SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '4791', '3096', '3084', '3099', '3093', 
    '3101', '3092', '3090', '3087', '3686', 
    '3089', '3104', '3094', '3100', '1856', 
    '1870', 'Y'
  ) 
  AND lang_code = 'en'

Query time 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variant_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "rows": 17,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('4791','3096','3084','3099','3093','3101','3092','3090','3087','3686','3089','3104','3094','3100','1856','1870','Y') and cscart_product_feature_variant_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}