SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN ('837', '905', '1856', 'Y') 
  AND lang_code = 'en'

Query time 0.00034

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": 4,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('837','905','1856','Y') and cscart_product_feature_variant_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}