SELECT 
  c.company_id 
FROM 
  cscart_products_categories pc 
  LEFT JOIN cscart_categories c ON c.category_id = pc.category_id 
WHERE 
  pc.product_id = 290 
GROUP BY 
  c.company_id 
LIMIT 
  2

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.004136432,
    "filesort": {
      "sort_key": "c.company_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "pc",
              "access_type": "ref",
              "possible_keys": ["pt"],
              "key": "pt",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "loops": 1,
              "rows": 1,
              "cost": 0.001792605,
              "filtered": 100,
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "c",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "p_category_id"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["category_id"],
              "ref": ["u358358020_main.pc.category_id"],
              "loops": 1,
              "rows": 1,
              "cost": 0.00171364,
              "filtered": 100
            }
          }
        ]
      }
    }
  }
}