SELECT 
  product_ids 
FROM 
  cscart_ec_em_viewed_products 
WHERE 
  user_id = 0

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.20"
    },
    "table": {
      "table_name": "cscart_ec_em_viewed_products",
      "access_type": "ALL",
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "1.00",
        "eval_cost": "0.20",
        "prefix_cost": "1.20",
        "data_read_per_join": "280"
      },
      "used_columns": [
        "user_id",
        "product_ids"
      ],
      "attached_condition": "(`rajansecarter_15_april_mve`.`cscart_ec_em_viewed_products`.`user_id` = 0)"
    }
  }
}