Reproducing Mongoid 7.3.3 simple find possible performance issue

idhack-vs-IXSCAN.rb

require 'mongoid'

Mongoid.configure do |config|
  config.clients.default = {
    hosts: ['localhost:27017'],
    database: 'junk_db',
  }
  config.raise_not_found_error = false
end

class Cat
  include Mongoid::Document
end

logger = Logger.new(STDOUT)
logger.level = :debug
Mongoid.logger = logger
Mongo::Logger.logger = logger


Cat.collection.drop
cat_id = BSON::ObjectId.from_string('620e1d1e4ca89036e25d2b6f')
Cat.collection.insert_one({ _id: cat_id })

# According to STDOUT logger, the following will use:
# {"find"=>"cats", "filter"=>{"_id"=>BSON::ObjectId('620e1d1e4ca89036e25d2b6f'), "$and"=>[{"_id"=>BSON::ObjectId('620e1d1e4ca89036e25d2b6f')}]}
#
# corresponding mongodb log, shows IXSCAN usage:
# "msg":"Only one plan is available; it will be run but will not be cached","attr":{"query":"ns: junk_db.cats query: { _id: ObjectId('620e1d1e4ca89036e25d2b6f'), $and: [ { _id: ObjectId('620e1d1e4ca89036e25d2b6f') } ] } sort: {} projection: {}","planSummary":"IXSCAN { _id: 1 }"}}
Cat.find(cat_id)

# According to STDOUT logger, the following will use:
# {"find"=>"cats", "filter"=>{"_id"=>BSON::ObjectId('620e1d1e4ca89036e25d2b6f')}
#
# corresponding mongodb log, shows idhack usage:
# msg":"Using idhack: {canonicalQuery_Short}","attr":{"canonicalQuery_Short":"ns: junk_db.cats query: { _id: ObjectId('620e1d1e4ca89036e25d2b6f') } sort: {} projection: {}"}}
Cat.collection.find({_id: cat_id}).to_a