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Filter results in query before they are returned with Neo4j Cypher

I have a query that can return a potentially huge number of rows. It is extremely critical that it is as fast as possible.

Is there a way to change what is RETURNED based on what is matched? Here is a simplified version of the query:

MATCH p=(x)-[*]->(y) WHERE x.n="a"
WITH p,y OPTIONAL MATCH (y)<-[]-(z)
RETURN DISTINCT p,z

If the OPTIONAL MATCH finds results then there is no need for the large set 'p' to be returned.

What I would like to do with the results is:

IF z has results THEN RETURN z ELSE RETURN p

Thanks!

Solution

  • This query should work (it essentially produces 2 kinds of results):

    MATCH p=(x)-[*]->(y) WHERE x.n="a"
OPTIONAL MATCH (y)<--(z)
RETURN DISTINCT
  CASE WHEN z IS NULL THEN {p: p} ELSE {z: z} END AS res;

    Note that unbounded patterns like [*] can take a long time (or never finish, or run out of memory) if you have a lot of data.