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

Query time 0.00914

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "8.05"
    },
    "table": {
      "table_name": "cscart_cp_video_product_links",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 23,
      "rows_produced_per_join": 0,
      "filtered": "1.00",
      "index_condition": "(`cscart_db`.`cscart_cp_video_product_links`.`type` = 'I')",
      "cost_info": {
        "read_cost": "5.75",
        "eval_cost": "0.02",
        "prefix_cost": "8.05",
        "data_read_per_join": "5"
      },
      "used_columns": [
        "video_id",
        "product_id",
        "type",
        "is_main_img"
      ],
      "attached_condition": "(`cscart_db`.`cscart_cp_video_product_links`.`is_main_img` = 'Y')"
    }
  }
}