (ms) | Pipeline | +---------------------+------------------------------------------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+ | +ProduceResults | p | 2 | 1 | 0 | | 1/0 | 0.252 | | | | +------------------------------------------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+ | | +ShortestPath | p = (KevinB)-[anon_0:ACTED_IN*]-(Al) WHERE all(r IN relationships(p) WHERE r.role IS NOT NULL) | 2 | 1 | 23 | 1688 | | | In Pipeline 1 | | | +------------------------------------------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+ | +MultiNodeIndexSeek | RANGE INDEX KevinB:Person(name) WHERE name = $autostring_0, | 2 | 1 | 4 | 120 | 1/1 | 0.916 | In Pipeline 0 | | | RANGE INDEX Al:Person(name) WHERE name = $autostring_1 | | | | | | | | +---------------------+------------------------------------------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+ Total database accesses: 27, total allocated memory: 1752
Shortest path — additional predicate checks on the paths
Predicates used in the WHERE clause that apply to the shortest path pattern are evaluated before deciding what the shortest matching path is.
MATCH
(KevinB:Person {name: 'Kevin Bacon'}),
(Al:Person {name: 'Al Pacino'}),
p = shortestPath((KevinB)-[*]-(Al))
WHERE length(p) > 1
RETURN pThis query, in contrast with the one above, needs to check that the whole path follows the predicate before we know if it is valid or not, and so the query plan will also include the fallback to the slower exhaustive search algorithm.
Planner COST
Runtime PIPELINED
Runtime version 5.25
Batch size 1024
+--------------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| Operator | Details | Estimated Rows | Rows | DB Hits | Memory (Bytes) | Page Cache Hits/Misses | Time (ms) | Pipeline |
+--------------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| +ProduceResults | p | 1 | 1 | 0 | | | | |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+ | | |
| +AntiConditionalApply | | 1 | 1 | 0 | 41464 | 0/0 | 0.332 | Fused in Pipeline 6 |
| |\ +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| | +Top | anon_1 ASC LIMIT 1 | 2 | 0 | 0 | 4280 | 0/0 | 0.000 | In Pipeline 5 |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| | +Projection | length(p) AS anon_1 | 7966 | 0 | 0 | | | | |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+ | | |
| | +Filter | length(p) > $autoint_2 | 7966 | 0 | 0 | | | | |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+ | | |
| | +Projection | (KevinB)-[anon_0*]-(Al) AS p | 26554 | 0 | 0 | | | | |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+ | | |
| | +VarLengthExpand(Into) | (KevinB)-[anon_0*]-(Al) | 26554 | 0 | 0 | | | | |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+ | | |
| | +Argument | KevinB, Al | 2 | 0 | 0 | 0 | 0/0 | 0.000 | Fused in Pipeline 4 |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| +Apply | | 2 | 1 | 0 | | 0/0 | 0.026 | |
| |\ +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| | +Optional | KevinB, Al | 2 | 1 | 0 | 4840 | 0/0 | 0.134 | In Pipeline 3 |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| | +ShortestPath | p = (KevinB)-[anon_0*]-(Al) WHERE length(p) > $autoint_2 | 1 | 1 | 1 | 1760 | | | In Pipeline 2 |
| | | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| | +Argument | KevinB, Al | 2 | 1 | 0 | 24680 | 0/0 | 0.056 | In Pipeline 1 |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
| +MultiNodeIndexSeek | RANGE INDEX KevinB:Person(name) WHERE name = $autostring_0, | 2 | 1 | 4 | 120 | 2/0 | 0.644 | In Pipeline 0 |
| | RANGE INDEX Al:Person(name) WHERE name = $autostring_1 | | | | | | | |
+--------------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------------+
Total database accesses: 5, total allocated memory: 50152The way the bigger exhaustive query plan works is by using Apply/Optional to ensure that when the fast algorithm does not find any results, a null result is generated instead of simply stopping the result stream.
On top of this, the planner will issue an AntiConditionalApply, which will run the exhaustive search if the path variable is pointing to null instead of a path.
An ErrorPlan operator will appear in the execution plan in cases where:
-
dbms.cypher.forbid_exhaustive_shortestpathis set totrue. -
The fast algorithm is not able to find the shortest path.
MATCH
(KevinB:Person {name: 'Kevin Bacon'}),
(Al:Person {name: 'Al Pacino'}),
p = shortestPath((KevinB)-[*]-(Al))
WITH p
WHERE length(p) > 1
RETURN pThis query, just like the one above, needs to check that the whole path follows the predicate before we know if it is valid or not.
However, the inclusion of the WITH clause means that the query plan will not include the fallback to the slower exhaustive search algorithm.
Instead, any paths found by the fast algorithm will subsequently be filtered, which may result in no answers being returned.
Planner COST
Runtime PIPELINED
Runtime version 5.25
Batch size 128
+---------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+
| Operator | Details | Estimated Rows | Rows | DB Hits | Memory (Bytes) | Page Cache Hits/Misses | Time (ms) | Pipeline |
+---------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+
| +ProduceResults | p | 1 | 1 | 0 | | 1/0 | 0.353 | |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+ |
| +Filter | length(p) > $autoint_2 | 1 | 1 | 0 | | 0/0 | 0.255 | |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+ |
| +ShortestPath | p = (KevinB)-[anon_0*]-(Al) | 2 | 1 | 1 | 1760 | | | In Pipeline 1 |
| | +-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+
| +MultiNodeIndexSeek | RANGE INDEX KevinB:Person(name) WHERE name = $autostring_0, | 2 | 1 | 4 | 120 | 2/0 | 0.371 | In Pipeline 0 |
| | RANGE INDEX Al:Person(name) WHERE name = $autostring_1 | | | | | | | |
+---------------------+-------------------------------------------------------------+----------------+------+---------+----------------+------------------------+-----------+---------------+
Total database accesses: 5, total allocated memory: 1824