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`