SELECT 
  DISTINCT product_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    20589, 20590, 20591, 20592, 20593, 20594, 
    20595, 20596, 20597, 20598, 20599, 
    20600, 20601, 20602, 20603, 20604
  ) 
  AND feature_id IN (116) 
  AND (
    UPPER(value) IN ('Y', 'YES', 'TRUE', '1') 
    OR value IN (
      '해외구매대행', '수입품', 
      'Imported'
    ) 
    OR value_int > 0
  )

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "fpl",
      "key_length": "6",
      "used_key_parts": ["feature_id", "product_id"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_features_values.feature_id = 116 and cscart_product_features_values.product_id in (20589,20590,20591,20592,20593,20594,20595,20596,20597,20598,20599,20600,20601,20602,20603,20604)",
      "attached_condition": "ucase(cscart_product_features_values.`value`) in ('Y','YES','TRUE','1') or cscart_product_features_values.`value` in ('해외구매대행','수입품','Imported') or cscart_product_features_values.value_int > 0"
    }
  }
}