SELECT 
  cscart_products_categories.category_id 
FROM 
  cscart_products_categories 
  LEFT JOIN cscart_discussion ON cscart_discussion.object_id = cscart_products_categories.product_id 
WHERE 
  cscart_discussion.thread_id = 13200 
  AND cscart_products_categories.link_type = 'M'

Query time 0.00038

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_discussion",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "object_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["thread_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_products_categories",
          "access_type": "ref",
          "possible_keys": ["link_type", "pt"],
          "key": "pt",
          "key_length": "6",
          "used_key_parts": ["product_id", "link_type"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "cscart_products_categories.link_type = 'M'"
        }
      }
    ]
  }
}