使用materialization

explain select `countries`.`id` AS `id`,`countries`.`sortname` AS `sortname`,`countries`.`name` AS `name`,`countries`.`phonecode` AS `phonecode` from `countries` where `countries`.`id` in (/* select#2 */ select `states`.`country_id` from `states`);

+----+--------------+-------------+--------+---------------+------------+---------+-------------------+------+-------------+
| id | select_type  | table       | type   | possible_keys | key        | key_len | ref               | rows | Extra       |
+----+--------------+-------------+--------+---------------+------------+---------+-------------------+------+-------------+
|  1 | SIMPLE       | countries   | ALL    | PRIMARY       | NULL       | NULL    | NULL              |  246 | Using where |
|  1 | SIMPLE       | <subquery2> | eq_ref | <auto_key>    | <auto_key> | 4       | test.countries.id |    1 | NULL        |
|  2 | MATERIALIZED | states      | ALL    | NULL          | NULL       | NULL    | NULL              | 4120 | NULL        |
+----+--------------+-------------+--------+---------------+------------+---------+-------------------+------+-------------+

QUERY: explain select `countries`.`id` AS `id`,`countries`.`sortname` AS `sortname`,`countries`.`name` AS `name`,`countries`.`phonecode` AS `phonecode` from `countries` where `countries`.`id` in ( select `states`.`country_id` from `states`)
                            TRACE: {
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "join_preparation": {
              "select#": 2,
              "steps": [
                {
                  "expanded_query": "/* select#2 */ select `states`.`country_id` from `states`"
                },
                {
                  "transformation": {
                    "select#": 2,
                    "from": "IN (SELECT)",
                    "to": "semijoin",
                    "chosen": true
                  }
                }
              ]
            }
          },
          {
            "expanded_query": "/* select#1 */ select `countries`.`id` AS `id`,`countries`.`sortname` AS `sortname`,`countries`.`name` AS `name`,`countries`.`phonecode` AS `phonecode` from `countries` where `countries`.`id` in (/* select#2 */ select `states`.`country_id` from `states`)"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "transformation": {
              "select#": 2,
              "from": "IN (SELECT)",
              "to": "semijoin",
              "chosen": true,
              "evaluating_constant_semijoin_conditions": [
              ]
            }
          },
          {
            "transformations_to_nested_joins": {
              "transformations": [
                "semijoin"
              ],
              "expanded_query": "/* select#1 */ select `countries`.`id` AS `id`,`countries`.`sortname` AS `sortname`,`countries`.`name` AS `name`,`countries`.`phonecode` AS `phonecode` from `countries` semi join (`states`) where (1 and (`countries`.`id` = `states`.`country_id`))"
            }
          },
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "(1 and (`countries`.`id` = `states`.`country_id`))",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(1 and multiple equal(`countries`.`id`, `states`.`country_id`))"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(1 and multiple equal(`countries`.`id`, `states`.`country_id`))"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "multiple equal(`countries`.`id`, `states`.`country_id`)"
                }
              ]
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`countries`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              },
              {
                "table": "`states`",
                "row_may_be_null": false,
                "map_bit": 1,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`countries`",
                "field": "id",
                "equals": "`states`.`country_id`",
                "null_rejecting": false
              }
            ]
          },
          {
            "pulled_out_semijoin_tables": [
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`countries`",
                "table_scan": {
                  "rows": 246,
                  "cost": 1
                }
              },
              {
                "table": "`states`",
                "table_scan": {
                  "rows": 4120,
                  "cost": 12
                }
              }
            ]
          },
          {
            "execution_plan_for_potential_materialization": {
              "steps": [
                {
                  "considered_execution_plans": [
                    {
                      "plan_prefix": [
                      ],
                      "table": "`states`",
                      "best_access_path": {
                        "considered_access_paths": [
                          {
                            "access_type": "scan",
                            "rows": 4120,
                            "cost": 836,
                            "chosen": true
                          }
                        ]
                      },
                      "cost_for_plan": 836,
                      "rows_for_plan": 4120,
                      "chosen": true
                    }
                  ]
                }
              ]
            }
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`states`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "scan",
                      "rows": 4120,
                      "cost": 836,
                      "chosen": true
                    }
                  ]
                },
                "cost_for_plan": 836,
                "rows_for_plan": 4120,
                "semijoin_strategy_choice": [
                  {
                    "strategy": "MaterializeScan",
                    "choice": "deferred"
                  }
                ],
                "rest_of_plan": [
                  {
                    "plan_prefix": [
                      "`states`"
                    ],
                    "table": "`countries`",
                    "best_access_path": {
                      "considered_access_paths": [
                        {
                          "access_type": "ref",
                          "index": "PRIMARY",
                          "rows": 1,
                          "cost": 4120.2,
                          "chosen": true
                        },
                        {
                          "access_type": "scan",
                          "using_join_cache": true,
                          "rows": 185,
                          "cost": 152453,
                          "chosen": false
                        }
                      ]
                    },
                    "cost_for_plan": 5780,
                    "rows_for_plan": 4120,
                    "semijoin_strategy_choice": [
                      {
                        "strategy": "MaterializeScan",
                        "recalculate_access_paths_and_cost": {
                          "tables": [
                            {
                              "table": "`countries`",
                              "best_access_path": {
                                "considered_access_paths": [
                                  {
                                    "access_type": "ref",
                                    "index": "PRIMARY",
                                    "rows": 1,
                                    "cost": 4120.2,
                                    "chosen": true
                                  },
                                  {
                                    "access_type": "scan",
                                    "using_join_cache": true,
                                    "rows": 185,
                                    "cost": 152453,
                                    "chosen": false
                                  }
                                ]
                              }
                            }
                          ]
                        },
                        "cost": 7430,
                        "rows": 1,
                        "duplicate_tables_left": true,
                        "chosen": true
                      },
                      {
                        "strategy": "DuplicatesWeedout",
                        "cost": 6606.2,
                        "rows": 1,
                        "duplicate_tables_left": false,
                        "chosen": true
                      }
                    ],
                    "chosen": true
                  }
                ]
              },
              {
                "plan_prefix": [
                ],
                "table": "`countries`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "ref",
                      "index": "PRIMARY",
                      "usable": false,
                      "chosen": false
                    },
                    {
                      "access_type": "scan",
                      "rows": 246,
                      "cost": 50.2,
                      "chosen": true
                    }
                  ]
                },
                "cost_for_plan": 50.2,
                "rows_for_plan": 246,
                "semijoin_strategy_choice": [
                ],
                "rest_of_plan": [
                  {
                    "plan_prefix": [
                      "`countries`"
                    ],
                    "table": "`states`",
                    "best_access_path": {
                      "considered_access_paths": [
                        {
                          "access_type": "scan",
                          "using_join_cache": true,
                          "rows": 4120,
                          "cost": 202721,
                          "chosen": true
                        }
                      ]
                    },
                    "cost_for_plan": 202771,
                    "rows_for_plan": 1.01e6,
                    "semijoin_strategy_choice": [
                      {
                        "strategy": "FirstMatch",
                        "recalculate_access_paths_and_cost": {
                          "tables": [
                            {
                              "table": "`states`",
                              "best_access_path": {
                                "considered_access_paths": [
                                  {
                                    "access_type": "scan",
                                    "using_join_cache": true,
                                    "rows": 4120,
                                    "cost": 202721,
                                    "chosen": true
                                  }
                                ]
                              }
                            }
                          ]
                        },
                        "cost": 202771,
                        "rows": 246,
                        "chosen": true
                      },
                      {
                        "strategy": "MaterializeLookup",
                        "cost": 1761.4,
                        "rows": 246,
                        "duplicate_tables_left": false,
                        "chosen": true
                      },
                      {
                        "strategy": "DuplicatesWeedout",
                        "cost": 405527,
                        "rows": 246,
                        "duplicate_tables_left": false,
                        "chosen": false
                      }
                    ],
                    "chosen": true
                  }
                ]
              },
              {
                "final_semijoin_strategy": "MaterializeLookup"
              }
            ]
          },
          {
            "creating_tmp_table": {
              "tmp_table_info": {
                "row_length": 5,
                "key_length": 4,
                "unique_constraint": false,
                "location": "memory (heap)",
                "row_limit_estimate": 3355443
              }
            }
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`<subquery2>`.`country_id` = `countries`.`id`)",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`countries`",
                  "attached": "(`countries`.`id` is not null)"
                },
                {
                  "table": "``.`<subquery2>`",
                  "attached": null
                },
                {
                  "table": "`states`",
                  "attached": null
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`countries`",
                "access_type": "table_scan"
              },
              {
                "table": "``.`<subquery2>`"
              },
              {
                "table": "`states`",
                "access_type": "table_scan"
              }
            ]
          }
        ]
      }
    },
    {
      "join_explain": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}