Optimizer Trace

{
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query":
            "/* select#1 */ select `departments`.`dept_no` AS `dept_no`
            from (`departments` join `dept_manager`
            on((`departments`.`dept_no` = `dept_manager`.`dept_no`)))
            where (`dept_manager`.`to_date` > now())
            group by `departments`.`dept_no`
            having (count(`dept_manager`.`emp_no`) > 1)
            limit 0,1000"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "transformations_to_nested_joins": {
              "transformations": [
                "JOIN_condition_to_WHERE",
                "parenthesis_removal"
              ],
              "expanded_query": "/* select#1 */ select `departments`.`dept_no` AS `dept_no` from `departments` join `dept_manager` where ((`dept_manager`.`to_date` > now()) and (`departments`.`dept_no` = `dept_manager`.`dept_no`)) group by `departments`.`dept_no` having (count(`dept_manager`.`emp_no`) > 1) limit 0,1000"
            }
          },
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "((`dept_manager`.`to_date` > now()) and (`departments`.`dept_no` = `dept_manager`.`dept_no`))",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "((`dept_manager`.`to_date` > now()) and multiple equal(`departments`.`dept_no`, `dept_manager`.`dept_no`))"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "((`dept_manager`.`to_date` > now()) and multiple equal(`departments`.`dept_no`, `dept_manager`.`dept_no`))"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "((`dept_manager`.`to_date` > now()) and multiple equal(`departments`.`dept_no`, `dept_manager`.`dept_no`))"
                }
              ]
            }
          },
          {
            "condition_processing": {
              "condition": "HAVING",
              "original_condition": "(count(`dept_manager`.`emp_no`) > 1)",
              "steps": [
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(count(`dept_manager`.`emp_no`) > 1)"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(count(`dept_manager`.`emp_no`) > 1)"
                }
              ]
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`departments`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              },
              {
                "table": "`dept_manager`",
                "row_may_be_null": false,
                "map_bit": 1,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`departments`",
                "field": "dept_no",
                "equals": "`dept_manager`.`dept_no`",
                "null_rejecting": false
              },
              {
                "table": "`dept_manager`",
                "field": "dept_no",
                "equals": "`departments`.`dept_no`",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`departments`",
                "const_keys_added": {
                  "keys": [
                    "PRIMARY",
                    "dept_name"
                  ],
                  "cause": "group_by"
                },
                "range_analysis": {
                  "table_scan": {
                    "rows": 9,
                    "cost": 4.9
                  },
                  "potential_range_indices": [
                    {
                      "index": "PRIMARY",
                      "usable": true,
                      "key_parts": [
                        "dept_no"
                      ]
                    },
                    {
                      "index": "dept_name",
                      "usable": true,
                      "key_parts": [
                        "dept_name"
                      ]
                    }
                  ],
                  "best_covering_index_scan": {
                    "index": "dept_name",
                    "cost": 2.929,
                    "chosen": true
                  },
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_single_table"
                  }
                }
              },
              {
                "table": "`dept_manager`",
                "table_scan": {
                  "rows": 24,
                  "cost": 1
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`departments`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "ref",
                      "index": "PRIMARY",
                      "usable": false,
                      "chosen": false
                    },
                    {
                      "access_type": "scan",
                      "rows": 9,
                      "cost": 2.8,
                      "chosen": true,
                      "use_tmp_table": true
                    }
                  ]
                },
                "cost_for_plan": 2.8,
                "rows_for_plan": 9,
                "rest_of_plan": [
                  {
                    "plan_prefix": [
                      "`departments`"
                    ],
                    "table": "`dept_manager`",
                    "best_access_path": {
                      "considered_access_paths": [
                        {
                          "access_type": "ref",
                          "index": "dept_no",
                          "rows": 1,
                          "cost": 9.2,
                          "chosen": true
                        },
                        {
                          "access_type": "scan",
                          "using_join_cache": true,
                          "rows": 18,
                          "cost": 34.6,
                          "chosen": false
                        }
                      ]
                    },
                    "cost_for_plan": 13.6,
                    "rows_for_plan": 9,
                    "sort_cost": 9,
                    "new_cost_for_plan": 22.6,
                    "chosen": true
                  }
                ]
              },
              {
                "plan_prefix": [
                ],
                "table": "`dept_manager`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "ref",
                      "index": "dept_no",
                      "usable": false,
                      "chosen": false
                    },
                    {
                      "access_type": "scan",
                      "rows": 24,
                      "cost": 5.8,
                      "chosen": true
                    }
                  ]
                },
                "cost_for_plan": 5.8,
                "rows_for_plan": 24,
                "rest_of_plan": [
                  {
                    "plan_prefix": [
                      "`dept_manager`"
                    ],
                    "table": "`departments`",
                    "best_access_path": {
                      "considered_access_paths": [
                        {
                          "access_type": "ref",
                          "index": "PRIMARY",
                          "rows": 1,
                          "cost": 24.2,
                          "chosen": true
                        },
                        {
                          "access_type": "scan",
                          "cause": "covering_index_better_than_full_scan",
                          "chosen": false
                        }
                      ]
                    },
                    "cost_for_plan": 34.6,
                    "rows_for_plan": 24,
                    "pruned_by_cost": true
                  }
                ]
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "((`dept_manager`.`dept_no` = `departments`.`dept_no`) and (`dept_manager`.`to_date` > now()))",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`departments`",
                  "attached": null
                },
                {
                  "table": "`dept_manager`",
                  "attached": "(`dept_manager`.`to_date` > now())"
                }
              ]
            }
          },
          {
            "clause_processing": {
              "clause": "GROUP BY",
              "original_clause": "`departments`.`dept_no`",
              "items": [
                {
                  "item": "`departments`.`dept_no`"
                }
              ],
              "resulting_clause_is_simple": true,
              "resulting_clause": "`departments`.`dept_no`"
            }
          },
          {
            "refine_plan": [
              {
                "table": "`departments`",
                "access_type": "index_scan"
              },
              {
                "table": "`dept_manager`"
              }
            ]
          },
          {
            "reconsidering_access_paths_for_index_ordering": {
              "clause": "GROUP BY",
              "index_order_summary": {
                "table": "`departments`",
                "index_provides_order": true,
                "order_direction": "asc",
                "index": "PRIMARY",
                "plan_changed": true,
                "access_type": "index_scan"
              }
            }
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}