SELECT 
  position 
FROM 
  cscart_dosiin_post_categories 
WHERE 
  post_id = 422 
  AND category_id = 3

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_dosiin_post_categories",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["post_id", "category_id"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}