Hi, Thanks, Dawei and Jialin.
> Is last an aggregation function or is the result format of last the same with aggregation queries? Ok... let me agree with you temporarily. Now we have some syntax like `select last s1, s2`... There are two concerns: (1) Do we support select last *? I think it is needed in some applications. (2) I do not think it is good to let the result data type of `select last s1 from root ` as "｜ path | max_time |. last_value. |" As we are designing something like SQL, we are not designing API. We need to let the semantic clear enough. So, my suggestion, if user write `select last s1 from root`, then return a wide-column table; if user write `select last s1 s2 from root disable align`, then return narrow table. Best, ----------------------------------- Xiangdong Huang School of Software, Tsinghua University 黄向东 清华大学 软件学院 Dawei Liu <[email protected]> 于2020年2月21日周五 下午8:37写道： > Hi， > > I would like to introduce the current requirements first. Imagine when the > user executes an SQL: > > Select last_value(*),max_time(*) from root.sg1.* > > The result is: > > > +-------------+-------------+------------+---------------+-----------+-------------+ > ｜last_value(s1) | max_time(s1)| last_time(s2) | > max_value(s2) | last_time(N) | max_value(N) | > > +-------------+-------------+------------+---------------+-----------+-------------+ > > > We all know that in the Internet of things, a device typically contains > multiple sensors, and a set of requirements contains multiple devices. > The result set above will have N * M columns, sometimes up to 50,000 > columns, which is a very unfriendly way to return data. > > So the way we expect to return the data is： > > +-------+-------------+-------------+ > ｜ path | max_time |. last_value. | > +-------+-------------+-------------+ > | s1. |. 1 | 1 | > +-------+-------------+-------------+ > |. s2 | 2 | 2 | > +-------+-------------+-------------+ > ……. > > Business processes max_time and last_value in many cases come in pairs, so > we would like to see a simplified query function named last, so we have > listed several candidates for SQL to support the new return format > conversion. Which one do you prefer? > > 1. select last(s1), last(s2) from root.sg.d > 2. select last(s1), last(s2) from root.sg.d align by sensor > 3. select last(s1, s2) from root.sg.d > 4. select last s1, s2 from root.sg.d > 5. last s1, s2 from root.sg.d > 6. select s1, s2 from root.sg.d last > 7. select s1, s2 from root.sg.d onlylast > > > > Thanks > --- > Dawei Liu > > > > > 2020年2月21日 下午8:09，Jialin Qiao <[email protected]> 写道： > > > > Hi, > > > > Actually we do not have "last" syntax now. There are two ways to treat > last: > > > > (1) Treat it as a special case of raw data query > > (2)Treat it as a aggregation query > > > > Before deciding, let's consider two questions: > > > > (1) Is last an aggregation function or is the result format of last the > same with aggregation queries? > > > > I thinks it's not. One aggregation function should return one column. > For example, the result of "select count(s1) from xx" has one column: > count(s1). > > However, the last should return three columns: path, time, value > > > > (2) Could "last" be used with other aggregations at the same time? > > > > NO, if users issue a last query, he should query the last of all > selected series. "select last(s1), count(s2)" is not allowed. > > i.e., it could only be this form: select last(s1), last(s2), last(s3), > last(s4) from root.sg.d1. And if so, why we duplicate last so many > times... > > > > Therefor, I prefer treat last as a special case of raw data query. > i.e., 4. select last s1, s2 from root.sg.d1 > > > > Thanks, > > -- > > Jialin Qiao > > School of Software, Tsinghua University > > > > 乔嘉林 > > 清华大学 软件学院 > > > >> -----原始邮件----- > >> 发件人: "Xiangdong Huang" <[email protected]> > >> 发送时间: 2020-02-21 19:44:46 (星期五) > >> 收件人: [email protected] > >> 抄送: > >> 主题: Re: Last Query syntax vote > >> > >> Hi, > >> > >> +1 for select last(s1), last(s2) from root.sg.d > >> > >> I think this vote description does not show why we need a new syntax and > >> the data result format we will get... > >> > >> For example, the first SQL is a common query statement, but why it is > not > >> suitable and you want a new syntax? > >> > >> It is hard to let other guys joining the vote. > >> > >> Best, > >> ----------------------------------- > >> Xiangdong Huang > >> School of Software, Tsinghua University > >> > >> 黄向东 > >> 清华大学 软件学院 > >> > >> > >> Dawei Liu <[email protected]> 于2020年2月21日周五 下午5:35写道： > >> > >>> Hi, > >>> > >>> I’d vote for > >>> > >>> [2] select last(s1), last(s2) from root.sg.d align by sensor > >>> > >>> It looks simple, it's easy to use, it's human logic > >>> > >>> > >>> Thanks > >>> --- > >>> Dawei Liu > >>> > >>> > >>> > >>>> 2020年2月21日 下午5:26，Wei Shao <[email protected]> 写道： > >>>> > >>>> Hi, > >>>> > >>>> > >>>> I'd vote for > >>>> [4]. select last s1, s2 from root.sg.d > >>>> > >>>> > >>>> Thanks, > >>>> Wei > >>>> > >>>> > >>>> > >>>> At 2020-02-21 17:08:14, "Wei Shao" <[email protected]> wrote: > >>>>> Hi, > >>>>> > >>>>> > >>>>> We are to determine the exact syntax of Last Query for IoTDB. Last > >>> query is to select the latest time-value pair result for a given > timeseries. > >>>>> You can choose from the following syntax options. > >>>>> 1. select last(s1), last(s2) from root.sg.d > >>>>> 2. select last(s1), last(s2) from root.sg.d align by sensor > >>>>> 3. select last(s1, s2) from root.sg.d > >>>>> 4. select last s1, s2 from root.sg.d > >>>>> 5. last s1, s2 from root.sg.d > >>>>> 6. select s1, s2 from root.sg.d last > >>>>> 7. select s1, s2 from root.sg.d onlylast > >>>>> > >>>>> > >>>>> Please vote for the option you are comfortable with. > >>>>> > >>>>> > >>>>> Thanks, > >>>>> Wei > >>> > >>> > >
