三、optimizer_trace分析

{
  "steps": [
    {
      "join_preparation": { //这是连接的准备工作
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `student`.`s_name` AS `s_name` from `student` where (`student`.`s_id` = 3)"
            //上面是连接准备过程中的查询:字段已解析为它们的数据库和表，每个SELECT都用它的编号标注(对于子查询很有用)。
          }
        ]
      }
    },
    {
      "join_optimization": {//优化
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "(`student`.`s_id` = 3)",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "multiple equal(3, `student`.`s_id`)"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "multiple equal(3, `student`.`s_id`)"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "multiple equal(3, `student`.`s_id`)"
                }
              ]
            }
          },
          {
            "substitute_generated_columns": {
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`student`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`student`",
                "field": "s_id",
                "equals": "3",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`student`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 5,
                    "cost": 2.85
                  },
                  "potential_range_indexes": [
                    {
                      "index": "s_index_1",
                      "usable": true,
                      "key_parts": [
                        "s_id"
                      ]
                    }
                  ],
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  },
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [
                      {
                        "index": "s_index_1",
                        "ranges": [
                          "3 <= s_id <= 3"
                        ],
                        "index_dives_for_eq_ranges": true,
                        "rowid_ordered": true,
                        "using_mrr": false,
                        "index_only": false,
                        "rows": 1,
                        "cost": 0.61,
                        "chosen": true
                      }
                    ],
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    }
                  },
                  "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "range_scan",
                      "index": "s_index_1",
                      "rows": 1,
                      "ranges": [
                        "3 <= s_id <= 3"
                      ]
                    },
                    "rows_for_plan": 1,
                    "cost_for_plan": 0.61,
                    "chosen": true
                  }
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`student`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "ref",
                      "index": "s_index_1",
                      "rows": 1,
                      "cost": 0.35,
                      "chosen": true
                    },
                    {
                      "access_type": "range",
                      "range_details": {
                        "used_index": "s_index_1"
                      },
                      "chosen": false,
                      "cause": "heuristic_index_cheaper"
                    }
                  ]
                },
                "condition_filtering_pct": 100,
                "rows_for_plan": 1,
                "cost_for_plan": 0.35,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`student`.`s_id` = 3)",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`student`",
                  "attached": null
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`student`"
              }
            ]
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}