需要加上插件名，例如signal::fft
q = select signal::fft(Volume) as fft from trans
我从https://gitee.com/dolphindb/DolphinDBPlugin/tree/release200.9/signal下载插件，然后
loadPlugin("/opt/dolphindb/server/plugins/signal/PluginSignal.txt")
但运行
q = select fft(Volume) as fft from trans
报错：
