SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    50281, 50282, 50283, 6642, 50279, 50280
  ) 
  AND feature_id IN (25134) 
  AND lang_code = 'vi'

Query time 0.00356

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["feature_id", "product_id"],
          "rows": 6,
          "filtered": 51.52878952,
          "attached_condition": "cscart_product_features_values.feature_id = 25134 and cscart_product_features_values.product_id in (50281,50282,50283,6642,50279,50280) and cscart_product_features_values.lang_code = 'vi'",
          "using_index": true
        }
      }
    ]
  }
}