Neo4j Query Tuning Skill

When to Use

• Query takes unexpectedly long; need root-cause analysis
• EXPLAIN/PROFILE output in hand — needs interpretation
• Identifying which index is missing or unused
• Deciding between slotted / pipelined / parallel runtimes
• Monitoring live queries: SHOW QUERIES, SHOW TRANSACTIONS
• Cardinality estimates wrong (plan replanning needed)

When NOT to Use

• Writing Cypher from scratch → neo4j-cypher-skill
• GDS algorithm performance → neo4j-gds-skill
• Schema design / data modelling → neo4j-modeling-skill

EXPLAIN vs PROFILE

Run PROFILE twice — first run warms page cache; second gives representative metrics.

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EXPLAIN MATCH (p:Person {email: $email}) RETURN p.name
PROFILE MATCH (p:Person {email: $email}) RETURN p.name

Query API alternative (no driver):

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curl -X POST https://<host>/db/<db>/query/v2 \
  -u <user>:<pass> -H "Content-Type: application/json" \
  -d '{"statement": "EXPLAIN MATCH (p:Person {email: $email}) RETURN p.name", "parameters": {"email": "a@b.com"}}'

Key Plan Metrics

Read plans bottom-up — leaf operators at bottom initiate data retrieval.

Operator Reference

Full operator reference → references/plan-operators.md

Diagnostic Workflow (Agent Runbook)

Step 1 — Baseline Plan

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EXPLAIN <query>

Scan output for AllNodesScan, NodeByLabelScan, CartesianProduct, Eager.

Step 2 — Check Indexes

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SHOW INDEXES YIELD name, type, labelsOrTypes, properties, state
WHERE state = 'ONLINE'

Find whether the label/property from the bad operator has an index.

Step 3 — Create Missing Index

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// RANGE index for equality/range predicates:
CREATE INDEX person_email IF NOT EXISTS FOR (n:Person) ON (n.email)
// TEXT index for CONTAINS/ENDS WITH:
CREATE TEXT INDEX person_bio IF NOT EXISTS FOR (n:Person) ON (n.bio)
// Composite for multi-property lookup:
CREATE INDEX order_status_date IF NOT EXISTS FOR (n:Order) ON (n.status, n.createdAt)

Wait for state = 'ONLINE' before measuring.

Step 4 — Profile After Fix

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PROFILE <query>

Compare dbHits and elapsed ms before/after. Target: NodeIndexSeek replaces scan operators.

Step 5 — Stale Statistics (if estimatedRows wildly off)

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CALL db.prepareForReplanning()
// or resample a specific index:
CALL db.resampleIndex("person_email")
// or resample all outdated:
CALL db.resampleOutdatedIndexes()

Config: dbms.cypher.statistics_divergence_threshold (default 0.75 — plan expires when stat changes >75%).

Fixing Common Plan Problems

Missing Index → NodeByLabelScan / AllNodesScan

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// Force index hint when planner ignores it:
MATCH (p:Person {email: $email})
USING INDEX p:Person(email)
RETURN p.name
// Force label scan (sometimes faster for high selectivity):
MATCH (p:Person {email: $email})
USING SCAN p:Person
RETURN p.name

Wrong Anchor — Planner Picks Wrong Starting Node

Reorder MATCH or use hints:

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// Force join at specific node:
MATCH (a:Author)-[:WROTE]->(b:Book)-[:IN_CATEGORY]->(c:Category {name: $cat})
USING JOIN ON b
RETURN a.name, b.title

CartesianProduct — Two Unconnected MATCHes

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// Bad (Cartesian product):
MATCH (a:Author {id: $aid})
MATCH (b:Book  {id: $bid})
RETURN a.name, b.title

// Good (explicit join or WITH):
MATCH (a:Author {id: $aid})-[:WROTE]->(b:Book {id: $bid})
RETURN a.name, b.title
// Or: WITH between them to reset planning context

Eager — Read/Write Conflict

Three strategies (pick simplest):

1. Add specific labels to MATCH nodes so planner distinguishes read/write sets
2. Collect-then-write: WITH collect(n) AS nodes UNWIND nodes AS n SET n.x = 1
3. CALL IN TRANSACTIONS: isolates each batch in its own transaction

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CYPHER 25
MATCH (p:Person) WHERE p.score > 100
CALL (p) { SET p.tier = 'gold' } IN TRANSACTIONS OF 1000 ROWS

Expensive CONTAINS / ENDS WITH

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// Needs TEXT index (RANGE does NOT support these):
CREATE TEXT INDEX person_bio IF NOT EXISTS FOR (n:Person) ON (n.bio)
MATCH (p:Person) WHERE p.bio CONTAINS $keyword RETURN p.name

Over-Traversal — Push LIMIT Early

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// Bad: LIMIT after expensive join
MATCH (a:Author)-[:WROTE]->(b:Book)-[:REVIEWED_BY]->(r:Review)
RETURN a.name, b.title, r.text LIMIT 10

// Good: anchor limit before fan-out
MATCH (a:Author)-[:WROTE]->(b:Book)
WITH a, b LIMIT 10
MATCH (b)-[:REVIEWED_BY]->(r:Review)
RETURN a.name, b.title, r.text

Cypher Runtime Selection

Pipelined is default for most queries. Parallel requires dbms.cypher.parallel.worker_limit configured; available on Enterprise and Aura Pro 2025+.

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// Force parallel for large aggregation:
CYPHER 25 runtime=parallel
MATCH (n:Transaction) WHERE n.amount > 1000
RETURN n.currency, count(*), sum(n.amount)

Query Monitoring Commands

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// Live queries + resource usage:
SHOW QUERIES YIELD query, queryId, elapsedTimeMillis, allocatedBytes, status, username

// Running transactions:
SHOW TRANSACTIONS YIELD transactionId, currentQuery, currentQueryProgress, elapsedTime, status, username, cpuTime, activeLockCount  // currentQueryProgress added [2026.03]

// Kill a specific transaction:
TERMINATE TRANSACTION $transactionId

// Kill a query:
TERMINATE QUERY $queryId

// Graph count stats (node/rel counts by label/type — feed into planner):
CALL db.stats.retrieve('GRAPH COUNTS') YIELD section, data RETURN section, data

// Token stats (label/property/rel-type IDs):
CALL db.stats.retrieve('TOKENS') YIELD section, data RETURN section, data

Full monitoring reference → references/stats-and-monitoring.md

Checklist

• Run EXPLAIN first — identifies plan problems without execution cost
• Check for AllNodesScan / NodeByLabelScan — missing index
• Check for CartesianProduct — missing join predicate
• Check for Eager — read/write conflict
• SHOW INDEXES — confirm relevant index exists and state = 'ONLINE'
• Create missing index; wait for ONLINE
• Run PROFILE twice — first warms cache, second is representative
• Compare dbHits before/after fix
• If estimatedRows wildly off → CALL db.prepareForReplanning()
• Push LIMIT / WITH n LIMIT k before high-fanout operations
• For CONTAINS/ENDS WITH — TEXT index, not RANGE
• For large analytical queries — consider runtime=parallel
• Kill long-running queries with TERMINATE TRANSACTION