joins - is a mechanism in Ruby on Rails that allows you to combine data from different database tables using SQL queries. They (joins) are used to retrieve information from associated models with related data.

An example of using the joins method:

class User < ApplicationRecord
  has_many :posts
end

class Post < ApplicationRecord
  belongs_to :user
end

# Get all users and all their posts

users = User.joins(:posts)

users.each do |user|
  puts "User: #{user.name}"
  user.posts.each do |post|
    puts "Post: #{post.title}"
  end
end

In the example, we use joins(:posts) to get all users along with their associated posts. This construct will create and execute an SQL query that joins the users and posts tables by the foreign key user_id.

joins allows you to efficiently retrieve data from different tables using relationships between models. joins makes your code more efficient and convenient for working with data from various tables. This may sound abstract. But to optimize (reduce) the number of database queries, you need to use the best SQL queries. joins is a query method of the Active Record Query Interface built into Ruby on Rails. In short, it is a set of methods that allows you not to write pure SQL. To understand how the methods of the Active Record Query Interface work, you need to have at least a basic knowledge of SQL.