Question on MATCH query (cartesian product)

[andrija78]

Hi,

I'm testing some MATCH queries, and found weird behavior with Cartesian product.
Am I doing something wrong, my understanding is wrong, or is this a bug?

BTW - I'm testing this since multiple match statements are not supported yet - something like:

MATCH (a:person {name:"name A"})
MATCH (b:city {name:"city 1"})
CREATE (a)-[:visited]->(:b)

I have a simple graph: 4 persons, 3 cities. Persons A, B and C visited 2 cities. Person D visited all 3 cities.

CREATE (:person{name:'name A', last_name:'last A'})
CREATE (:person{name:'name B', last_name:'last B'})
CREATE (:person{name:'name C', last_name:'last C'})
CREATE (:person{name:'name D', last_name:'last D'})

CREATE (:city{name:'city 1', country:'country 1'})
CREATE (:city{name:'city 2', country:'country 2'})
CREATE (:city{name:'city 3', country:'country 3'})


MATCH (a:person), (b:city) WHERE (a.name = 'name A' AND b.name='city 1') CREATE (a)-[:visited]->(b)
MATCH (a:person), (b:city) WHERE (a.name = 'name A' AND b.name='city 2') CREATE (a)-[:visited]->(b)


MATCH (a:person), (b:city) WHERE (a.name = 'name B' AND b.name='city 1') CREATE (a)-[:visited]->(b)
MATCH (a:person), (b:city) WHERE (a.name = 'name B' AND b.name='city 3') CREATE (a)-[:visited]->(b)


MATCH (a:person), (b:city) WHERE (a.name = 'name C' AND b.name='city 2') CREATE (a)-[:visited]->(b)
MATCH (a:person), (b:city) WHERE (a.name = 'name C' AND b.name='city 3') CREATE (a)-[:visited]->(b)

MATCH (a:person), (b:city) WHERE (a.name = 'name D' AND b.name='city 1') CREATE (a)-[:visited]->(b)
MATCH (a:person), (b:city) WHERE (a.name = 'name D' AND b.name='city 2') CREATE (a)-[:visited]->(b)
MATCH (a:person), (b:city) WHERE (a.name = 'name D' AND b.name='city 3') CREATE (a)-[:visited]->(b)

These 2 queries have a result that does not look correct:

Here I'm expecting to get all 12 visits, joined with 3 cities - 36 rows in total.
But only person D gets cartesian right with all 3 cities; persons A, B and C only get matched with city 1.
MATCH (a:person)-[:visited]->(b:city), (c:city) return a,b,c
[last A, name A, country 1, city 1, country 1, city 1]
[last A, name A, country 2, city 2, country 1, city 1]

[last B, name B, country 1, city 1, country 1, city 1]
[last B, name B, country 3, city 3, country 1, city 1]

[last C, name C, country 2, city 2, country 1, city 1]
[last C, name C, country 3, city 3, country 1, city 1]

[last D, name D, country 1, city 1, country 1, city 1]
[last D, name D, country 2, city 2, country 1, city 1]
[last D, name D, country 3, city 3, country 1, city 1]
[last D, name D, country 1, city 1, country 2, city 2]
[last D, name D, country 2, city 2, country 2, city 2]
[last D, name D, country 3, city 3, country 2, city 2]
[last D, name D, country 1, city 1, country 3, city 3]
[last D, name D, country 2, city 2, country 3, city 3]
[last D, name D, country 3, city 3, country 3, city 3]

Similarly, when applying additional filter:
MATCH (a:person)-[:visited]->(b:city), (c:city) where b.name=c.name return a,b,c
[last A, name A, country 1, city 1, country 1, city 1]
[last B, name B, country 1, city 1, country 1, city 1]
[last D, name D, country 1, city 1, country 1, city 1]
[last B, name B, country 3, city 3, country 3, city 3]
[last C, name C, country 3, city 3, country 3, city 3]
[last D, name D, country 3, city 3, country 3, city 3]

[swilly22]

Hi @andrija78
You're right RedisGraph behaviour isn't correct here, and it seems like indeed it is a problem with the cartesian product.

But one think to notice, this graph contains 9 edges, and so each edge will appear 4 times and so I would expect 27 results,

when reversing the order: MATCH (c:city), (a:person)-[:visited]->(b:city) return a,b,c
we do get the expected result-set.

Thank you for reporting!

[andrija78]

You are right; 27 rows are expected. I ran a different test before, I mixed up the numbers.

When can we expect support for multiple MATCH queries - so I can avoid Cartesian product like this:

MATCH (a:person {name:"name A"})
MATCH (b:city {name:"city 1"})
CREATE (a)-[:visited]->(:b)

[swilly22]

I'm not sure exactly when multiple MATCH clauses will be supported, as we're finalising our roadmap these days, I can say that this is defiantly on the roadmap, I think it wouldn't take too long to implement,
But please notice that multiple MATCH clauses translate into Cartesian product when there's no shared entity between the two, executing:

MATCH (a:person {name:"name A"})
MATCH (b:city {name:"city 1"})

Is the as executing:

MATCH (a:person {name:"name A"}), (b:city {name:"city 1"})

Unless I'm missing something,
I'll update here with a better estimation, as soon as out roadmap is finalised.

[swilly22]

@andrija78 just pushed a fix, could you please verify.

[andrija78]

This was fast! Thanks!
Got the right results:

MATCH (a:person)-[:visited]->(b:city), (c:city) return a,b,c
[last A, name A, country 1, city 1, country 1, city 1]
[last A, name A, country 2, city 2, country 1, city 1]
[last B, name B, country 1, city 1, country 1, city 1]
[last B, name B, country 3, city 3, country 1, city 1]
[last C, name C, country 2, city 2, country 1, city 1]
[last C, name C, country 3, city 3, country 1, city 1]
[last D, name D, country 1, city 1, country 1, city 1]
[last D, name D, country 2, city 2, country 1, city 1]
[last D, name D, country 3, city 3, country 1, city 1]
[last A, name A, country 1, city 1, country 2, city 2]
[last A, name A, country 2, city 2, country 2, city 2]
[last B, name B, country 1, city 1, country 2, city 2]
[last B, name B, country 3, city 3, country 2, city 2]
[last C, name C, country 2, city 2, country 2, city 2]
[last C, name C, country 3, city 3, country 2, city 2]
[last D, name D, country 1, city 1, country 2, city 2]
[last D, name D, country 2, city 2, country 2, city 2]
[last D, name D, country 3, city 3, country 2, city 2]
[last A, name A, country 1, city 1, country 3, city 3]
[last A, name A, country 2, city 2, country 3, city 3]
[last B, name B, country 1, city 1, country 3, city 3]
[last B, name B, country 3, city 3, country 3, city 3]
[last C, name C, country 2, city 2, country 3, city 3]
[last C, name C, country 3, city 3, country 3, city 3]
[last D, name D, country 1, city 1, country 3, city 3]
[last D, name D, country 2, city 2, country 3, city 3]
[last D, name D, country 3, city 3, country 3, city 3]

MATCH (a:person)-[:visited]->(b:city), (c:city) where b.name=c.name return a,b,c
[last A, name A, country 1, city 1, country 1, city 1]
[last B, name B, country 1, city 1, country 1, city 1]
[last D, name D, country 1, city 1, country 1, city 1]
[last A, name A, country 2, city 2, country 2, city 2]
[last C, name C, country 2, city 2, country 2, city 2]
[last D, name D, country 2, city 2, country 2, city 2]
[last B, name B, country 3, city 3, country 3, city 3]
[last C, name C, country 3, city 3, country 3, city 3]
[last D, name D, country 3, city 3, country 3, city 3]