SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(1, 2, 3, 4, 5, 6, 7)

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "possible_keys": [
        "parent"
      ],
      "rows": 7,
      "filtered": 100,
      "attached_condition": "(`_testcscart`.`cscart_categories`.`parent_id` in (1,2,3,4,5,6,7))"
    }
  }
}