使用union 外加count

explain extended select count(*) from (select id from states where country_id < 100 union (select id from countries where id > 10 and name='aaa')) t;
+----+--------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type  | table      | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | ALL   | NULL          | NULL    | NULL    | NULL | 4356 |   100.00 | NULL            |
|  2 | DERIVED      | states     | ALL   | NULL          | NULL    | NULL    | NULL | 4120 |   100.00 | Using where     |
|  3 | UNION        | countries  | range | PRIMARY       | PRIMARY | 4       | NULL |  236 |   100.00 | Using where     |
| NULL | UNION RESULT | <union2,3> | ALL   | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+

QUERY: explain extended select count(*) from (select id from states where country_id < 100 union (select id from countries where id > 10 and name='aaa')) t
                            TRACE: {
  "steps": [
    {
      "join_preparation": {
        "select#": 2,
        "steps": [
          {
            "expanded_query": "/* select#2 */ select `states`.`id` AS `id` from `states` where (`states`.`country_id` < 100)"
          }
        ]
      }
    },
    {
      "join_preparation": {
        "select#": 3,
        "steps": [
          {
            "expanded_query": "/* select#3 */ select `countries`.`id` AS `id` from `countries` where ((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))"
          }
        ]
      }
    },
    {
      "creating_tmp_table": {
        "tmp_table_info": {
          "table": "intermediate_tmp_table",
          "row_length": 5,
          "key_length": 4,
          "unique_constraint": false,
          "location": "memory (heap)",
          "row_limit_estimate": 3355443
        }
      }
    },
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select count(0) AS `count(*)` from (/* select#2 */ select `states`.`id` AS `id` from `states` where (`states`.`country_id` < 100) union (/* select#3 */ select `countries`.`id` AS `id` from `countries` where ((`countries`.`id` > 10) and (`countries`.`name` = 'aaa')))) `t`"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "join_optimization": {
              "select#": 2,
              "steps": [
                {
                  "condition_processing": {
                    "condition": "WHERE",
                    "original_condition": "(`states`.`country_id` < 100)",
                    "steps": [
                      {
                        "transformation": "equality_propagation",
                        "resulting_condition": "(`states`.`country_id` < 100)"
                      },
                      {
                        "transformation": "constant_propagation",
                        "resulting_condition": "(`states`.`country_id` < 100)"
                      },
                      {
                        "transformation": "trivial_condition_removal",
                        "resulting_condition": "(`states`.`country_id` < 100)"
                      }
                    ]
                  }
                },
                {
                  "table_dependencies": [
                    {
                      "table": "`states`",
                      "row_may_be_null": false,
                      "map_bit": 0,
                      "depends_on_map_bits": [
                      ]
                    }
                  ]
                },
                {
                  "ref_optimizer_key_uses": [
                  ]
                },
                {
                  "rows_estimation": [
                    {
                      "table": "`states`",
                      "table_scan": {
                        "rows": 4120,
                        "cost": 12
                      }
                    }
                  ]
                },
                {
                  "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
                    }
                  ]
                },
                {
                  "attaching_conditions_to_tables": {
                    "original_condition": "(`states`.`country_id` < 100)",
                    "attached_conditions_computation": [
                    ],
                    "attached_conditions_summary": [
                      {
                        "table": "`states`",
                        "attached": "(`states`.`country_id` < 100)"
                      }
                    ]
                  }
                },
                {
                  "refine_plan": [
                    {
                      "table": "`states`",
                      "access_type": "table_scan"
                    }
                  ]
                }
              ]
            }
          },
          {
            "join_optimization": {
              "select#": 3,
              "steps": [
                {
                  "condition_processing": {
                    "condition": "WHERE",
                    "original_condition": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))",
                    "steps": [
                      {
                        "transformation": "equality_propagation",
                        "resulting_condition": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))"
                      },
                      {
                        "transformation": "constant_propagation",
                        "resulting_condition": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))"
                      },
                      {
                        "transformation": "trivial_condition_removal",
                        "resulting_condition": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))"
                      }
                    ]
                  }
                },
                {
                  "table_dependencies": [
                    {
                      "table": "`countries`",
                      "row_may_be_null": false,
                      "map_bit": 0,
                      "depends_on_map_bits": [
                      ]
                    }
                  ]
                },
                {
                  "ref_optimizer_key_uses": [
                  ]
                },
                {
                  "rows_estimation": [
                    {
                      "table": "`countries`",
                      "range_analysis": {
                        "table_scan": {
                          "rows": 246,
                          "cost": 52.3
                        },
                        "potential_range_indices": [
                          {
                            "index": "PRIMARY",
                            "usable": true,
                            "key_parts": [
                              "id"
                            ]
                          }
                        ],
                        "setup_range_conditions": [
                        ],
                        "group_index_range": {
                          "chosen": false,
                          "cause": "not_group_by_or_distinct"
                        },
                        "analyzing_range_alternatives": {
                          "range_scan_alternatives": [
                            {
                              "index": "PRIMARY",
                              "ranges": [
                                "10 < id"
                              ],
                              "index_dives_for_eq_ranges": true,
                              "rowid_ordered": true,
                              "using_mrr": false,
                              "index_only": false,
                              "rows": 236,
                              "cost": 48.419,
                              "chosen": true
                            }
                          ],
                          "analyzing_roworder_intersect": {
                            "usable": false,
                            "cause": "too_few_roworder_scans"
                          }
                        },
                        "chosen_range_access_summary": {
                          "range_access_plan": {
                            "type": "range_scan",
                            "index": "PRIMARY",
                            "rows": 236,
                            "ranges": [
                              "10 < id"
                            ]
                          },
                          "rows_for_plan": 236,
                          "cost_for_plan": 48.419,
                          "chosen": true
                        }
                      }
                    }
                  ]
                },
                {
                  "considered_execution_plans": [
                    {
                      "plan_prefix": [
                      ],
                      "table": "`countries`",
                      "best_access_path": {
                        "considered_access_paths": [
                          {
                            "access_type": "range",
                            "rows": 236,
                            "cost": 95.619,
                            "chosen": true
                          }
                        ]
                      },
                      "cost_for_plan": 95.619,
                      "rows_for_plan": 236,
                      "chosen": true
                    }
                  ]
                },
                {
                  "attaching_conditions_to_tables": {
                    "original_condition": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))",
                    "attached_conditions_computation": [
                    ],
                    "attached_conditions_summary": [
                      {
                        "table": "`countries`",
                        "attached": "((`countries`.`id` > 10) and (`countries`.`name` = 'aaa'))"
                      }
                    ]
                  }
                },
                {
                  "refine_plan": [
                    {
                      "table": "`countries`",
                      "access_type": "range"
                    }
                  ]
                }
              ]
            }
          },
          {
            "table_dependencies": [
              {
                "table": " `t`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": " `t`",
                "table_scan": {
                  "rows": 4356,
                  "cost": 227
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": " `t`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "scan",
                      "rows": 4356,
                      "cost": 1099,
                      "chosen": true
                    }
                  ]
                },
                "cost_for_plan": 1099,
                "rows_for_plan": 4356,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": null,
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": " `t`",
                  "attached": null
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": " `t`",
                "access_type": "table_scan"
              }
            ]
          }
        ]
      }
    },
    {
      "join_explain": {
        "select#": 1,
        "steps": [
          {
            "creating_tmp_table": {
              "tmp_table_info": {
                "table": " `t`",
                "row_length": 5,
                "key_length": 0,
                "unique_constraint": false,
                "location": "memory (heap)",
                "row_limit_estimate": 3355443
              }
            }
          },
          {
            "join_explain": {
              "select#": 2,
              "steps": [
              ]
            }
          },
          {
            "join_explain": {
              "select#": 3,
              "steps": [
              ]
            }
          },
          {
            "join_preparation": {
              "select#": "fake",
              "steps": [
                {
                  "expanded_query": "/* select#fake */ select `id` AS `id` from dual"
                }
              ]
            }
          },
          {
            "join_optimization": {
              "select#": "fake",
              "steps": [
                {
                  "table_dependencies": [
                    {
                      "table": "``.``",
                      "row_may_be_null": false,
                      "map_bit": 0,
                      "depends_on_map_bits": [
                      ]
                    }
                  ]
                },
                {
                  "rows_estimation": [
                    {
                      "table": "``.``",
                      "table_scan": {
                        "rows": 0,
                        "cost": 10
                      }
                    }
                  ]
                },
                {
                  "considered_execution_plans": [
                    {
                      "plan_prefix": [
                      ],
                      "table": "``.``",
                      "best_access_path": {
                        "considered_access_paths": [
                          {
                            "access_type": "scan",
                            "rows": 0,
                            "cost": 10,
                            "chosen": true
                          }
                        ]
                      },
                      "cost_for_plan": 10,
                      "rows_for_plan": 0,
                      "chosen": true
                    }
                  ]
                },
                {
                  "attaching_conditions_to_tables": {
                    "original_condition": null,
                    "attached_conditions_computation": [
                    ],
                    "attached_conditions_summary": [
                      {
                        "table": "``.``",
                        "attached": null
                      }
                    ]
                  }
                },
                {
                  "refine_plan": [
                    {
                      "table": "``.``",
                      "access_type": "table_scan"
                    }
                  ]
                }
              ]
            }
          },
          {
            "join_explain": {
              "select#": "fake",
              "steps": [
              ]
            }
          }
        ]
      }
    }
  ]
}