Made this mistake today while writing some code to delete data out of Redis.

def delete_keys
  cursor = "0"

  loop do
    cursor, keys = redis.scan(cursor, count: SCAN_COUNT)
    keys.reject! { |key| keep_key?(key) }

    redis.del(keys) unless keys.empty?

    break if cursor == "0"
  end
end

def keep_key?(key)
  keep_keys.any? { |keep_key| key.include?(keep_key) }
end

def keep_keys
  flattened_keys << transaction_id
end

The mistake was in the final method keep_keys. Every check to see if a key should be rejected I was adding an element to the flattened_keys array over and over again, causing my deletion to slow down over time. A simple change to memoize the keep keys made the process go from never finishing to, completing in a few seconds.

def keep_keys
  @keep_keys ||= (flattened_keys << transaction_id)
end