Sandbox

SELECT 
  category_id, 
  parent_id 
FROM 
  jmj_categories 
WHERE 
  parent_id IN(79, 80, 81) 
  AND category_id != 1

Query time 0.00052

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "24.01"
    },
    "table": {
      "table_name": "jmj_categories",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "parent",
        "p_category_id"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 15,
      "rows_produced_per_join": 15,
      "filtered": "100.00",
      "index_condition": "(`jmj_test`.`jmj_categories`.`parent_id` in (79,80,81))",
      "cost_info": {
        "read_cost": "21.01",
        "eval_cost": "3.00",
        "prefix_cost": "24.01",
        "data_read_per_join": "58K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`jmj_test`.`jmj_categories`.`category_id` <> 1)"
    }
  }
}

Result

category_id	parent_id
88	79
109	80
110	80
111	80
112	80
113	80
114	80
115	80
116	81
117	81
118	81
119	81
120	81
121	81
122	81

category_id	parent_id
88	79
109	80
110	80
111	80
112	80
113	80
114	80
115	80
116	81
117	81
118	81
119	81
120	81
121	81
122	81

category_id	parent_id
88	79
109	80
110	80
111	80
112	80
113	80
114	80
115	80
116	81
117	81
118	81
119	81
120	81
121	81
122	81

category_id	parent_id
88	79
109	80
110	80
111	80
112	80
113	80
114	80
115	80
116	81
117	81
118	81
119	81
120	81
121	81
122	81