SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (
    2202, 1855, 1874, 1856, 0, 389, 388, 428, 
    17
  )

Query time 0.00162

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": 9,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variants.variant_id in (2202,1855,1874,1856,0,389,388,428,17)"
        }
      }
    ]
  }
}