SELECT 
  video_id 
FROM 
  cscart_cp_video_product_links 
WHERE 
  product_id = 3963 
  AND type = 'I' 
  AND is_main_img = 'Y'

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.70"
    },
    "table": {
      "table_name": "cscart_cp_video_product_links",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "idx_product_type_position"
      ],
      "key": "idx_product_type_position",
      "used_key_parts": [
        "product_id",
        "type"
      ],
      "key_length": "9",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 0,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "0.50",
        "eval_cost": "0.02",
        "prefix_cost": "0.70",
        "data_read_per_join": "4"
      },
      "used_columns": [
        "video_id",
        "product_id",
        "type",
        "is_main_img"
      ],
      "attached_condition": "(`cscart_db`.`cscart_cp_video_product_links`.`is_main_img` = 'Y')"
    }
  }
}