Hi, Glad to see this discussion.
I am on my travel so I have no enough time these days to join with you but I will never miss this discussion.. The discussion is important because the query is the highest frequent function that user use IoTDB and make data valuable. Besides, it will impact how we transfer ioTDB's schema view to a relational schema view (e.g., be integrated with Calcite.). I think Lei Rui got the key difference: the wide, the narrow and the narrowest table depends on "how to align the data". - If we want to align all timeseries according to the timestamp, then it is a wide table. (minor issue: I think for sql "select * from root.sg_1", the printed result should be "d1.s1, d1.s2..." rather than "root.sg1.d1.s1, root.sg1.d1.s2". So the table's head row can be concise) - If we want to align all timeseries that belong to the same device (i.e., the data source) according to the time stamp, then it is a narrow table. - If we do not want to align data, then it is the narrowest table. I do not know which one that users like. If we decide support all the three format, maybe an ALIGN clause can be introduced in our SQL. (Well, Jialin said it as "Group By", I am not sure which one is better). Best, 在 2019年9月7日星期六，Rui, Lei <nezhaleg...@163.com> 写道： > Sorry, pictures cannot be attached in the last email I sent . So I > supplement them here in text. > The "wide" table is: > | time | root.sg_1.device_1.sensor_1 | root.sg_1.device_1.sensor_2 | > root.sg_1.device_2.sensor_1 | root.sg_1.device_2.sensor_2 | > | 1 | 100 | 2.5 | 99 | 1.3 | > | ... | ... | ... | ... | ... | > > > The "narrow" table is : > | time | device_Id | sensor_1 | sensor_2 | > | 1 | root.sg_1.device_1 | 100 | 2.5 | > | ... | ... | ... | ... | > | 1 | root.sg_1.device_2 | 99 | 1.3 | > | ... | ... | ... | ... | > On 9/7/2019 15:51，Rui, Lei<nezhaleg...@163.com> wrote： > Hi, > > > I try to make this proposal more concrete from a semantic perspective. > > > Consider the sql "select * from root.sg_1". The following format is the > "wide" table: > > > The following format is the "narrow" table: > > > The levels of data from low to high are: > - sensor data, or series data, e.g., from root.sg_1.device_1.sensor_1 > - device data, e.g., from root.sg_1.device_1 > - storage group data , e.g., from root.sg_1 > > > So, the sql "select * from root.sg_1" queries data at the storage group > level. To present the results, > the wide table aligns all series data across multiple devices in the > storage group by timestamp, > while the narrow table aligns series data in a single device by timestamp, > and does the same for other devices in the storage group. > > > By the way, I guess the "narrowest" table is for a single sensor's data, > without the need to align with any other series data. > > > I have one question: > Why not make full use of sql and just use "select * from > root.sg_1.device_1" to specify the device (or the data level) they care > about? > Why use "select * from root.sg_1" with a narrow table format? > > > Lastly, I think the better query execution efficiency that a narrow table > may sometimes has is not the drive purpose, > because presenting the query result in a wide table and in a narrow table > are two different tasks. > > > Sincerely, > Lei Rui > > > From： Jialin Qiao <qj...@mails.tsinghua.edu.cn> > Date： 9/7/2019 15:26 > To： <dev@iotdb.apache.org> > Subject： Re: A new result set format > Hi Julian， > > He is my friend and contacted me offline, because I advertise IoTDB in my > weChat(like facebook or twitter). > > Next time I will try to let him put issue in the mail list himself :) > > Best, > -- > 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: "Julian Feinauer" <j.feina...@pragmaticminds.de> > 发送时间: 2019-09-07 13:52:17 (星期六) > 收件人: "dev@iotdb.apache.org" <dev@iotdb.apache.org> > 抄送: > 主题: Re: A new result set format > > Hi Jialin, > > perhaps one question about "wanted by users" means (as I didn’t see > anything on the list). > How do these users get in contact with you? > > Julian > > Am 07.09.19, 04:29 schrieb "Jialin Qiao" <qj...@mails.tsinghua.edu.cn>: > > Hi, > > As described in this issue, a new result set format is wanted by users. > I'd like to open a discussion here. > > For simplicity, I refer this format "time, root.sg1.d1.s1, root.sg1.d2.s1" > to wide table, and "time, deviceId, s1" as narrow table. > > This issue is not only about how to organize the results, but also the > query process. > > There are some advantages about narrow table. > > (1) For wide table, we need to open a SeriesReader for each series at the > same time, each SeriesReader holds some ChunkMetadatas. For narrow table, > we only need to open SeriesReaders for one device at one time, then return > results and open SeriesReaders for the next device, which occupies less > memory compared to the wide table. > (2) Avoid reading all series at once may also improve the query latency. > > There is also a question: > > (1) If we show result in the narrow table format for users, do we need to > highlight the concept of table and device? > (2) If the answer of the first question is yes, do we need to support sql: > "select time, deviceId, s1, s2, s3 from root.sg1 where deviceId=d1"? This > may involve a lot of work... > > From my side, I prefer the answers of the two questions are all NO. Then > we do not need to change the sql grammar and only use a new query process > to organize the result set. > > Best, > -- > 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: "Jialin Qiao (Jira)" <j...@apache.org> > 发送时间: 2019-09-07 09:40:00 (星期六) > 收件人: dev@iotdb.apache.org > 抄送: > 主题: [jira] [Created] (IOTDB-203) A new result set format > > Jialin Qiao created IOTDB-203: > --------------------------------- > > Summary: A new result set format > Key: IOTDB-203 > URL: https://issues.apache.org/jira/browse/IOTDB-203 > Project: Apache IoTDB > Issue Type: New Feature > Reporter: Jialin Qiao > > > When executing a SQL like "select d1.s1, d2.s1 from root.sg1", the default > result set format in IoTDB is > > "time, root.sg1.d1.s1, root.sg1.d2.s1" > > 1 , 1, 1 > > 2, 2, 2 > > However, some users want to get another format, The results could be > grouped by device, then sorted by time. > > "time, deviceId, s1". > > 1, root.sg1.d1, 1 > > 2, root.sg1.d2, 2 > > > > This can be done in the client, but it would be better if we support this > format in the server. > > > > > > -- > This message was sent by Atlassian Jira > (v8.3.2#803003) > > -- ----------------------------------- Xiangdong Huang School of Software, Tsinghua University 黄向东 清华大学 软件学院
