gistfile1.txt

explain select /*+ SKIP_SCAN(t) */ x,y,z from t where x=1 and z=1\G'

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: range
possible_keys: x1
          key: x1
      key_len: 12
          ref: NULL
         rows: 10
     filtered: 100.00
        Extra: Using where; Using index for skip scan
        
Interesting parts of optimizer trace:
                  "skip_scan_range": {
                    "potential_skip_scan_indexes": [
                      {
                        "index": "x1",
                        "tree_travel_cost": 0.4,
                        "num_groups": 11,
                        "rows": 10,
                        "cost": 10.555
                      }
                    ]
                  },
                  "best_skip_scan_summary": {
                    "type": "skip_scan",
                    "index": "x1",
                    "key_parts_used_for_access": [
                      "x",
                      "y",
                      "z"
                    ],
                    "prefix ranges": [
                      "1 <= x <= 1"
                    ],
                    "range": [
                      "1 <= z <= 1"
                    ],
                    "chosen": true
                  },
                  
             
             "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "skip_scan",
                      "index": "x1",
                      "key_parts_used_for_access": [
                        "x",
                        "y",
                        "z"
                      ],
                      "prefix ranges": [
                        "1 <= x <= 1"
                      ],
                      "range": [
                        "1 <= z <= 1"
                      ]
                    },
                    "rows_for_plan": 10,
                    "cost_for_plan": 10.555,
                    "chosen": true
                  }


Full optimizer trace:
trace: {
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select /*+ SKIP_SCAN(`t`@`select#1`) */ `t`.`x` AS `x`,`t`.`y` AS `y`,`t`.`z` AS `z` from `t` where ((`t`.`x` = 1) and (`t`.`z` = 1))"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "((`t`.`x` = 1) and (`t`.`z` = 1))",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "(multiple equal(1, `t`.`x`) and multiple equal(1, `t`.`z`))"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "(multiple equal(1, `t`.`x`) and multiple equal(1, `t`.`z`))"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "(multiple equal(1, `t`.`x`) and multiple equal(1, `t`.`z`))"
                }
              ]
            }
          },
          {
            "substitute_generated_columns": {
            }
          },
          {
            "table_dependencies": [
              {
                "table": "`t`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`t`",
                "field": "x",
                "equals": "1",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`t`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 200,
                    "cost": 23.35
                  },
                  "potential_range_indexes": [
                    {
                      "index": "x1",
                      "usable": true,
                      "key_parts": [
                        "x",
                        "y",
                        "z"
                      ]
                    }
                  ],
                  "best_covering_index_scan": {
                    "index": "x1",
                    "cost": 20.359,
                    "chosen": true
                  },
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  },
                  "skip_scan_range": {
                    "potential_skip_scan_indexes": [
                      {
                        "index": "x1",
                        "tree_travel_cost": 0.4,
                        "num_groups": 11,
                        "rows": 10,
                        "cost": 10.555
                      }
                    ]
                  },
                  "best_skip_scan_summary": {
                    "type": "skip_scan",
                    "index": "x1",
                    "key_parts_used_for_access": [
                      "x",
                      "y",
                      "z"
                    ],
                    "prefix ranges": [
                      "1 <= x <= 1"
                    ],
                    "range": [
                      "1 <= z <= 1"
                    ],
                    "chosen": true
                  },
                  "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "skip_scan",
                      "index": "x1",
                      "key_parts_used_for_access": [
                        "x",
                        "y",
                        "z"
                      ],
                      "prefix ranges": [
                        "1 <= x <= 1"
                      ],
                      "range": [
                        "1 <= z <= 1"
                      ]
                    },
                    "rows_for_plan": 10,
                    "cost_for_plan": 10.555,
                    "chosen": true
                  }
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`t`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "rows_to_scan": 10,
                      "filtering_effect": [
                      ],
                      "final_filtering_effect": 1,
                      "access_type": "range",
                      "range_details": {
                        "used_index": "index_for_skip_scan(x1)"
                      },
                      "resulting_rows": 10,
                      "cost": 11.555,
                      "chosen": true
                    }
                  ]
                },
                "condition_filtering_pct": 100,
                "rows_for_plan": 10,
                "cost_for_plan": 11.555,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "((`t`.`z` = 1) and (`t`.`x` = 1))",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`t`",
                  "attached": "((`t`.`z` = 1) and (`t`.`x` = 1))"
                }
              ]
            }
          },
          {
            "finalizing_table_conditions": [
              {
                "table": "`t`",
                "original_table_condition": "((`t`.`z` = 1) and (`t`.`x` = 1))",
                "final_table_condition   ": "((`t`.`z` = 1) and (`t`.`x` = 1))"
              }
            ]
          },
          {
            "refine_plan": [
              {
                "table": "`t`"
              }
            ]
          }
        ]
      }
    },
    {
      "join_explain": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}