SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (2099, 2230, 1870, 62, 79, 0, 1520)

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variants",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 7,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variants.variant_id in (2099,2230,1870,62,79,0,1520)"
        }
      }
    ]
  }
}