SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    9387, 6975, 6979, 7095, 7243, 7250, 7291, 
    7293, 7334, 7348, 7363, 7372, 7387, 
    7577, 7662, 7719, 7727, 7748, 7766, 
    7797, 7891, 7899, 7905, 7911, 7962, 
    8053, 8209, 8308, 8324
  )

Query time 0.00123

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "45.09"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 110,
      "rows_produced_per_join": 110,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "23.09",
        "eval_cost": "22.00",
        "prefix_cost": "45.09",
        "data_read_per_join": "652K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (9387,6975,6979,7095,7243,7250,7291,7293,7334,7348,7363,7372,7387,7577,7662,7719,7727,7748,7766,7797,7891,7899,7905,7911,7962,8053,8209,8308,8324))"
    }
  }
}