Neo4j

MERGE Trap

• MERGE matches the FULL pattern—MERGE (a)-[:KNOWS]->(b) creates duplicates if relationship missing
• Safe upsert: merge nodes separately, then merge relationship
• Use ON CREATE SET and ON MATCH SET for conditional properties—without these, nothing updates on match
• For simple node upsert: MERGE (n:User {id: $id}) with unique constraint on id

Indexes

• No index on property = full label scan—always index properties used in WHERE
• Unique constraint auto-creates index—prefer constraint over plain index when applicable
• Check plan with EXPLAIN before production—look for "NodeByLabelScan" without filter pushdown
• Text search needs full-text index: CREATE FULLTEXT INDEX FOR (n:Post) ON EACH [n.title, n.body]

Variable-Length Paths

• Unbounded [*] explodes on connected graphs—always set upper bound [*1..5]
• [*0..] includes start node—usually unintended, start from [*1..]
• shortestPath() returns one path only—use allShortestPaths() for all equally short paths
• Filter inside path is expensive: [r:KNOWS* WHERE r.active] scans then filters—consider data model change

Cartesian Product

• Two disconnected patterns multiply: MATCH (a:User), (b:Product) returns rows × rows
• Connect patterns or split with WITH—unintended cartesian kills performance
• Same variable in two patterns = implicit join, no cartesian
• PROFILE query shows "CartesianProduct" operator when it happens

WITH Scope Reset

• Only variables in WITH carry forward—MATCH (a)--(b) WITH a loses b
• Aggregation forces WITH: MATCH (u:User) WITH u.country AS c, count(*) AS n
• Common mistake: filtering after aggregation requires second WITH
• Pagination: WITH n ORDER BY n.created SKIP 10 LIMIT 10

NULL Propagation

• OPTIONAL MATCH returns NULL for missing patterns—NULLs propagate through expressions
• WHERE after OPTIONAL MATCH filters out NULLs—use COALESCE() to preserve rows
• count(NULL) returns 0—useful: OPTIONAL MATCH (u)-[:REVIEWED]->(p) RETURN count(p)
• Property access on NULL throws no error, returns NULL—silent data loss

Direction

• Query direction ignored with no arrow: (a)-[:KNOWS]-(b) matches both ways
• Creation requires direction—must pick one, can't create undirected
• Wrong direction = empty results—if relationship is (a)-[:OWNS]->(b), query (b)-[:OWNS]->(a) finds nothing

Batch Operations

• Large creates in single transaction exhaust heap—use CALL {} IN TRANSACTIONS OF 1000 ROWS
• UNWIND $list AS item CREATE (n:Node {id: item.id}) for batch inserts
• apoc.periodic.iterate() for complex batch logic with progress
• Delete in batches: MATCH (n:Old) WITH n LIMIT 10000 DETACH DELETE n in loop

Parameter Injection

• Always use parameters $param not string concatenation—prevents Cypher injection
• Parameters also enable query plan caching—literal values recompile each time
• Pass as map: {param: value} in driver, :param {param: value} in browser
• List parameter for IN: WHERE n.id IN $ids