Class: RailsErrorDashboard::Services::NplusOneDetector

Inherits:
Object
  • Object
show all
Defined in:
lib/rails_error_dashboard/services/n_plus_one_detector.rb

Overview

Pure service: Detect N+1 query patterns from SQL breadcrumbs

Analyzes already-captured breadcrumbs at display time (NOT on every request). Groups SQL queries by normalized fingerprint and flags patterns where the same query shape appears >= threshold times.

SAFETY: O(n) over max 40 breadcrumbs, wrapped in rescue => [].

Class Method Summary collapse

Class Method Details

.call(breadcrumbs, threshold: nil) ⇒ Array<Hash>

Detect N+1 patterns in breadcrumbs

Parameters:

  • breadcrumbs (Array<Hash>) —

    Parsed breadcrumb array from JSON

  • threshold (Integer, nil) (defaults to: nil) —

    Minimum repetitions to flag (default: config or 3)

Returns:

  • (Array<Hash>) —

    Detected patterns sorted by count desc



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# File 'lib/rails_error_dashboard/services/n_plus_one_detector.rb', line 18

def call(breadcrumbs, threshold: nil)
  return [] unless breadcrumbs.is_a?(Array) && breadcrumbs.any?

  threshold ||= RailsErrorDashboard.configuration.n_plus_one_threshold || 3

  # Extract SQL breadcrumbs only
  sql_crumbs = breadcrumbs.select { |c| c["c"] == "sql" }
  return [] if sql_crumbs.empty?

  # Group by normalized fingerprint
  groups = Hash.new { |h, k| h[k] = { count: 0, total_duration_ms: 0.0, sample_query: nil } }

  sql_crumbs.each do |crumb|
    sql = crumb["m"].to_s
    next if sql.empty?

    fingerprint = normalize_sql(sql)
    group = groups[fingerprint]
    group[:count] += 1
    group[:total_duration_ms] += crumb["d"].to_f
    group[:sample_query] ||= sql
  end

  # Filter by threshold and sort by count desc
  groups
    .select { |_, v| v[:count] >= threshold }
    .map { |fingerprint, v| { fingerprint: fingerprint, count: v[:count], total_duration_ms: v[:total_duration_ms].round(2), sample_query: v[:sample_query] } }
    .sort_by { |p| -p[:count] }
rescue => e
  []
end

.normalize_sql(sql) ⇒ String

Normalize SQL for fingerprinting Replaces literal values with ? placeholders while preserving structure

Parameters:

  • sql (String) —

    Raw SQL query

Returns:

  • (String) —

    Normalized query



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# File 'lib/rails_error_dashboard/services/n_plus_one_detector.rb', line 54

def normalize_sql(sql)
  normalized = sql.to_s.dup

  # Replace single-quoted string literals with ?
  normalized.gsub!(/'[^']*'/, "?")

  # Replace IN (...) contents with single ?
  normalized.gsub!(/\bIN\s*\([^)]+\)/i, "IN (?)")

  # Replace standalone numeric literals with ?
  # Negative lookbehind for " to avoid replacing inside double-quoted identifiers
  normalized.gsub!(/(?<!")(?<!\w)\d+(?!\w)(?!")/, "?")

  normalized
rescue => e
  sql.to_s
end