"explain SELECT `ServerID`,
`ServerIP`, `ServerPort`, ROUND(IFNULL(CAST(`SessionCount` AS UNSIGNED),CONVERT(0,UNSIGNED)) / 10) AS `SessionBucket`
FROM `servers` 
LEFT JOIN (SELECT `Servers_ServerID`,CONVERT(COUNT(1),UNSIGNED) AS `SessionCount` FROM `session` GROUP BY `Servers_ServerID`) `SC` ON (`ServerID` = `SC`.`Servers_ServerID`)"	"{
  "steps": [
    {
      "join_preparation": {
        "select_id": 1,
        "steps": [
          {
            "derived": {
              "table": "SC",
              "select_id": 2,
              "algorithm": "materialized"
            }
          },
          {
            "join_preparation": {
              "select_id": 2,
              "steps": [
                {
                  "expanded_query": "/* select#2 */ select `session`.Servers_ServerID AS Servers_ServerID,cast(count(1) as unsigned) AS SessionCount from `session` group by `session`.Servers_ServerID"
                }
              ]
            }
          },
          {
            "expanded_query": "/* select#1 */ select servers.ServerID AS ServerID,servers.ServerIP AS ServerIP,servers.ServerPort AS ServerPort,round(ifnull(cast(sc.SessionCount as unsigned),cast(0 as unsigned)) / 10,0) AS SessionBucket from (servers left join (/* select#2 */ select `session`.Servers_ServerID AS Servers_ServerID,cast(count(1) as unsigned) AS SessionCount from `session` group by `session`.Servers_ServerID) sc on(servers.ServerID = sc.Servers_ServerID))"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select_id": 1,
        "steps": [
          {
            "build_equal_items": {
              "condition": "ON expr",
              "attached_to": "SC",
              "resulting_condition": "multiple equal(servers.ServerID, sc.Servers_ServerID)"
            }
          },
          {
            "join_optimization": {
              "select_id": 2,
              "steps": [
                {
                  "table_dependencies": [
                    {
                      "table": "session",
                      "row_may_be_null": false,
                      "map_bit": 0,
                      "depends_on_map_bits": []
                    }
                  ]
                },
                {
                  "rows_estimation": [
                    {
                      "table": "session",
                      "range_analysis": {
                        "table_scan": {
                          "rows": 12489,
                          "cost": 2596.8
                        },
                        "potential_range_indexes": [
                          {
                            "index": "PRIMARY",
                            "usable": false,
                            "cause": "not applicable"
                          },
                          {
                            "index": "fk_Session_Servers_idx",
                            "usable": true,
                            "key_parts": ["Servers_ServerID", "SessionID"]
                          }
                        ],
                        "best_covering_index_scan": {
                          "index": "fk_Session_Servers_idx",
                          "cost": 2654.227537,
                          "chosen": false,
                          "cause": "cost"
                        },
                        "group_index_range": {
                          "chosen": false,
                          "cause": "not applicable aggregate function"
                        }
                      }
                    }
                  ]
                },
                {
                  "considered_execution_plans": [
                    {
                      "plan_prefix": [],
                      "get_costs_for_tables": [
                        {
                          "best_access_path": {
                            "table": "session",
                            "considered_access_paths": [
                              {
                                "access_type": "scan",
                                "resulting_rows": 12489,
                                "cost": 97,
                                "chosen": true,
                                "use_tmp_table": true
                              }
                            ],
                            "chosen_access_method": {
                              "type": "scan",
                              "records": 12489,
                              "cost": 97,
                              "uses_join_buffering": false
                            }
                          }
                        }
                      ]
                    },
                    {
                      "plan_prefix": [],
                      "table": "session",
                      "rows_for_plan": 12489,
                      "cost_for_plan": 2594.8,
                      "cost_for_sorting": 12489
                    }
                  ]
                },
                {
                  "check_split_materialized": {
                    "split_candidates": ["`session`.Servers_ServerID"]
                  }
                }
              ]
            }
          },
          {
            "table_dependencies": [
              {
                "table": "servers",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": []
              },
              {
                "table": "<derived2>",
                "row_may_be_null": true,
                "map_bit": 1,
                "depends_on_map_bits": ["0"]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "<derived2>",
                "index": "key0",
                "field": "Servers_ServerID",
                "equals": "servers.ServerID",
                "null_rejecting": true
              }
            ]
          },
          {
            "eliminated_tables": []
          },
          {
            "rows_estimation": [
              {
                "table": "servers",
                "table_scan": {
                  "rows": 244,
                  "cost": 1
                }
              },
              {
                "table": "<derived2>",
                "table_scan": {
                  "rows": 12489,
                  "cost": 12489
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [],
                "get_costs_for_tables": [
                  {
                    "best_access_path": {
                      "table": "servers",
                      "considered_access_paths": [
                        {
                          "access_type": "scan",
                          "resulting_rows": 244,
                          "cost": 1,
                          "chosen": true
                        }
                      ],
                      "chosen_access_method": {
                        "type": "scan",
                        "records": 244,
                        "cost": 1,
                        "uses_join_buffering": false
                      }
                    }
                  }
                ]
              },
              {
                "plan_prefix": [],
                "table": "servers",
                "rows_for_plan": 244,
                "cost_for_plan": 49.8,
                "rest_of_plan": [
                  {
                    "plan_prefix": ["servers"],
                    "get_costs_for_tables": [
                      {
                        "best_access_path": {
                          "table": "<derived2>",
                          "choose_best_splitting": {
                            "considered_keys": [
                              {
                                "table_name": "session",
                                "index": "fk_Session_Servers_idx",
                                "rec_per_key": 1,
                                "param_tables": 1
                              }
                            ],
                            "refills": 244,
                            "spl_pd_boundary": 2,
                            "split_plan_search": [
                              {
                                "considered_execution_plans": [
                                  {
                                    "plan_prefix": [],
                                    "get_costs_for_tables": [
                                      {
                                        "best_access_path": {
                                          "table": "session",
                                          "considered_access_paths": [
                                            {
                                              "access_type": "ref",
                                              "index": "fk_Session_Servers_idx",
                                              "rows": 1,
                                              "cost": 1.001635642,
                                              "chosen": true
                                            },
                                            {
                                              "type": "scan",
                                              "chosen": false,
                                              "cause": "cost"
                                            }
                                          ],
                                          "chosen_access_method": {
                                            "type": "ref",
                                            "records": 1,
                                            "cost": 1.001635642,
                                            "uses_join_buffering": false
                                          }
                                        }
                                      }
                                    ]
                                  },
                                  {
                                    "plan_prefix": [],
                                    "table": "session",
                                    "rows_for_plan": 1,
                                    "cost_for_plan": 1.201635642,
                                    "cost_for_sorting": 1
                                  }
                                ]
                              }
                            ],
                            "lead_table": "session",
                            "index": "fk_Session_Servers_idx",
                            "parts": 1,
                            "split_sel": 8.007046e-5,
                            "cost": 1.151635642,
                            "unsplit_cost": 4745.048744,
                            "records": 1,
                            "chosen": true
                          },
                          "considered_access_paths": [
                            {
                              "access_type": "ref",
                              "index": "key0",
                              "rec_per_key_stats_missing": true,
                              "used_range_estimates": false,
                              "reason": "not available",
                              "rows": 2,
                              "cost": 341.9990967,
                              "chosen": true
                            },
                            {
                              "type": "scan",
                              "chosen": false,
                              "cause": "cost"
                            }
                          ],
                          "chosen_access_method": {
                            "type": "ref",
                            "records": 2,
                            "cost": 341.9990967,
                            "uses_join_buffering": false
                          }
                        }
                      }
                    ]
                  },
                  {
                    "plan_prefix": ["servers"],
                    "table": "<derived2>",
                    "rows_for_plan": 488,
                    "cost_for_plan": 489.3990967
                  }
                ]
              }
            ]
          },
          {
            "best_join_order": ["servers", "<derived2>"]
          },
          {
            "best_join_order": ["session"]
          },
          {
            "substitute_best_equal": {
              "condition": "WHERE",
              "resulting_condition": "`session`.Servers_ServerID = servers.ServerID"
            }
          },
          {
            "attaching_conditions_to_tables": {
              "attached_conditions_computation": [],
              "attached_conditions_summary": [
                {
                  "table": "session",
                  "attached": null
                }
              ]
            }
          },
          {
            "prepare_sum_aggregators": {
              "function": "count(1)",
              "aggregator_type": "simple"
            }
          },
          {
            "substitute_best_equal": {
              "condition": "WHERE",
              "resulting_condition": "1"
            }
          },
          {
            "substitute_best_equal": {
              "condition": "ON expr",
              "attached_to": "SC",
              "resulting_condition": "sc.Servers_ServerID = servers.ServerID"
            }
          },
          {
            "condition_on_constant_tables": "1",
            "computing_condition": []
          },
          {
            "attaching_conditions_to_tables": {
              "attached_conditions_computation": [],
              "attached_conditions_summary": [
                {
                  "table": "servers",
                  "attached": null
                },
                {
                  "table": "<derived2>",
                  "attached": null
                }
              ]
            }
          }
        ]
      }
    },
    {
      "join_execution": {
        "select_id": 1,
        "steps": [
          {
            "join_execution": {
              "select_id": 2,
              "steps": []
            }
          }
        ]
      }
    }
  ]
}"	0	false