Inconsistent MIN aggregation function behavior with conditional MATCH clauses

[dracoooooo]

Description:

When executing Cypher queries that involve the MIN aggregation function with conditional MATCH clauses, the function exhibits inconsistent behavior. Specifically, under certain conditions where no matches are found, MIN returns null in one scenario and an empty set in another, contrary to the documented behavior.

Steps to Reproduce:

1. Create Nodes:

   CREATE (n1:N1 {v: 1});
   CREATE (n2:N2 {v: 2});

2. Execute First MATCH Query:

   MATCH (n1:N1), (n2:N2)
   WHERE (1 = 2)
   RETURN MIN(1);

3. Execute Second MATCH Query:

   MATCH (n1:N1), (n2:N2)
   WHERE (1 = 2 AND n1.v = n2.v)
   RETURN MIN(1);

Expected Behavior:

Both queries should return null when the WHERE condition evaluates to false.

Actual Behavior:

• The first query returns null as expected.
• The second query returns an empty set, which is inconsistent with the documentation.

[swilly22]

Hi @dracoooooo thank you for sharing this with us,
Looking at the generated execution plan for the second query:

GRAPH.explain g "MATCH (n1:N1), (n2:N2) WHERE (1 = 2 AND n1.v = n2.v) RETURN MIN(1)"
1) "Results"
2) "    Filter"
3) "        Aggregate"
4) "            Value Hash Join | property(n1,v,0) = property(n2,v,0)"
5) "                Node By Label Scan | (n1:N1)"
6) "                Node By Label Scan | (n2:N2)"

We see an unnecessary operation Value Hash Join as the condition after reduction evaluates to simply false.
We'll look into it and make sure the Value Hash Join operation isn't utilized in such cases.