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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: INLINECODE6

Variable-Length Paths

• - Unbounded [*] explodes on connected graphs—always set upper bound INLINECODE8
• INLINECODE9 includes start node—usually unintended, start from INLINECODE10
• INLINECODE11 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
• INLINECODE15 query shows "CartesianProduct" operator when it happens

WITH Scope Reset

• - Only variables in WITH carry forward—MATCH (a)--(b) WITH a loses INLINECODE17
• Aggregation forces WITH: INLINECODE18
• Common mistake: filtering after aggregation requires second WITH
• Pagination: INLINECODE19

NULL Propagation

• - OPTIONAL MATCH returns NULL for missing patterns—NULLs propagate through expressions
• INLINECODE21 after OPTIONAL MATCH filters out NULLs—use COALESCE() to preserve rows
• INLINECODE23 returns 0—useful: INLINECODE24
• 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 INLINECODE28
• INLINECODE29 for batch inserts
• INLINECODE30 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: INLINECODE35