SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6937, 7208, 7105, 5690, 5725, 7218, 6954, 
    7118, 5818, 5749, 5737, 6971, 7177, 
    7233, 5759, 5826, 6990, 7252, 7195, 
    5765, 5837, 7005, 7259, 5774, 5850, 
    7024, 5786, 5792, 5801
  )

Query time 0.00072

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "19.04"
    },
    "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": 45,
      "rows_produced_per_join": 45,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "10.04",
        "eval_cost": "9.00",
        "prefix_cost": "19.04",
        "data_read_per_join": "266K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6937,7208,7105,5690,5725,7218,6954,7118,5818,5749,5737,6971,7177,7233,5759,5826,6990,7252,7195,5765,5837,7005,7259,5774,5850,7024,5786,5792,5801))"
    }
  }
}