建议用context by +limit -1的方式查询，以下是我本地执行效率比较，两者相差800余倍：
//用context by + limit -1 ,耗时9.6ms
timer select SecurityID from loadTable("dfs://SH_TSDB_tick","tick") where date(TradeTime) = 2021.01.04 context by SecurityID limit -1
//用distinct方式，耗时842.5ms
timer select distinct SecurityID from loadTable("dfs://SH_TSDB_tick","tick") where date(TradeTime) = 2021.01.04;
