SELECT 
  thread_id 
FROM 
  cscart_discussion 
WHERE 
  object_type = 'A' 
  AND object_id = 94

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.40"
    },
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "ref",
      "possible_keys": [
        "object_id"
      ],
      "key": "object_id",
      "used_key_parts": [
        "object_id",
        "object_type"
      ],
      "key_length": "6",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 4,
      "rows_produced_per_join": 4,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "1.00",
        "eval_cost": "0.40",
        "prefix_cost": "1.40",
        "data_read_per_join": "96"
      },
      "used_columns": [
        "thread_id",
        "object_id",
        "object_type"
      ]
    }
  }
}