SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    976, 978, 968, 971, 636, 969, 972, 977, 
    1022, 1023, 1106, 1013, 1105, 1101, 
    1104, 979, 989, 632, 613, 617, 621, 622, 
    1015, 1016, 1018, 1019, 611, 612, 980, 
    1007, 1002, 638, 997, 990, 992, 985, 
    1012, 1020
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00090

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "91.21"
    },
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "product_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "product_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 38,
      "rows_produced_per_join": 38,
      "filtered": "100.00",
      "index_condition": "((`cscart`.`cscart_product_descriptions`.`product_id` in (976,978,968,971,636,969,972,977,1022,1023,1106,1013,1105,1101,1104,979,989,632,613,617,621,622,1015,1016,1018,1019,611,612,980,1007,1002,638,997,990,992,985,1012,1020)) and (`cscart`.`cscart_product_descriptions`.`lang_code` = 'ru'))",
      "cost_info": {
        "read_cost": "83.61",
        "eval_cost": "7.60",
        "prefix_cost": "91.21",
        "data_read_per_join": "173K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description",
        "unit_name"
      ]
    }
  }
}