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PromQL Best Practices

Comprehensive guide to writing efficient, correct, and maintainable PromQL queries.

Claude Code Knowledge Pack7/10/2026

Overview

PromQL Best Practices

Comprehensive guide to writing efficient, correct, and maintainable PromQL queries.

Metric Types and Functions
Label Filtering
Aggregations
Time Ranges
Performance Optimization
Recording Rules
Histograms and Summaries
Alerting Queries
Common Patterns

Metric Types and Functions

Counters

What they are: Metrics that only increase (or reset to zero). Examples: http_requests_total, errors_count.

Best practices:

✅ Always use rate() or increase() with counters
✅ Use rate() for per-second rates: rate(http_requests_total[5m])
✅ Use increase() for total increase: increase(http_requests_total[1h])
❌ Never use raw counter values (they always increase, not useful)
❌ Never use rate() or increase() without a range vector

Naming convention: Counters typically end with _total, _count, _sum, or _bucket.

Examples:

# Good: Calculate requests per second
rate(http_requests_total{job="api"}[5m])

# Good: Total requests in last hour
increase(http_requests_total{job="api"}[1h])

# Bad: Raw counter value
http_requests_total{job="api"}

Gauges

What they are: Metrics that can go up and down. Examples: memory_usage_bytes, temperature_celsius.

Best practices:

✅ Use gauge values directly
✅ Use avg_over_time(), max_over_time(), min_over_time() for time windows
✅ Can use delta() for change over time (but not common)
❌ Never use rate(), irate(), or increase() on gauges
❌ These functions assume monotonically increasing values

Examples:

# Good: Current memory usage
node_memory_usage_bytes{instance="prod-1"}

# Good: Average over time
avg_over_time(node_memory_usage_bytes{instance="prod-1"}[5m])

# Good: Maximum in last hour
max_over_time(node_cpu_percent{instance="prod-1"}[1h])

# Bad: Rate on gauge
rate(memory_usage_bytes[5m])

Histograms

What they are: Multiple time series representing bucketed observations. Metrics end with _bucket, _sum, _count.

Best practices:

✅ Use histogram_quantile() to calculate quantiles
✅ Always include le label in by() clause
✅ Use rate() on bucket metrics
✅ Aggregate before calculating quantiles
❌ Never average pre-calculated quantiles

Examples:

# Good: Calculate 95th percentile latency
histogram_quantile(0.95,
  sum by (job, le) (
    rate(http_request_duration_seconds_bucket{job="api"}[5m])
  )
)

# Good: Calculate average from histogram
rate(http_request_duration_seconds_sum{job="api"}[5m])
/
rate(http_request_duration_seconds_count{job="api"}[5m])

# Bad: Missing rate()
histogram_quantile(0.95, sum by (le) (http_request_duration_seconds_bucket))

# Bad: Missing 'le' in aggregation
histogram_quantile(0.95, sum by (job) (rate(http_request_duration_seconds_bucket[5m])))

Summaries

What they are: Pre-calculated quantiles with _sum and _count. Includes labels like quantile="0.95".

Best practices:

✅ Use _sum and _count to calculate averages
❌ Never average or aggregate pre-calculated quantiles
❌ Quantiles from summaries cannot be aggregated across instances

Examples:

# Good: Calculate average from summary
rate(http_request_duration_seconds_sum[5m])
/
rate(http_request_duration_seconds_count[5m])

# Bad: Averaging quantiles (mathematically invalid!)
avg(http_request_duration_seconds{quantile="0.95"})

Label Filtering

Always Use Specific Label Filters

Why: Reduces cardinality, improves query performance, and makes intent clear.

# Bad: No filters
http_requests_total

# Good: Specific filters
http_requests_total{job="api-service", environment="production"}

# Good: Multiple filters for precision
http_requests_total{
  job="api-service",
  environment="production",
  datacenter="us-east-1",
  instance="prod-api-1"
}

Use Exact Matches Over Regex When Possible

Why: Exact matches are faster (index lookups) vs regex (pattern matching).

# Bad: Regex for exact match
http_requests_total{status=~"200"}

# Good: Exact match
http_requests_total{status="200"}

# Regex is fine when you need it:
http_requests_total{status=~"2[0-9]{2}"}  # All 2xx status codes

Efficient Regex Patterns

# Bad: Multiple OR queries
sum(http_requests_total{path="/api/users"})
or
sum(http_requests_total{path="/api/products"})
or
sum(http_requests_total{path="/api/orders"})

# Good: Single regex with alternation
sum by (path) (
  http_requests_total{path=~"/api/(users|products|orders)"}
)

# Good: Negative regex for exclusions
http_requests_total{path!~"/health|/metrics"}

Label Matcher Operators

= : Equal to
!= : Not equal to
=~ : Regex match (fully anchored)
!~ : Regex does not match

Aggregations

Always Use `by()` or `without()` Clauses

Why: Makes output labels explicit and prevents confusion.

# Unclear: What labels will remain?
sum(rate(http_requests_total[5m]))

# Clear: Group by these labels
sum by (job, instance) (rate(http_requests_total[5m]))

# Clear: Remove only these labels
sum without (pod, container) (rate(http_requests_total[5m]))

Use `without()` for High-Cardinality Labels

Why: More maintainable when you want to keep many labels.

# Verbose: List all labels to keep
sum by (job, instance, environment, datacenter, region, cluster, zone) (metric)

# Better: Drop only the high-cardinality labels
sum without (pod, container, node) (metric)

Common Aggregation Operators

sum: Total across series
avg: Average value
min: Minimum value
max: Maximum value
count: Count of series
stddev: Standard deviation
stdvar: Standard variance
topk(N, ...): Top N series
bottomk(N, ...): Bottom N series
quantile(φ, ...): φ-quantile (0 ≤ φ ≤ 1)

Aggregation Examples

# Sum request rate per service
sum by (service) (rate(http_requests_total[5m]))

# Average CPU across all cores per node
avg by (instance) (rate(node_cpu_seconds_total[5m]))

# Top 10 pods by memory usage
topk(10, container_memory_usage_bytes)

# Count running instances per job
count by (job) (up == 1)

Time Ranges

rate() Range Selection

Rule of thumb: Use at least 4x your scrape interval.

Typical scrape interval: 15s
Minimum rate() range: [1m] (preferably [2m])

# Bad: Too short (less than 4x scrape interval)
rate(http_requests_total[30s])

# Good: At least 2 minutes
rate(http_requests_total[2m])

# Common: 5 minutes (good balance of responsiveness and stability)
rate(http_requests_total[5m])

# Longer ranges: More stable, less sensitive to spikes
rate(http_requests_total[15m])

irate() vs rate()

irate(): Instant rate, only uses last two samples.

✅ Use for high-frequency, short-range monitoring
✅ Good for rapidly changing metrics
✅ Range: [2m] to [5m] typically
❌ Don't use for long ranges (wasted range)

rate(): Average rate over entire range.

✅ Use for most cases
✅ More stable and accurate for longer ranges
✅ Better for alerting (less noisy)

# Good: irate with short range
irate(http_requests_total[2m])

# Good: rate for longer range
rate(http_requests_total[5m])

# Bad: irate with long range (only uses last 2 samples anyway!)
irate(http_requests_total[1h])

Subqueries

Syntax: query[range:resolution]

Use sparingly: Subqueries can be very expensive.

# Calculate max rate over 30 minutes with 1-minute resolution
max_over_time(
  rate(http_requests_total[5m])[30m:1m]
)

# Bad: Excessive range
max_over_time(
  rate(http_requests_total[5m])[95d:1m]
)  # Processes millions of samples!

# Better: Use recording rules for long ranges

Performance Optimization

1. Filter Early, Aggregate Late

# Good: Filter before expensive operations
sum(rate(http_requests_total{job="api", status="200"}[5m]))

# Bad: Filter after aggregation (processes more data)
sum(rate(http_requests_total[5m])) and {job="api", status="200"}

2. Use topk/bottomk to Limit Results

# Instead of processing all series:
sum by (pod) (rate(container_cpu_usage[5m]))

# Limit to top 10 in query:
topk(10, sum by (pod) (rate(container_cpu_usage[5m])))

3. Avoid High-Cardinality Labels

User IDs, request IDs, timestamps as labels = BAD
Job, instance, path, status code = OK
Keep label cardinality under 10-100 unique values when possible

4. Use Recording Rules for Complex Queries

See Recording Rules section below.

5. Minimize Regex Usage

# Slower: Regex match
{label=~"value"}

# Faster: Exact match
{label="value"}

6. Share Common Subexpressions

# Bad: Same rate calculated twice
rate(metric[5m]) / rate(metric[5m] offset 1h)

# Can't be optimized in PromQL directly, but use recording rules:
# - record: metric:rate5m
#   expr: rate(metric[5m])

# Then:
metric:rate5m / (metric:rate5m offset 1h)

Recording Rules

Purpose: Pre-compute frequently-used or expensive queries.

Benefits:

Faster dashboard loads
Lower query latency
Reduced Prometheus CPU usage
Easier to maintain complex expressions

When to use:

Query runs frequently (multiple dashboards, alerts)
Query is computationally expensive
Query spans long time ranges (subqueries)
Query is complex (multiple aggregations, joins)

Naming convention:

level:metric:operations

Examples:

job:http_requests:rate5m
instance:node_cpu:rate1m
job_instance:request_latency_seconds:mean5m

Configuration example:

groups:
  - name: example_recording_rules
    interval: 30s
    rules:
      # Basic rate recording
      - record: job:http_requests:rate5m
        expr: sum by (job) (rate(http_requests_total[5m]))

      # Error rate recording
      - record: job:http_requests:error_rate5m
        expr: |
          sum by (job) (rate(http_requests_total{status=~"5.."}[5m]))
          /
          sum by (job) (rate(http_requests_total[5m]))

      # Average latency recording
      - record: job:http_request_latency_seconds:mean5m
        expr: |
          sum by (job) (rate(http_request_duration_seconds_sum[5m]))
          /
          sum by (job) (rate(http_request_duration_seconds_count[5m]))

Histograms and Summaries

Histogram Best Practices

# Calculate quantile
histogram_quantile(0.95,
  sum by (le, job) (
    rate(http_request_duration_seconds_bucket{job="api"}[5m])
  )
)

# Always include 'le' in aggregation
sum by (job, le) (...)  # ✅ Correct
sum by (job) (...)      # ❌ Wrong - missing 'le'

# Use rate() on bucket metrics
rate(http_request_duration_seconds_bucket[5m])  # ✅ Correct
http_request_duration_seconds_bucket            # ❌ Wrong - missing rate()

Calculate Average from Histogram

rate(http_request_duration_seconds_sum[5m])
/
rate(http_request_duration_seconds_count[5m])

Count Observations

rate(http_request_duration_seconds_count[5m])

Native Histograms (Prometheus 2.40+/3.0)

Native histograms are a newer histogram format introduced in Prometheus 2.40 and made stable in 3.0. They offer significant storage and query efficiency improvements over classic histograms.

Key Differences from Classic Histograms

Classic Histograms	Native Histograms
Separate `_bucket`, `_sum`, `_count` time series	Single time series containing all data
Fixed bucket boundaries defined at instrumentation	Dynamic bucket resolution
Requires `_bucket` suffix in queries	No `_bucket` suffix needed
Always need `le` label in aggregation	No `le` label manipulation

Native Histogram Query Syntax

# Classic histogram (old way)
histogram_quantile(0.9, sum by (job, le) (rate(http_request_duration_seconds_bucket[10m])))

# Native histogram (simpler - no _bucket suffix, no 'le' label needed)
histogram_quantile(0.9, sum by (job) (rate(http_request_duration_seconds[10m])))

Native Histogram Functions

Prometheus provides special functions for native histograms:

# Calculate average from native histogram
histogram_avg(rate(http_request_duration_seconds[5m]))

# Calculate standard deviation
histogram_stddev(rate(http_request_duration_seconds[5m]))

# Calculate standard variance
histogram_stdvar(rate(http_request_duration_seconds[5m]))

# Get observation count
histogram_count(rate(http_request_duration_seconds[5m]))

# Get sum of observations
histogram_sum(rate(http_request_duration_seconds[5m]))

# Get fraction of observations in a range
histogram_fraction(0.1, 0.5, rate(http_request_duration_seconds[5m]))

Best Practices for Native Histograms

Still use rate() with native histograms - The histogram functions work with rate-aggregated data

# ✅ Correct
histogram_avg(rate(http_request_duration_seconds[5m]))

# ❌ Wrong - missing rate()
histogram_avg(http_request_duration_seconds)

Simpler aggregation - No need for le label in by() clause

# Classic histogram - need 'le'
histogram_quantile(0.95, sum by (job, le) (rate(metric_bucket[5m])))

# Native histogram - no 'le' needed
histogram_quantile(0.95, sum by (job) (rate(metric[5m])))

Enable native histograms in Prometheus - Requires configuration:

# prometheus.yml
global:
  scrape_native_histograms: true

Check if metrics are native or classic - Query the metric directly to see its format in the response

When to Use Native Histograms

✅ New projects starting with Prometheus 2.40+
✅ High-cardinality histogram data (storage efficiency)
✅ When you need many quantile calculations (query efficiency)
❌ Legacy systems that don't support native histograms
❌ When you need exact bucket boundaries for compliance

Alerting Queries

Keep Alert Expressions Simple

# Bad: Complex alert expression
alert: HighErrorRate
expr: |
  (
    sum by (job) (rate(http_requests_total{status=~"5.."}[5m]))
    /
    sum by (job) (rate(http_requests_total[5m]))
  ) > 0.05

# Better: Use recording rule, simple alert
# Recording rule:
- record: job:http_requests:error_rate5m
  expr: ...

# Alert:
alert: HighErrorRate
expr: job:http_requests:error_rate5m > 0.05

Use `for` Clause to Avoid Flapping

- alert: HighMemoryUsage
  expr: node_memory_usage_percent > 90
  for: 5m  # Must be true for 5 minutes
  annotations:
    summary: "High memory usage on {{ $labels.instance }}"

Alert on Rate of Change

# Alert if request rate drops sudd

PromQL Best Practices

Overview

PromQL Best Practices

Table of Contents

Metric Types and Functions

Counters

Gauges

Histograms

Summaries

Label Filtering

Always Use Specific Label Filters

Use Exact Matches Over Regex When Possible

Efficient Regex Patterns

Label Matcher Operators

Aggregations

Always Use by() or without() Clauses

Use without() for High-Cardinality Labels

Common Aggregation Operators

Aggregation Examples

Time Ranges

rate() Range Selection

irate() vs rate()

Subqueries

Performance Optimization

1. Filter Early, Aggregate Late

2. Use topk/bottomk to Limit Results

3. Avoid High-Cardinality Labels

4. Use Recording Rules for Complex Queries

5. Minimize Regex Usage

6. Share Common Subexpressions

Recording Rules

Histograms and Summaries

Histogram Best Practices

Calculate Average from Histogram

Count Observations

Native Histograms (Prometheus 2.40+/3.0)

Key Differences from Classic Histograms

Native Histogram Query Syntax

Native Histogram Functions

Best Practices for Native Histograms

When to Use Native Histograms

Alerting Queries

Keep Alert Expressions Simple

Use for Clause to Avoid Flapping

Alert on Rate of Change

Always Use `by()` or `without()` Clauses

Use `without()` for High-Cardinality Labels

Use `for` Clause to Avoid Flapping