SELECT 
  product_id 
FROM 
  cscart_products 
WHERE 
  product_type = 'D'

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "index",
      "key": "product_id",
      "key_length": "782",
      "used_key_parts": [
        "product_id",
        "product_type",
        "company_id",
        "usergroup_ids",
        "parent_product_id"
      ],
      "rows": 107,
      "filtered": 100,
      "attached_condition": "cscart_products.product_type = 'D'",
      "using_index": true
    }
  }
}