SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '9070', '9063', '9056', '8815', '8819', 
    '8820', '9039', '8821', '8822', '8823', 
    '9068', '9061', '8846', '8850', '8851', 
    '9044', '8853', '9055', '9062', '9078', 
    '9030', '9060', '9091', '9087', '9082', 
    '9090', '9086', '9084', '6967', '6970', 
    '6965', '6966', '9139', '9097', '7959', 
    '9142', '9109', '9153', '9158', '9155', 
    '9157', '9150', '9147', '9172', '9167', 
    '9165', '9162', '9170', '9168'
  ) 
  AND lang_code = 'ru'

Query time 0.00124

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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": 49,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('9070','9063','9056','8815','8819','8820','9039','8821','8822','8823','9068','9061','8846','8850','8851','9044','8853','9055','9062','9078','9030','9060','9091','9087','9082','9090','9086','9084','6967','6970','6965','6966','9139','9097','7959','9142','9109','9153','9158','9155','9157','9150','9147','9172','9167','9165','9162','9170','9168') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
    }
  }
}