问题
I have some measurement data in my influxdb database which I can query with:
select * from E_real_con
name: E_real_con
time value
---- -----
1537920001044785525 | 57160036.00
1538006401069651036 | 57227208.00
1538092800108297103 | 57294112.00
1538179200697333731 | 57366108.00
However, "value" is a cumulative value and I would like to get the delta/difference between two consecutive values.
I tried the following:
SELECT difference(last(value)) FROM E_real_con WHERE time >= now() - 7d GROUP BY time(1d) fill(null)
However, I get the following error message:
ERR: unsupported difference iterator type: *query.stringInterruptIterator
I would be happy to get some hints and feedback how to solve my issue.
I am using influxdb 1.6.1
Thanks a lot! Christoph
回答1:
I found the solution. The following two mistakes had to be corrected:
1) The values in the measurement were of type "string" and not "float". As the data was coming from nodered, I cleared the database and used parseFloat() in nodered before writing the data to influxdb. BTW: you can check the datatype of your measurement field by:
SHOW FIELD KEYS FROM E_real_con
2) It seems that the query command requires a "where"
This works:
SELECT difference(last(value)) FROM E_real_del WHERE time >= now() - 7d GROUP BY time(1d)
whereas:
SELECT difference(last(value)) FROM E_real_del GROUP BY time(1d)
does not work.
I hope this might help someone else.
回答2:
2) It seems that the query command requires a "where"
It's even more restricted than that. It requires a where with a minimum timestamp.
For example, the following gives me no result:
select difference(last(value)) from WaterConsumption_Total where time < now() - 1d group by time(1d) fill(previous)
While this does:
select difference(last(value)) from WaterConsumption_Total where time > '2019-08-23T00:00:00Z' group by time(1d) fill(previous)
This effectively makes it impossible to use such a query as a continuous query.
来源：https://stackoverflow.com/questions/52566289/using-influxdb-difference-function