SELECT 
  IFNULL(position, 0) 
FROM 
  cscart_cp_video_product_links 
WHERE 
  video_id = 34 
  AND product_id = 501 
  AND type = 'T'

Query time 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_cp_video_product_links",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "12",
      "used_key_parts": ["product_id", "video_id", "type"],
      "ref": ["const", "const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}