I have the following query in Neo4J Cypher 2.0:
MATCH (u:User{uid:'1111111'}), (c1:Concept), (c2:Concept),
c1-[:BY]->u, c2-[:BY]->u, c1-[rel:TO]->c2
WITH c1,c2,rel
MATCH c1-[:AT]->ctx, c2-[:AT]-ctx
WHERE ctx.uid = rel.context
RETURN c1.uid AS source_id, c1.name AS source_name,
c2.uid AS target_id, c2.name AS target_name,
rel.uid AS edge_id,
rel.context AS context_id, ctx.name AS context_name;
What it does is that it looks for all the nodes of the Concept label (c1 and c2) connected to User node u, finds their (c1 to c2) connections to one another (rel), then it tries to find which different contexts (ctx) those concept nodes (c1 and c2) appear in, but only those, whose uid matches the uid of the .context property of the relationships rel (rel.context) and then returns them in a table, where we have the source id and name, the target id and name, the connection id, as well as the .context id property of that relation and the name of the context with that id.
So all works fine, but the question is: WHY?
I mean how does Cypher matches so neatly the right ctx.uid to the right rel.context to know that it should be inserted exactly at the right place of the results table?
Can somebody explain me the magic behind this?
Or am I completely wrong and just getting messy results?
Thank you!
It creates a pattern-graph that represents your combined match patterns. And then it uses indexes to find bound nodes that it starts to apply the pattern graph to and returns a result row for every match found.
While applying the pattern graph it uses your WHERE conditions to filter out paths that you don't want eagerly as early as possible.
If it can't find bound nodes it has to go over all nodes of a label (like :Concept) or over all nodes of the graph (if you haven't specify any label or lookup condition).
Related
I'm wondering, when I have read the data of a node and I want to match it in another query, which way will have the best performance? Using id like this:
MATCH (n) WHERE ID(n) = 1234
or using indices of the node:
MATCH (n:Label {SomeIndexProperty: 3456})
Which one is better?
IDs are a technical ID for Neo4j, and those should not be used as a primary key for your application.
Every node (and relationship) has a technical ID, and it's stable over time.
But if you delete a node, for example the node 32, Neo4j will reuse this ID for a new node.
So you can use it in your queries inside the same transaction (there is no problem), otherwise you should know what you are doing.
The only way to retrieve the technical ID, is to use the function ID like you do on your first query : MATCH (n) WHERE ID(n) = 1234 RETURN n.
The ID is not exposed as a node's property, so you can't do MATCH (n {ID:1234}) RETURN n.
You have noticed that if you want to do a WHERE on a strict equality, you can do put the condition directly on the node.
For example :
MATCH (n:Node) WHERE n.name = 'logisima' RETURN n
MATCH (n:Node {name:'logisima'}) RETURN n
Those two queries are identicals, they generate the same query plan, it's just a syntactic sugar.
Is it faster to retrieve a node by its ID or by an indexed property ?
The easier way to know the answer to this question is to profile the two queries.
On the one based on the ID, you will see the box NodeByIdSeek that cost 1 db hit, and on the one with a unique constrainst you will see the box NodeUniqueIndexSeek with 2 db hits.
So searching a node by its ID is faster.
I have a huge database of size 260GB, which is storing a ton of transaction information. It has Agent, Customer,Phone,ID_Card as the nodes. Relationships are as follows:
Agent_Send, Customer_Send,Customer_at_Agent, Customer_used_Phone,Customer_used_ID.
A single agent is connected to many customers .And hence hitting the agent node while querying a path is not feasible. Below is my query:
match p=((ph: Phone {Phone_ID : "3851308.0"})-[r:Customer_Send
| Customer_used_ID | Customer_used_Phone *1..5]-(n2))
with nodes(p) as ns
return extract (node in ns | Labels(node) ) as Labels
I am starting with a phone number and trying to extract a big "Customer" network. I am intentionally not touching the "Customer_at_Agent" relationship in the above networked query as it is not optimal as far as performance is concerned.
So, the idea is to extract all the "Customer" labeled nodes from the path and match it with [Customer_at_Agent] relationship.
For instance , something like:
match p=((ph: Phone {Phone_ID : "3851308.0"})-[r:Customer_Send
| Customer_used_ID | Customer_used_Phone *1..5]-(n2))
with nodes(p) as ns
return extract (node in ns | Labels(node) ) as Labels
of "type customer as c "
optional match (c)-[r1:Customer_at_Agent]-(n3)
return distinct p,r1
I am still new to neo4j and cypher and I am not able to figure out a hack to extract only "customer" nodes from the path and use that in the optional match.
Thanks in advance.
Use filter notation instead of extract and you can drop any nodes that aren't labelled right. Try out this query instead:
MATCH p = (ph:Phone {Phone_ID : "3851308.0"}) - [:Customer_Send|:Customer_used_ID|:Customer_used_Phone*1..5] - ()
WITH ph, [node IN NODES(p) WHERE node:Customer] AS customer_nodes
UNWIND customer_nodes AS c_node
OPTIONAL MATCH (c_node) - [r1:Customer_at_Agent] - ()
RETURN ph, COLLECT(DISTINCT r1)
So the second line takes the phone number and the path generated and gives you a list of nodes that have the Customer label as customer_nodes. You then unwind this list so you have individual nodes you can use in path matching. Line 4 performs your optional match and finds the r1 you're interested in, then line 5 will return the phone number node you started with and a collection of all of the r1 relationships that you found on customer nodes hooked up to that phone number.
UPDATE: I added some modifications to clean up your first query line as well. If you aren't going to use an alias (like r or n2 in the first line), then don't assign them in the first place; they can affect performance and cause confusion. Empty nodes and relationships are totally fine if you don't actually have any restrictions to place on them. You also don't need parentheses to mark off a path; they are used as a core part of Cypher's ASCII art to signify nodes, so I find they are more confusing than helpful.
In a graph where the following nodes
A,B,C,D
have a relationship with each nodes successor
(A->B)
and
(B->C)
etc.
How do i make a query that starts with A and gives me all nodes (and relationships) from that and outwards.
I do not know the end node (C).
All i know is to start from A, and traverse the whole connected graph (with conditions on relationship and node type)
I think, you need to use this pattern:
(n)-[*]->(m) - variable length path of any number of relationships from n to m. (see Refcard)
A sample query would be:
MATCH path = (a:A)-[*]->()
RETURN path
Have also a look at the path functions in the refcard to expand your cypher query (I don't know what exact conditions you'll need to apply).
To get all the nodes / relationships starting at a node:
MATCH (a:A {id: "id"})-[r*]-(b)
RETURN a, r, b
This will return all the graphs originating with node A / Label A where id = "id".
One caveat - if this graph is large the query will take a long time to run.
I'm struggling with a problem despite having read a lot of documentation... I'm trying to find my graph root node (or nodes, they may be several top nodes) and counting their immediate children (all relations are typed :BELONGS_TO)
My graph looks like this (cf. attached screenshot). I have been trying the following query which works as long as the root node only has ONE incomming relationship, and it doesn not when it has more than one. (i'm not realy familiar with the cyhper language yet).
MATCH (n:Somelabel) WHERE NOT (()-[:BELONGS_TO]->(n:Somelabel)) RETURN n
Any help would be much appreciated ! (i haven't even tried to count the root nodes immediate children yet...which would be "2" according to my graph)
Correct query was given by cybersam
MATCH (n:Somelabel) WHERE NOT (n)-[:BELONGS_TO]->() RETURN n;
MATCH (n:Somelabel)<-[:BELONGS_TO]-(c:Somelabel)
WHERE NOT (n)-[:BELONGS_TO]->() RETURN n, count(c);
Based on your diagram, it looks like you are actually looking for "leaf" nodes. This query will search for all Somelabel nodes that have no outgoing relationships, and return each such node along with a count of the number of distinct nodes that have a relationship pointing to that node.
MATCH (n:Somelabel)
WHERE NOT (n)-[:BELONGS_TO]->()
OPTIONAL MATCH (m)-[:BELONGS_TO]->(n)
RETURN n, COUNT(DISTINCT m);
If you are actually looking for all "root" nodes, your original query would have worked.
As a sanity check, if you have a specific node that you believe is a "leaf" node (let's say it has an id value of 123), this query should return a single row with null values for r and m. If you get non-null results, then you actually have outgoing relationships.
MATCH (n {id:123})
OPTIONAL MATCH (n)-[r]->(m)
RETURN r, m
I'm struggling to find a single clean, efficient Cypher query that will let me identify all distinct paths emanating from a start node such that every relationship in the path is of the same type when there are many relationship types.
Here's a simple version of the model:
CREATE (a), (b), (c), (d), (e), (f), (g),
(a)-[:X]->(b)-[:X]->(c)-[:X]->(d)-[:X]->(e),
(a)-[:Y]->(c)-[:Y]->(f)-[:Y]->(g)
In this model (a) has two outgoing relationship types, X and Y. I would like to retrieve all the paths that link nodes along relationship X as well as all the paths that link nodes along relationship Y.
I can do this programmatically outside of cypher by making a series of queries, the first to
retrieve the list of outgoing relationships from the start node, and then a single query (submitted together as a batch) for each relationship. That looks like:
START n=node(1)
MATCH n-[r]->()
RETURN COLLECT(DISTINCT TYPE(r)) as rels;
followed by:
START n=node(1)
MATCH n-[:`reltype_param`*]->()
RETURN p as path;
The above satisfies my need, but requires at minimum 2 round trips to the server (again, assuming I batch together the second set of queries in one transaction).
A single-query approach that works, but is horribly inefficient is the following single Cypher query:
START n=node(1)
MATCH p = n-[r*]->() WHERE
ALL (x in RELATIONSHIPS(p) WHERE TYPE(x) = TYPE(HEAD(RELATIONSHIPS(p))))
RETURN p as path;
That query uses the ALL predicate to filter the relationships along the paths enforcing that each relationship in the path matches the first relationship in the path. This, however, is really just a filter operation on what it essentially a combinatorial explosion of all possible paths --- much less efficient than traversing a relationship of a known, given type first.
I feel like this should be possible with a single Cypher query, but I have not been able to get it right.
Here's a minor optimization, at least non-matching the paths will fail fast:
MATCH n-[r]->()
WITH distinct type(r) AS t
MATCH p = n-[r*]->()
WHERE type(r[-1]) = t // last entry matches
RETURN p AS path
This is probably one of those things that should be in the Java API if you want it to be really performant, though.