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SELECT 
  variant_id, 
  yml2_unit 
FROM 
  cscart_product_feature_variant_descriptions 
WHERE 
  variant_id IN (
    '91', '108', '109', '110', '89', '84', 
    '55', '66', '73', '76', '78', '69', 
    '39', '77', '83', '80', '65', '71', 
    '67', '68', '74', '70', '45', '79', 
    '81', '54', '75', '82', '72'
  ) 
  AND lang_code = 'ru'

Query time 0.00318

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": 29,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variant_descriptions.variant_id in ('91','108','109','110','89','84','55','66','73','76','78','69','39','77','83','80','65','71','67','68','74','70','45','79','81','54','75','82','72') and cscart_product_feature_variant_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}

Result

variant_id	yml2_unit
39
45
54
55
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
89
91
108
109
110

variant_id	yml2_unit
39
45
54
55
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
89
91
108
109
110

variant_id	yml2_unit
39
45
54
55
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
89
91
108
109
110

variant_id	yml2_unit
39
45
54
55
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
89
91
108
109
110