namespace :insert do task do: :environment do carriers = ['a', 'b', 'c'] (1..1000000).each do |i| u = User.create(name: "name#{i}", carrier: carriers[rand(3)]) u.created_at = DateTime.now - rand(365*24*3600).second u.save end end end レコード数: 100万件 carrier(簡単のためにとりあえず"a","b","c"の3種類を取るということにした) created_atはとりあえず現在から一年以内とした(DateTime.now - rand(365*24*3600).second) 結果 クエリ select carrier, date(created_a
![MySQLのexplainとかについてしらべたときのメモ - Qiita](https://cdn-ak-scissors.b.st-hatena.com/image/square/376b9a1987716a9df90d7aca22338b49ae498236/height=288;version=1;width=512/https%3A%2F%2Fqiita-user-contents.imgix.net%2Fhttps%253A%252F%252Fcdn.qiita.com%252Fassets%252Fpublic%252Farticle-ogp-background-412672c5f0600ab9a64263b751f1bc81.png%3Fixlib%3Drb-4.0.0%26w%3D1200%26mark64%3DaHR0cHM6Ly9xaWl0YS11c2VyLWNvbnRlbnRzLmltZ2l4Lm5ldC9-dGV4dD9peGxpYj1yYi00LjAuMCZ3PTk3MiZoPTM3OCZ0eHQ9TXlTUUwlRTMlODElQUVleHBsYWluJUUzJTgxJUE4JUUzJTgxJThCJUUzJTgxJUFCJUUzJTgxJUE0JUUzJTgxJTg0JUUzJTgxJUE2JUUzJTgxJTk3JUUzJTgyJTg5JUUzJTgxJUI5JUUzJTgxJTlGJUUzJTgxJUE4JUUzJTgxJThEJUUzJTgxJUFFJUUzJTgzJUExJUUzJTgzJUEyJnR4dC1hbGlnbj1sZWZ0JTJDdG9wJnR4dC1jb2xvcj0lMjMyMTIxMjEmdHh0LWZvbnQ9SGlyYWdpbm8lMjBTYW5zJTIwVzYmdHh0LXNpemU9NTYmcz0zZDZiMmNmNTBjNmE3NmY0NGYyYjAzZDI3MzQ4NWY5YQ%26mark-x%3D142%26mark-y%3D57%26blend64%3DaHR0cHM6Ly9xaWl0YS11c2VyLWNvbnRlbnRzLmltZ2l4Lm5ldC9-dGV4dD9peGxpYj1yYi00LjAuMCZoPTc2Jnc9NzcwJnR4dD0lNDBsYXN0Y2F0XyZ0eHQtY29sb3I9JTIzMjEyMTIxJnR4dC1mb250PUhpcmFnaW5vJTIwU2FucyUyMFc2JnR4dC1zaXplPTM2JnR4dC1hbGlnbj1sZWZ0JTJDdG9wJnM9ZDE0NTI3MGQzYTdkZDUzMzU1NWIwYTgxZDdlYzQwMjk%26blend-x%3D142%26blend-y%3D486%26blend-mode%3Dnormal%26s%3D506deae3a1c454917934d75e3c82f8a8)