SELECT 
  data 
FROM 
  cscart_storage_data 
WHERE 
  data_key = 'dev_mode'

Query time 0.00606

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "391.04"
    },
    "table": {
      "table_name": "cscart_storage_data",
      "access_type": "ALL",
      "rows_examined_per_scan": 3803,
      "rows_produced_per_join": 380,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "353.01",
        "eval_cost": "38.03",
        "prefix_cost": "391.04",
        "data_read_per_join": "291K"
      },
      "used_columns": [
        "data_key",
        "data"
      ],
      "attached_condition": "(`mobikul_delivery_boy_livedemo`.`cscart_storage_data`.`data_key` = 'dev_mode')"
    }
  }
}