ruby 43 lines · 7 steps

Logging slow SQL queries in Rails

A module that subscribes to ActiveRecord's SQL notifications and warns whenever a query crosses a duration threshold.

Explained by highlit

Rails Intermediate instrumentation logging pub-sub observability performance

1module SlowQueryLogger
SLOW_QUERY_THRESHOLD_MS = 200.0
IGNORED_PAYLOAD_NAMES = %w[SCHEMA TRANSACTION].freeze
4 
module_function
6 
def subscribe!
  ActiveSupport::Notifications.subscribe("sql.active_record") do |*args|
    event = ActiveSupport::Notifications::Event.new(*args)
    log_if_slow(event)
  end
end
13 
def log_if_slow(event)
  return if event.duration < SLOW_QUERY_THRESHOLD_MS
  return if IGNORED_PAYLOAD_NAMES.include?(event.payload[:name])
  return if event.payload[:cached]
18 
  Rails.logger.warn do
    binds = format_binds(event.payload[:type_casted_binds])
    <<~LOG.squish
      [SlowQuery] #{event.duration.round(1)}ms
      name=#{event.payload[:name].inspect}
      sql=#{event.payload[:sql].squish.truncate(500)}
      binds=#{binds}
      source=#{query_source}
    LOG
  end
end
30 
def format_binds(binds)
  return "[]" if binds.blank?
33 
  casted = binds.respond_to?(:call) ? binds.call : binds
  casted.map { |value| value.inspect.truncate(50) }.join(", ")
end
37 
def query_source
  Rails.backtrace_cleaner.clean(caller).find do |line|
    line.include?("/app/")
  end || "unknown"
end
43end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1ActiveSupport::Notifications lets you observe SQL without patching ActiveRecord internals.
2Guard clauses filter out noise like cached hits and schema queries before you pay logging costs.
3Passing a block to Rails.logger.warn defers expensive string building until the level is actually enabled.

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